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Mon.Not.R.Astron.Soc.000,1–15(2012) Printed10January2012 (MNLATEXstylefilev2.2) The angular momentum of disc galaxies: implications for gas accretion, outflows, and dynamical friction Aaron A. Dutton1⋆ , Frank C. van den Bosch2 1Department of Physics and A†stronomy, University of Victoria, Victoria, BC, V8P 5C2, Canada 2Astronomy Department, Yale University,P.O. Box 208101, New Haven, CT 06520-8101, USA 2 Accepted 2011December 5.Received2011October29;inoriginalform2011June2 1 0 2 ABSTRACT n Wecombineconstraintsonthegalaxy-darkmatterconnectionwithstructuralanddy- a namical scaling relations to investigate the angular momentum content of disc galax- J ies. For haloes with masses in the interval 1011.3M⊙ < Mvir < 1012.7M⊙ we find 7 that the galaxy spin parameters are basically independe∼nt of hal∼o mass with λ′ (J /M )/(√2R V ) = 0.019+0.004(1σ). This is significantly lower thanh fgoarlire≡- ] gal gal vir vir −0.003 O laxed ΛCDM haloes, which have an average spin parameter λ′ = 0.031 0.001. h haloi ± C The average ratio between the specific angular momentum of disc galaxies and their hostdarkmatter haloes is therefore λ′ /λ′ =0.61+0.13.This calls into ques- . Rj ≡ gal halo −0.11 h tion a standard assumption made in the majority of all (semi-analytical) models for p (disc) galaxy formation, namely that = 1. Using simple disc formation models j - R o we show that it is particularly challenging to understand why j is independent of R r halo mass, while the galaxy formation efficiency (ǫGF, proportional to the ratio of st galaxy mass to halo mass) reveals a strong halo mass dependence. We argue that a the empirical scaling relations between ǫ , and halo mass require both feedback GF j [ (i.e., galacticoutflows)andangularmomentuRm transferfrom the baryonsto the dark matter (i.e., dynamical friction). Most importantly, the efficiency of angular momen- 2 v tum loss need to decrease with increasing halo mass. Such a mass dependence may 3 reflecta biasagainstformingstablediscs inhighmass,lowspinhaloesoratransition 6 from cold-mode accretion in low mass haloes to hot-mode accretion at the massive 6 end. However,currenthydrodynamicalsimulations ofgalaxyformation,which should 0 include these processes,seem unable to reproduce the empirical relation between ǫ GF 8. and j. We conclude that the angular momentum build-up of galactic discs remains R 0 poorly understood. 1 Key words: galaxies: formation — galaxies: fundamental parameters — galaxies: 1 : spirals — galaxies: structure v i X r a 1 INTRODUCTION make galaxy discs with the observed sizes (e.g., Dalcanton etal.1997;Mo,Mao,&White1998;Firmani&Avila-Reese In the current paradigm of galaxy formation, galaxy discs 2000; de Jong & Lacey 2000; Navarro & Steinmetz 2000). areconsideredtoformfromtheaccretion ofgasinsidehier- In fact, models in which galaxies have 30% lower spe- archicallygrowingcolddarkmatter(CDM)haloes(seeMo, ∼ cificangularmomentumthantheirdarkmatterhaloesmay vandenBosch&White2010foracomprehensiveoverview). be preferred (Dutton et al. 2007). Another success of this Thedarkmatter(DM)andgasacquireangularmomentum picture is that the large variation in the specific angular viatidaltorquesintheearlyUniverse(Peebles1969).When momentum of dark matter haloes, at fixed halo mass, ex- gasaccretesontothecentralgalaxy,thisangularmomentum plains the large scatter in disc sizes, or equivalently surface mayeventuallyhaltthecollapseandleadtotheformationof brightnesses, at fixed luminosity (Dalcanton et al. 1997). arotationallysupporteddisc(Fall&Efstathiou1980).Ifthe Thispicturealsoaccountsforthetimeevolutionofdiscsizes specificangularmomentumofthebaryonsisconserveddur- (Somerville et al. 2008; Firmani & Avila-Reese 2009; Dut- inggalaxyformation,thereisenoughangularmomentumto ton et al. 2011a), reproducing the observed weak evolution since redshift z 1 (Barden et al. 2005), and the stronger ≃ ⋆ [email protected] † CITANationalFellow (cid:13)c 2012RAS 2 A.A. Dutton & F.C. van den Bosch evolutionobservedfrom z 2toz 1(Trujilloetal.2006; attributed to lack of numerical resolution (e.g. Governato ∼ ∼ Williams et al. 2010). et al. 2004; Kaufmann et al. 2007). However, Piontek & Despitethemanysuccessesofthesemodels,allofwhich Steinmetz(2009) arguethatthedominantcauseisdynami- assumethatspecificangularmomentumisconservedduring calfriction betweenclumpsofbaryonsand thedarkmatter galaxy formation, thereare severalreasons, from aphysical halo. If, based on these findings, we assume that angular pointofview,astowhytheratiobetweenthespecificangu- momentumlossisanaturalfeatureofdiscgalaxyformation lar momentum of the baryons and that of the dark matter in a ΛCDM universe, a relevant question is the following: halo, denoted 1, might bedifferent from unity: If the baryons that make up a discy galaxy carry half the j R specificangularmomentum ofthedarkmatter haloinwhich (i) Selective accretion of baryons. If not all available they reside, is it possible to reproduce in detail the observed baryons make it into the galaxy, the specific angular mo- sizes of disc galaxies? mentumofthegalaxycanbeeitherhigher( >1)orlower Rj This question was addressed by Navarro & Steinmetz ( < 1) than that of all baryons (whether it is higher or Rj (2000). Using scaling relations between disc size, luminos- lowersimplydependsonthespecificangularmomentumdis- ity, and rotation velocity, these authors derived an expres- tribution ofthematerials that areselected). Anexample of sionfor (f intheirnotation)thatdependsonthegalaxy j j a selective accretion process is cooling. Cooling is generally R formation efficiency, ǫGF (Mgal/Mvir)/(Ωb/Ωm) (fbdsk in aninside-outprocess,withtheinnerregionscoolingpriorto ≡ theirnotation),cosmicmatterdensity,Ωm,andstellarmass- the more outer regions. Since the mass of ΛCDM haloes is to-light ratio, Υ . Theyfound that if therotation speedsof I moreconcentratedthanthespecificangularmomentum(see galaxy discs are approximately the same as the circular ve- e.g., Navarro & Steinmetz 1997), one expects that < 1 Rj locitiesoftheirsurroundinghaloes(whichhassubsequently if not all baryonsmanage tocool. been shown to be the case, at least on average, by Dutton (ii) Exchange of angular momentum between the dark etal.2010b),thendiscsmusthaveretainedabouthalfofthe matter and the baryons. An example of this is dynamical availablespecificangularmomentumduringtheirassembly. friction,whichtransfersorbitalangularmomentumfromthe Anotherresultfrom thisanalysis isthat discgalaxies form- baryons to the dark matter, thus resulting in <1 (e.g., Rj ing in low density cosmogonies, such as ΛCDM, need to Navarro&Benz1991;Navarro&White1994).Thestrength have j 2ǫGF, that is, they draw a larger fraction of the ofthisprocessdependsontheefficiencyofstarformationat R ≃ specific angular momentum than the galaxy formation effi- earlytimes.Iftheformationofbaryonicclumpscanbesup- ciency.Thisisatoddswiththeexpectation(see 4.1below) pressed, such as through strong feedback, then the angular § that j ǫGF if the galaxy formation efficiency is mainly momentumtransferisreduced(e.g.,Weil,Eke&Efstathiou R ≃ determined by theinefficiency of cooling. 1998;Sommer-Larsen,Gelato&Vedel1999;Eke,Efstathiou Thus observational measurements of the specific an- & Wright 2000). gular momentum in galaxies place constraints on the way (iii) Feedback.If feedback (from supernova, AGN or any galaxies acquire their mass and angular momentum. The other mechanism) results in the removal of gas, one can specific angular momentum of the dark matter, Jvir/Mvir, achieve values for that are very different from unity Rj where Jvir is the total angular momentum, and Mvir is the by simply removing a specific subset of the baryons. As an total virial mass, is commonly expressed in terms of thedi- example, several studies have argued that supernova feed- mensionless spin parameter (Peebles 1969): backaffectslowangularmomentummaterialmorethanhigh a(Mngaulllearr&moDmeekneltu2m002m;aDteurtitaol,nc&auvsainngdaennBetosinchcre2a0s0e9;oDf uRtj- λhalo= JGvirM|Ev5|i/1r3/2 = √J2viRr/vMirVvivrir fc1/2 =λ′halofc1/2 (1) ton 2009; Governato et al. 2010; Brook et al. 2011). Here E is the halo’s energy, Rvir is the virial radius, Vvir is Cosmological hydrodynamical simulations almost in- the circular velocity at the virial radius, and fc measures variably predict that disc galaxies have significantly less thedeviationofE fromthatofasingularisothermalsphere specific angular momentum than their dark matter haloes truncated at Rvir. It is common to set fc = 1 (Bullock ( < 1), suggesting that mechanisms (i) and (ii) dis- et al. 2001), so that the spin parameter only depends on j R cussed above play an important role. Early simulations the total angular momentum and mass of the halo. In this in a standard cold dark matter (SCDM) cosmology (i.e. casethespinparameterisdenotedλ′ .Theequivalentspin halo Ωm = 1,ΩΛ = 0) resulted in a catastrophic loss of angu- parameter for thegalaxy is defined as lar momentum (Navarro & Benz 1991; Navarro & White (J /M ) 1o9f9m4;agSntietiundmeettzoo&smNaalvla.rMroo1re99r9e)c,enptrosdimucuilnagtiodnisscsinanΛoCrdoeldr λ′gal = √g2aRlvirVgvalir , (2) Dark Matter (ΛCDM) cosmologies haveresulted in less an- (see Section 2.1 for details). gular momentum loss, but typical ratios between the spe- Dark matteronly cosmological simulations haveshown cificangularmomentumofthebaryonsandthatofthedark thatbothλhalo andλ′halo arelog-normally distributed(e.g., matter are 50% (Piontek & Steinmetz 2011; Sales Bullock et al. 2001) with median and scatter independent j R ≃ et al. 2010). This “angular momentum loss” is sometimes of halo mass (e.g., Maccio` et al. 2007; Bett et al. 2007; Mun˜oz-Cuartas et al. 2011) and independent of environ- ment (Lemson & Kauffmann 1999, Maccio` et al. 2007). 1 Notethat inthe notation of Mo,Mao,& White (1998), Rj = For relaxed haloes identified with a spherical overdensity (jd/md), where jd is the ratio between the angular momentum (SO) algorithm Maccio` et al. (2008) find a median value of ofthediscanddarkmatterhalo,andmdistheratiobetweenthe λ′halo = 0.031±0.001. Using a similar halo definition Bett massofthediscanddarkmatterhalo. et al. (2007) find a median λhalo = 0.0367. For the cosmol- (cid:13)c 2012RAS,MNRAS000,1–15 Angular momentum of disc galaxies 3 ogy adopted by Bett et al. , fc 1.3 for a 1012M⊙ halo, galaxy-galaxy lensing (e.g, Mandelbaum et al. 2006; Cac- ′ ≃ andthusλ 0.032,whichisinexcellentagreementwith ciatoetal.2009),abundancematching(e.g.,Conroy,Wech- halo ≃ the results of Maccio` et al. (2008). Simulations with gas in sler & Kravtsov 2006), and satellite kinematics (e.g., More whichthereisnocoolingfindsimilarspinparametersforthe et al. 2009). baryonsanddarkmatter(vandenBoschetal.2002;Sharma In this paper we compute the average specific angu- &Steinmetz2005).Thusinwhatfollowsweassumethatthe lar momentum and spin parameters of disc galaxies as a median spin parameter of thehalo is function of their halo mass. We improve on previous stud- ies in a number of ways. We constrain halo masses us- ′ λhalo=0.031±0.001. (3) ing recent results for the halo mass - stellar mass relation for disc galaxies obtained from weak gravitational lensing Themaingoalofthispaperistodeterminetheaverage and satellite kinematics (Dutton et al. 2010b), rather than valueof for alarge sample of discgalaxies as afunction j R abundance matching results as in Tonini et al. (2006), or of their halo mass, and to use semi-analytical models to the highly questionable assumption of a constant galaxy explore implications for how disc galaxies form. Since it is massfractionmadebyHernandez&Cervantes-Sodi(2006), impossible to measure the specific angular momentum (or Hernandez et al. (2007), and Berta et al. (2008). We also spin parameter) of a real (as opposed to simulated) dark improve upon these studies by taking account of realis- matterhalo,onecanonlyobtainanestimatefortheaverage tic uncertainties on halo masses. We consider both stel- value of by comparing the distribution of galaxy spin j parameteRrs, λ′ , obtained from a sample of galaxies to the lar and gaseous discs, rather than just stellar discs, which distribution ofgahlalo spin parameters, λ′ , obtained from is important for low mass galaxies which tend to be gas halo rich (Tonini et al. 2006). We calculate disc angular mo- numerical simulations of structure formation in a ΛCDM mentum by integrating a model that is constructed to re- cosmology.Thisistheapproachwewillfollowinthispaper. Measuring λ′ for an individual galaxy necessarily in- produce the size-mass and velocity-mass (i.e., Tully-Fisher, gal Tully & Fisher 1977) relations, rather than simple scaling volves thefollowing steps: arguments for exponential discs in isothermal haloes (i.e., (i) Determine the specific angular momentum of the Jgal/Mgal =2RdVrot),asusedbye.g.,Navarro&Steinmetz baryons. (2000) and Hernandez et al. (2007). We also discuss how (ii) Determine the mass (and virial radius) of the dark uncertainties related to stellar mass-to-light ratios and adi- matter halo in which thegalaxy resides. abaticcontractionimpacttheinferredvaluesof j,anduse R simple models of disc galaxy formation to discuss the im- Step (i) is relatively straightforward. Using that, for a disc plications of our findings for cooling, outflows and angular galaxy, momentum loss. Rvir We adopt a flat ΛCDM cosmology with matter den- Jgal =2πZ Σ(R)Vrot(R)RdR, (4) sity parameter Ωm =0.27, baryon density Ωb =0.044, and 0 Hubble parameter h = H0/100kms−1Mpc−1 = 0.7. Dark itisclearthatonecandeterminetheangularmomentumof matter halo masses are defined so that the mean density of adiscgalaxyfrommeasurementsofitssurfacemassdensity, the halo (assumed to be spherical) is 200 times the critical Σ(R), (accounting for both gas and stars), and its rotation density. curve Vrot(R) (see e.g., van den Bosch, Burkert & Swaters 2001). Alternative, since disc galaxies in general have ex- ponential surface density profiles and flat rotation curves, one can use simple scaling relations between mass (or lu- minosity), size and characteristic rotation velocity in order 2 DEFINITIONS AND MASS MODELS to obtain an estimate of J without having to measure gal This section gives a brief overview of the main parameters Σ(R) or Vrot(R) (e.g., Navarro & Steinmetz 2000; Tonini we discuss in this paper, and the constrained mass models et al. 2006; Hernandez & Cervantes-Sodi 2006; Hernandez we use toconstrain them. et al. 2007; Berta et al. 2008). Although less accurate than using Eq. (4), this method has the advantage that it can beapplied to much larger samples of galaxies. Step (ii) is a little bit more involved. Typically, accurate measurements 2.1 Mass and Angular Momentum Parameters of halo mass require probes on scales of the virial radius. Examples of such ‘probes’ are gravitational lensing, X-ray There are four main parameters that are relevant for what dataandsatellitekinematics.However,unlessthegalaxyin follows:galaxymass,Mgal,halovirialmass,Mvir,galaxyan- question is the central galaxy of a cluster-sized halo, such gular momentum, Jgal, and the angular momentum of the measurements are rarely available for individual systems, dark matter halo, Jvir. The galaxy mass is the sum of the andonehastorely onuncertainestimates ofthehalomass massinstarsandcoldgas.Thevirialmassisthetotalmass from thegalaxy’s rotation curve.Alternatively,onecan use (stars, cold gas, hot gas, dark matter) within the virial ra- variousstatisticaltechniquestoassigntoeachgalaxyanav- dius. The galaxy angular momentum is the sum of the an- erage halo mass. Recent years have seen dramatic progress gular momenta of the stars and of the cold gas. The virial inusingavarietyoftechniquestoconstraintheaveragerela- angular momentum is the total angular momentum within tionbetweengalaxyproperties(typicallyluminosityofstel- thevirial radius. larmass)andhalomass,inparticularfromgalaxyclustering Our main goal is to constrain the angular momentum (e.g., Yang, Mo & van den Bosch 2003; Tinker et al. 2005), ratio, , defined as the ratio between the galaxy specific j R (cid:13)c 2012RAS,MNRAS000,1–15 4 A.A. Dutton & F.C. van den Bosch angular momentum and the total specific angular momen- lensing, satellite kinematics, galaxy clustering, and abun- tum2: dancematching.Wewillusethescatteramongthesediffer- (J /M ) λ′ entstudies/techniques,asanestimatefortheuncertaintyin Rj ≡ (Jgvairl/Mgvairl) = λ′hgaallo , (5) tthieesMtovitrhvoss.eMonstaǫrGrFelaantidon,ja.nFdinparollpy,agtahteertehlaetsieonunbceetrwtaeienn- where the spin parameters λ′halo and λ′gal are defined by halo mass, Mvir, and haloRconcentration, c, is taken from Eqs. (1) and (2), respectively. Using that for disc galaxies Maccio` etal.(2008),andiscalibrated usinghigh-resolution Jgal RdMgalVrot, where Rd is the disc scale length and numerical simulations. ∝ Vrot isa characteristic rotation velocity of thedisc, onecan The usage of all these empirical and theoretical scal- use Eq.(5) to show that ingrelationsimpliesthattheninemodelparameterscanbe Rd ∝Rjλ′halo (cid:16)VVrvoirt(cid:17)−1 Rvir, (6) rfinuefldnuuctecienodcnets(oItMhtweFos)t,praurnicmdtuaarraylprpuernsokcprnieoprwttiinoessn:oftfohrtehhesotdwelalargkralmianxaiyttitagelrromhwaatslhos whichisthestandardapproachusedinanalyticalandsemi- (hereafter“thehalocontractionmodel”).Itiscommonprac- analytical models to predict the sizes of disc galaxies (e.g., tice to assume that disc formation is an adiabatic process Kauffmann1996;Mo,Mao&White1998;Somerville&Pri- and that the halo responds by contracting using the adi- mack 1999; van denBosch 2000; Cole et al. 2000). In many abatic contraction (hereafter AC) prescription of Blumen- casesthesestudiessimplifyEq.(5)byassumingthat j =1 thaletal.(1986;hereafterBFFP).However,numericalsim- R and that Vrot/Vvir is a constant. In this paper we examine ulations have shown that this prescription may not be suf- whether there is empirical support for the former assump- ficiently accurate, and several alternatives have been pro- tionandwhatthisimpliesfortheformation ofdiscgalaxies posed(e.g.,Gnedinetal.2004;Abadietal.2010).Wefollow in general. Aswe will demonstrate, it isinstructivetocom- Dutton et al. (2007) and use a generalized halo contraction pare j to thegalaxy formation efficiency parameter model, according to which a shell of dark matter initially R ǫGF ≡ (M(Ωgabl//ΩMmv)ir). (7) m(baetfioorne,daitscafroardmiuatsion) at radius ri ends up, after disc for- whichdescribeswhatfractionofthecosmologicallyavailable rf =ΓνBFFPri. (8) baryonshaveended up in a galaxy. HereΓBFFP istheamountofcontractionaspredictedbythe AC-model of BFFP, and ν is a free parameter. Note that 2.2 Mass Models ν = 1 corresponds to the standard adiabatic contraction model of BFFP, while the Gnedin et al. (2004) and Abadi Wecomputetheaveragevaluefor fordiscgalaxiesusing Rj et al. (2010) models can be well approximated by ν = 0.8 the mass models of Dutton et al. (2011b), which have been and ν =0.4, respectively. A model without contraction has constructed to reproduce the observed structural and dy- ν = 0, while ν < 0 indicates that the dark matter halo namical scaling relations of late-type(i.e., star-forming and responds todisc formation byexpanding. disc-dominated) galaxies. They consist of four components: AsshowninDuttonetal.(2011b),reproducingtheob- a stellar bulge, a stellar disc, a gas disc, and a dark matter servedTully-Fisherrelationimpliesdifferentvaluesforν for halo. Boththebulgeandthedarkmatterhaloareassumed adifferentchoiceof theIMF.Inthispaperwe will consider to be spherical and to follow a density distribution that is two different models that fit the Tully-Fisher relation (and given by a Hernquist profile (Hernquist 1990) and a NFW all other scaling relations mentioned above) equally well: profile (Navarro, Frenk, & White 1997), respectively. The modelAwhichadoptsaChabrier(2003)IMFandν = 0.5 density profile of the halo, however, is modified to account − (i.e., halo expansion), and model B, in which we adopt the forthegravitational responsetogalaxy formation (mostof- AC model of Gnedin et al. (2004), i.e., we set ν =0.8, and tenmodelledasadiabaticcontraction,seeBlumenthaletal. we assume an IMF that yields stellar mass-to-light ratios 1986). The disc is assumed to have an exponential surface that are 0.3 dex lower than for the Chabrier (2003) IMF. density and to be infinitesimally thin. AsshowninDuttonetal.(2011b) boththesemodelsarein Each halo-galaxy system is described by nine parame- excellent agreement with the data. As we will show below, ters: three for the halo (virial mass, Mvir, concentration, c, model B results in galaxy spin parameters that are 30 andaparameterthatdescribestheimpactofgalaxyforma- ∼ percent ( 0.12 dex) higher than those of model A. This tion on the halo density profile); and six for the baryons ∼ uncertainty is comparable to that arising from the uncer- (stellar mass, Mstar, gas mass, Mgas, bulge fraction, fb, tainty in the Mvir vs. Mstar relation. Throughout we will bulge size, R , stellar disc size, R , and gas disc size, b d mainly present the results for model A, while occasionally Rg). The 6 parameters for the baryons are constrained by showing, for comparison, the results for model B. the observed scaling relations: Rb vs. Mstar; Rd vs. Mstar; Foragiven choiceof IMFandν,themodel completely Rg/Rd vs.Mstar;fbvs.Mstar,andMgas/Mstar vs.Mstar (see specifiesthemassdistributionsofdisc-plus-bulgegalaxiesas Dutton et al. 2011b for details). For the relation between a function of halo mass. From these mass models we com- Mvir and Mstar we use the empirical relation from Dutton pute the angular momentum of the baryons by evaluating et al. (2010b), which is inferred from a large number of in- Eq. 4, where the surface density is the sum of the surface dependent studies and techniques, including galaxy-galaxy densities of the gas and stars: Σ(R) = Σgas(R)+Σstar(R). We assume that the bulge has no net angular momentum. 2 NotethatRj isnotanefficiencybecauseitcanbelargerthan Ifweassumedthatbulgesfollow thesamerotation curveas unity. the disc, then this would increase the specific angular mo- (cid:13)c 2012RAS,MNRAS000,1–15 Angular momentum of disc galaxies 5 Figure 1. Left: Galaxy specific angular momentum vs. virial mass for disc galaxies. The shaded regions indicate the one and two σ uncertainties on virial mass. The dashed line shows the relation between virial specific angular momentum and virial mass from cosmologicalN-bodysimulations,correspondingtoaspinparameterλ′=0.031.ThepentagonshowsestimatesfortheMilkyWay(MW) with the shaded region showing one and two σ uncertainties. Right: Specific angular momentum ratio (Rj) vs. virial mass. The solid line and shaded regions indicate the mean and the one and two σ uncertainties for our fiducial model A, which assumes a Chabrier (2003)IMFandahalocontractionmodelcharacterizedbyν=−0.5.ThedashedlineshowsthemeanformodelB,inwhichthestellar mass-to-lightratios are0.3dex smallerand ν =0.8. Note that, for both models, Rj isless than unity and approximately independent ofvirialmass. mentumofthegalaxyby<20%.Theeffectislargestforthe uncertaintiesinvirialmassesatfixedgalaxymass3,whichis most massive galaxies, bu∼tsince massivebulges, in general, the dominant source of uncertainty. The dashed line shows are not rotationally supported, we believe our assumption the relation between virial specific angular momentum and that bulges have no angular momentum to be of no major virial mass from cosmological N-body simulations (Maccio` concern for what follows. et al. 2008), which corresponds to a spin parameter of AshownbyToninietal.(2006)asignificantfractionof λ′ = 0.031. The galaxy specific angular momentum scales theangularmomentumofagalaxyisinthecoldgas.Inour withvirialmassasJ /M M0.70,whichisverysimilar modelsthecoldgasinhighmassgalaxies(Mgal ≃1011M⊙) tothevirialscalingogfaJlvir/gMalv∝ir ∝vMirv0i.r67.ThustheratioRj contains 30% of the angular momentum. This fraction betweenthegalaxyandvirialspecificangularmomentumis rises to ov≃er 50% for galaxy masses below Mgal 1010M⊙. roughly constant (right-handpanelof Fig. 1).However, the ≃ Thus if cold gas fractions increase at higher redshifts, as is galaxyspecificangularmomentumissignificantlylowerthan commonly thought (but see Dutton et al. 2010a), measur- the virial specific angular momentum with = 0.61+0.13 ingangularmomentumofhighredshiftgalaxieswillrequire (1σ)foravirialmassofMvir =1012M⊙.TheRdjashedli−ne0.1in1 measurements of the distribution of cold gas, and not just theright-hand panel of Fig. 1 corresponds to model B, and thestars. isshownforcomparison. Notethatthecombination ofhalo contractionandlowerstellarmass-to-lightratiosresultin j R values that are 0.1 higher than for our fiducial model A. ∼ 3 RESULTS This difference is comparable to the 1σ uncertainty on j R duetotheuncertaintiesintherelationbetweenstellarmass Usingourconstrainedmassmodelswenowdiscussanumber and halo mass. ofcorrelationsbetweenmassandangularmomentumofdisc The left-hand panel of Fig. 2 shows the correlation be- galaxiesanddarkmatterhaloes,andcomparewithprevious tweengalaxymassandvirialmassforthediscgalaxiescon- results. These correlations are presented in Figs. 1-3 and sideredhere.Thedashedlineshowstherelationforthecos- Table 1. mically available baryonic mass, i.e., Mgal =(Ωb/Ωm)Mvir, corresponding to ǫGF = 1. The right-hand panel of Fig. 2 3.1 Galaxy Mass and Specific Angular shows the galaxy formation efficiency, ǫGF, which is the ratio between the solid and dashed lines in the left-hand Momentum The left-hand panel of Fig. 1 shows the correlation be- tween galaxy specific angular momentum and virial mass 3 Notethatthesearetheuncertaintiesonthemean;theydonot for model A. The shaded regions show the one and two σ reflectthescatter. (cid:13)c 2012RAS,MNRAS000,1–15 6 A.A. Dutton & F.C. van den Bosch Figure2.Left:Galaxymassvs.virialmassfordiscgalaxies.Theshadedregionsreflecttheoneandtwoσuncertaintiesonvirialmasses. Thedashedlinecorrespondstotheuniversalbaryonfraction(Ωb/Ωm=0.163).ThepentagonshowsestimatesfortheMilkyWay(MW), withthe shaded regionshowing one andtwo σ uncertainties. Right:Galaxy formationefficiency (ǫGF)vs. virialmass.As inFig.1, we show the results for both models A and B. For clarity, we only show the impact of virial mass uncertainties for our fiducial model A. NotethatunlikeRj,whichrevealsnosignificantdependenceonhalomass(seeFig.1),thegalaxyformationefficiencyvariesbyafactor of∼3overthemassrangeprobed. panel. Unlike the angular momentum ratio, the galaxy for- An interpretation advocated by Tonini et al. (2006) is mationefficiencyisastrongfunctionofvirialmass,varying that if one restricts attention to haloes that havenot expe- by a factor of 3 from ǫGF 0.1 at the low mass end riencedamajormergersincez 3,theaveragespinparam- (Mvir 2 10∼11M⊙) to ǫGF ≃0.3 at the high mass end etersλ′ turnsout to bearound∼0.023 (D’Onghia& Burkert (Mvir ∼ 5×1012M⊙). Once a≃gain, the dashed line in the 2004). This is motivated by the idea that bulge-less disc ∼ × right-handpanelcorrespondstomodelB,whichyieldslower galaxies require a quiescent merger history. However, this galaxy formation efficiencies dueto thelighter IMF. simple expectation has been cast into doubt by numerical Aswedemonstrateanddiscussbelow,(semi)-analytical simulations which haveshown that (gas rich) discscan sur- modelsandhydrodynamicalsimulationsofgalaxyformation vive major mergers (e.g., Springel & Hernquist 2005; Hop- do generally not predict values of that are independent kinsetal.2009).Whilewecannotruleoutamergerhistory j R ofhalomass,andiftheydo,thecorrespondingvaluesofǫGF bias for massive disc galaxies, we argue that such a bias are inconsistent with theempirical results inferred here. cannot occur for low mass galaxies (Mgal < 3 1010M⊙, Mvir < 1012M⊙) because typical low ma∼ss g×alaxies are bulge-∼less,andthusmustforminhaloeswithtypicalmerger 3.2 Galaxy Spin Parameter histories (and hencetypical halo spins). Wenowdiscussthegalaxyspinparameter,asthisisaquan- tity that has been measured and interpreted by previous studies. Fig. 3 shows the average galaxy spin parameter as 3.3 Comparison with the Milky Way afunctionofhalovirialmass.Thisisthesameastheright- handpanelofFig.1,butwithazeropointshift.Asdiscussed As a reference, the black pentagons and light blue shaded above, there is a slight mass dependence to λ′ , but given regions in Figs. 1 & 2 show estimates for the specific an- gal the uncertainties in halo masses, λ′ is consistent with be- gular momentum and mass of the Milky Way (MW). Here gal ing independent of halo mass. The normalization is 60% we have estimated the specific angular momentum of the ≃ lowerthanpredictedforΛCDMhaloes(dashedline).There MW assuming Jgal/Mgal = 2RdVrot, which is appropriate aretwosimpleinterpretationsofthisresult.Oneisthatthe foranexponentialdiscwith aflatrotationcurve.Weadopt baryonsthatformdiscgalaxieshavelostabout40%oftheir R = 2.8 0.23kpc(1σ) based on the dynamical models d angular momentum during galaxy formation, which could of Widrow±et al. (2008) and Vrot = 219 20 kms−1(1σ) ± occur via dynamical friction. Another is that the baryons fromtheobservationsofReidetal.(1999).Forthebaryonic thatform discgalaxies have40% lowerspecificangularmo- mass of the MW we use the bulge and disc masses, again mentumthanthecosmically availablebaryons,whichcould fromthedynamicalmodelsofWidrowetal.(2008)toobtain occurduetofeedbackand/orinefficientcooling. Wediscuss log10(Mgal/M⊙)=10.71±0.075 (1σ).Forthevirialmassof these interpretations in more detail in 4. the MW we average the results of Smith et al. (2007) and § (cid:13)c 2012RAS,MNRAS000,1–15 Angular momentum of disc galaxies 7 Table 1. Relations between mass and angular momentum of galaxies and dark matter haloes for model A, as presented in Figs. 1-3. Uncertainties onhalomassesare1and2σ,andarepropagatedtootherparametersasappropriate. log10(Mgal) log10(Mvir) ǫGF log10(Jgal/Mgal) Rj λ′gal [M⊙] [M⊙] [kpckms−1] 9.484 11.318+0.125 +0.250 0.09−0.03 −0.07 2.417 0.57−0.10 −0.18 0.018−0.003 −0.006 −0.125 −0.250 +0.02 +0.04 +0.12 +0.27 +0.004 +0.008 9.631 11.414+0.125 +0.250 0.10−0.03 −0.08 2.482 0.57−0.10 −0.18 0.018−0.003 −0.006 −0.125 −0.250 +0.03 +0.04 +0.12 +0.27 +0.004 +0.008 9.784 11.513+0.125 +0.250 0.11−0.04 −0.09 2.550 0.57−0.10 −0.18 0.018−0.003 −0.006 −0.125 −0.250 +0.03 +0.05 +0.12 +0.27 +0.004 +0.008 9.941 11.615+0.125 +0.250 0.13−0.04 −0.10 2.621 0.58−0.10 −0.18 0.018−0.003 −0.006 −0.125 −0.250 +0.03 +0.06 +0.12 +0.27 +0.004 +0.008 10.103 11.720+0.125 +0.250 0.15−0.05 −0.12 2.697 0.59−0.10 −0.19 0.018−0.003 −0.006 −0.125 −0.250 +0.04 +0.06 +0.12 +0.27 +0.004 +0.008 10.270 11.832+0.125 +0.250 0.17−0.06 −0.13 2.779 0.60−0.10 −0.19 0.018−0.003 −0.006 −0.125 −0.250 +0.04 +0.07 +0.13 +0.28 +0.004 +0.009 10.442 11.950+0.125 +0.250 0.19−0.06 −0.15 2.868 0.61−0.11 −0.19 0.019−0.003 −0.006 −0.125 −0.250 +0.05 +0.08 +0.13 +0.29 +0.004 +0.009 10.618 12.078+0.125 +0.250 0.21−0.07 −0.17 2.960 0.62−0.11 −0.20 0.019−0.003 −0.006 −0.125 −0.250 +0.05 +0.09 +0.13 +0.29 +0.004 +0.009 10.796 12.215+0.125 +0.250 0.23−0.08 −0.18 3.050 0.62−0.11 −0.20 0.019−0.003 −0.006 −0.125 −0.250 +0.06 +0.10 +0.13 +0.29 +0.004 +0.009 10.979 12.361+0.125 +0.250 0.25−0.08 −0.20 3.154 0.63−0.11 −0.20 0.019−0.003 −0.006 −0.125 −0.250 +0.06 +0.11 +0.13 +0.29 +0.004 +0.009 11.164 12.517+0.125 +0.250 0.27−0.09 −0.21 3.261 0.63−0.11 −0.20 0.020−0.003 −0.006 −0.125 −0.250 +0.07 +0.12 +0.13 +0.30 +0.004 +0.009 11.352 12.680+0.125 +0.250 0.29−0.10 −0.22 3.391 0.66−0.12 −0.21 0.021−0.004 −0.007 −0.125 −0.250 +0.07 +0.13 +0.14 +0.31 +0.004 +0.010 3.4 Comparison with Previous Studies Several studies in the past used similar methods to deter- mine thespin parameters of disc galaxies. These studies all find higher spin parameters than we find here, but as we show the differences can be ascribed to differences in the estimation of halo masses, which is the dominant source of systematic uncertainty in measuring thespin parameter. Van den Bosch, Burkert & Swaters (2001; hereafter BBS01) used Eq. (4) and halo masses inferred from rota- tion curve modelling to determine the galaxy spin parame- tersforasampleof14disc-dominated dwarfgalaxies. They found that λ′ = 0.048 (see Fig. 4)4, albeit with a rel- h gali atively large uncertainty due to the small sample size and theuncertaintiesinthestellar mass-to-light ratios andhalo masses. At first sight, this is much larger than the average galaxy spin parameters inferred here; λ′ = 0.019. How- h gali ever,thereareseveralfactorsthatcanaccountforthislarge difference. BBS01 used dark halo masses and sizes derived from fitstorotation curvesassumingadiabatic contraction. Since therotation curves only extend to 10 20% of the ∼ − halo virial radius, this involves a significant extrapolation, and thus the potential for systematic errors. As discussed Figure 3. Average spin parameter vs. halo virial mass, for disc in detail in Dutton et al. (2007), the assumption that disc galaxies(solidlineandgreyshadedregion),andΛCDMdarkmat- formationresultsinadiabaticcontractionofitsdarkmatter ter haloes (dashed line, red shaded region). For the galaxy spin halomaynotberealistic.ThegalaxiesinBBS01haveame- parametertheshadedregionsshowtheoneandtwoσuncertain- tiesintroducedbythesystematicuncertaintyinhalomasses.The dianVlast/Vvir =1.55, whereVlast istherotation velocityat pentagon shows an estimate for the Milky Way (MW) with the the last measured point (i.e., at the largest radius) . Since shadedregionshowingoneandtwoσ uncertainties. dark matter haloes in the ΛCDM concordance cosmology have Vmax/Vvir <1.2, where Vmax is the maximum circular velocity, the der∼ived values from BBS01 are strongly influ- enced by the assumption of adiabatic contraction. Dutton Xue et al. (2008) yielding log10(Mvir/M⊙) = 11.97±0.22 etal.(2010b)haveshownthatthehalovirialvelocityis,on (2σ). average, approximately equal to the optical rotation veloc- The resulting galaxy spin parameter (see Eq.2) and ity. Using this assumption to derive halo masses results in g0Ba.o0lta3hx1+−yλ00′gf..ao00l11ra61mna(d2tiǫσoG)nF,eaaffinredciheǫingGchFyer(=stehe0a.nE03qfo3.7r+−)0t0y..o21pf85itc(ha2elσlMa),tWer-etsyappreeecλgtia′gvaleallx=y-. l1λo0′gga%1l0=u(Mn0cve.i0rr2t/aM3in(⊙try)ig=ohnt-1Vh1ra.o2ntd/±Vpv0ia.r3n(eaDlnuodtftaoFnmige.etd4ai)al..nI2fs0p1wi0neb)cpoatnhraseimndeetrhteiasr ies. This could signify that the MW is atypical, or it could simplybeareflection ofthemeasurementerrors(especially in halo masses) both for the MW and for samples of late- 4 Here we have converted the results from BBS01 to the spin typegalaxies. parameterdefinitionofBullocketal.(2001). (cid:13)c 2012RAS,MNRAS000,1–15 8 A.A. Dutton & F.C. van den Bosch Figure4.Histograms(hatched)ofthedistributionofgalaxyspin parameters (λ′ )forthe 14discdominated dwarfgalaxies from gal van den Bosch, Burkert, & Swaters (2001; hereafter BBS01). In the left-hand panel λ′ has been obtained using the halo virial gal velocitiesfromBBS01.Intheright-handpanel,thehalovirialve- locitieshavebeencalculatedassumingVvir=Vlast,whereVlastis therotationvelocityatthelastmeasuredpoint(i.e.,atthelargest radius).Thelatter resultsinspinparameters ingoodagreement withourresults(cf.Fig.3). resultsinarangeof0.019 0.028forλ′ ,whichisconsistent Figure 5. The cumulative, normalized fractions of mass (blue − gal with our measured values(see Fig. 3). line)andangularmomentum(dashedredline)forΛCDMhaloes Tonini et al. (2006) used average scaling relations be- (Navarro et al. 1997; Bullock et al. 2001). The shaded region tween stellar mass, gas mass, halo mass and disc size to shows the uncertainty on J(r)/Jvir assuming s = 1.3±0.3 in Eq. 9. This clearly demonstrates that mass is more centrally compute an average galaxy spin parameter as function of concentrated than angular momentum. For example, the inner halo mass, under the assumption that dark matter haloes 50%of themasscontains just20+5%oftheangular momentum follow a Burkert (1995) profile. They found that λ′ −4 h gali ≃ (blackdotted lines).Thusiftheefficiency ofgalaxyformationis 0.025 0.030 with no significant dependenceon halo mass. − regulated by the efficiency of cooling, we would expect a strong Acomparison between thehalomass -stellar massrelation correlationbetweengalaxyspinparameterandgalaxyformation usedbyTonini etal. (2006) and byourselvesindicates that efficiency,whichisnotobserved. their halo masses are a factor of 2 lower than ours. This ∼ differenceinhalomassesfullyaccountsfortheirhigherspin λ′ F, and thus their assumption of F = 0.04 accounts parameters. gal ∝ for at least half the discrepancy between our result and Finally, Hernandezet al. (2007) determined thegalaxy theirs. spin parameters for a large sample of galaxies taken from the Sloan Digital Sky Survey (SDSS). Rather than Tonini et al. (2006), they obtained estimates for λ for individ- gal ual galaxies using the method proposed by Hernandez & 4 COMPARISON WITH GALAXY Cervantes-Sodi (2006), which is based on the following as- FORMATION MODELS sumptions: (i) dark matter haloes are singular isothermal 4.1 Inside-Out Cooling Models spheres, (ii) the self-gravity of the disc can be ignored, and (iii) all galaxies havethesameratio ofdark mattermassto If the galaxy formation efficiency is determined by the ef- discmass.Inaddition,theyinferredtherotationvelocityof ficiency of cooling, then we expect that will be higher j R eachindividualgalaxyinastatisticalsense,usinganempir- for higher galaxy formation efficiencies. This is because in ical Tully-Fisherrelation. They found that for disc galaxies ΛCDMhaloesmassismorecentrallyconcentratedthanan- λ′ follows a log-normal distribution with λ′ 0.059, gularmomentum.ThisisillustratedinFig.5.Thebluesolid gal h gali ∼ whichisafactorof 3higherthanourresult.Themethod line shows the cumulative mass fraction, M(<r)/Mvir as a ∼ used by Hernandezet al. (2007) makes several highly ques- functionofradiusforaNFWhalowithconcentrationc=10. tionable assumptions, and thus we do not consider their The red dashed line shows the cumulative angular momen- result reliable. For example, Hernandez et al. (2007) as- tum fraction, J(<r)/Jvir, as a function of radius assuming sume a constant galaxy mass to dark matter mass ratio of F =0.04,whichcorrespondstoagalaxyformationefficiency J(<r)/Jvir =[M(<r)/Mvir]s, (9) ofǫGF 0.25.ComparisonwithFig.2showsthatwhilethis M(<r)/Mvir isagoo≃d assumption fora2 1012M⊙ halo,it significantly wheres=1.3 0.3(Bullocketal.2001).Undertheassump- × ± over-estimates the galaxy formation efficiency, and hence tionsthat (i) thebaryonsinitially acquirethesame specific spin parameters, of lower mass galaxies. Since Hernandez angularmomentumasthedarkmatter,and(ii)thebaryons et al. (2007) use a volume limited sample, their measure- accreteontothecentralgalaxyinside-out,thenEq.9implies ment of the spin parameter is biased towards lower mass =ǫs . Rj GF galaxies, where their assumption of F = 0.04 is high by a Wenow extend thissimple inside-out cooling model to factorof 2.InthemethodusedbyHernandezetal.(2007) cosmologically evolvingdarkmatterhaloes.Theredlinesin ≃ (cid:13)c 2012RAS,MNRAS000,1–15 Angular momentum of disc galaxies 9 Figure 6.Correlations between halo mass, Mvir, galaxy forma- Figure7.SameasFig.6,butthistimewecomparetheempirical tionefficiency,ǫGF,andspecificangularmomentumratio,Rj,for relations(solidblacklineswithshadedregions)withmodelsthat modelsandobservationsatredshiftz=0.Theempiricalrelations include feedback and/or angular momentum loss (red lines). All fordiscgalaxies areshownas solidblack lines,withthe shading models shown adopt a hot gas metallicity of Zhot =0.1Z⊙, but reflecting the one and two σ uncertainties on halo masses. Note theydifferinfeedbackefficiency,ǫFB,andthefractionofangular thatthegalaxyformationefficiencyincreaseswithincreasinghalo momentum lost by the baryons during galaxy formation, flost. mass,whilethespecific angularmomentum ratioisindependent Thedottedline(ǫFB=flost=0)isidenticaltothesimplecooling of halo mass. The corresponding relations for a simple cooling modelfromFig.6,andisshownforcomparison.Notethatnone model are shown with red lines, where the different line types of the models is able to simultaneously match the low galaxy correspondtodifferentmetallicities,asindicated. Inthiscooling formationefficienciesandtheconstantangularmomentumratio. modelRj ∼ǫGF (independent ofthemetallicityofthehotgas), Seetextforadetaileddiscussion. inconsistentwiththeempiricalfindings. pirical result. As for the parameter , the simple cooling j R modelspredictthat shoulddecreasewithincreasinghalo Fig.6showtherelationsbetween j,ǫGFandMvirforasce- Rj R mass, which is a consequenceof cooling beingan inside-out narioinwhichthegalaxyformationefficiencyisdetermined process combined with the fact that mass is more centrally bytheefficiencyofcooling.Thismodelisasimplifiedversion concentratedthanangularmomentum(cf.Fig.5).Hence,in ofthegalaxyformationmodelfromDutton&vandenBosch (2009).BrieflythismodelconsistsofNFWhaloesthatgrow massivehaloeswhereǫGF 1duetoinefficientcooling,the ≪ galaxyendsuphavinglessspecificangularmomentumthan bysmoothaccretionofbaryonsanddarkmatter.Theangu- its dark matter halo. This strong mass dependence of larmomentumofeachshellofaccretedgasisdeterminedby j R isagain inconsistent with ourempirical results. Finally, the assuming that the angular momentum distribution (AMD) coolingmodelspredictsanalmostlinearrelationbetween of a given halo is independent of time. The AMD is spec- j R ified by two parameters: the normalization (λ) and shape and ǫGF, which is independent of the gas metallicity (lower right-hand panel of Fig. 6). Clearly, this linear relation is (α).Whenthebaryonsenterthehalotheyareshockheated very different from its empirical equivalent, indicating that to the virial temperature. The cooling time depends on the additional mechanisms for accreting and/or expelling gas temperature,densityandmetallicity of thehotgas. Weuse are required. the metallicity dependent collisional ionization equilibrium coolingfunctionsofSutherland&Dopita(1993).Resultsfor four different metallicities from primordial to one third so- 4.2 Outflows and Angular Momentum Loss lar are shown. For low mass haloes (Mvir 1011M⊙) cool- ∼ ing is very efficient, so that 90% of the baryonic mass We now consider two modifications to the simple cooling is accreted (left-hand panels)≃5. For more massive haloes, model: outflows and angular momentum loss. We assume the galaxy formation efficiency, ǫGF, depends strongly on that outflows, which move at the local escape velocity, are the metallicity of the gas, with higher metallicities yielding driven by energy from supernovae (SNe). The fraction of larger values of ǫGF. Clearly, the trend that ǫGF decreases the SNe energy that goes into an outflow is specified by withincreasinghalomassiscompletelyoppositetotheem- the free parameter ǫFB (see van den Bosch 2002; Dutton & van den Bosch 2009). Other than inefficient cooling in massive haloes, and UV heating in low mass haloes, galac- 5 It is not 100% because it takes a free fall time for the most tic outflows are the primary mechanism for explaining the recentlyaccretedbaryonstoreachthegalaxy. observed low galaxy formation efficiencies (e.g., Dekel & (cid:13)c 2012RAS,MNRAS000,1–15 10 A.A. Dutton & F.C. van den Bosch Silk 1986; Cole et al. 1994). For simplicity, we assume that outflow gas leaves the disc and halo, and does not return. However, recent cosmological hydrodynamical simulations haveshown that ejected gas may returnto thegalaxy (Op- penheimer et al. 2010). Some of the ejected gas may mix with the halo gas and thus re-accrete with higher specific angular momentum (Brook et al. 2012). Since this process is a re-distribution (i.e., ejected gas gains angular momen- tum,halogaslosesangularmomentum),itdoesnotchange the specific angular momentum of the baryons. If the halo gas accretes onto the central galaxy (as is expected for low mass, Mvir < 1012M⊙, haloes) then there is no net change intheangul∼armomentumofthegalaxy.Ifthehalogasdoes not accrete (which is expected for high mass haloes) then the galaxy can gain angular momentum via this galactic fountain effect. Thus if we were to implement re-accretion intoourmodelwewouldneedhigherfeedbackefficienciesto achieve the same galaxy formation efficiencies, but to first order there would be no change to the specific angular mo- mentum. We assume that during the accretion process, a frac- tion f of theangular momentum of thegas is lost to the lost darkmatterhalo.Themechanismresponsibleforthistrans- Figure8.SameasFig.7,butthistimewehavetunedthefeed- fer is dynamical friction. In our models we donot takeinto back efficiency, ǫFB, and angular momentum loss, flost, of the account the effects of angular momentum transfer and out- model (red, soliddots) inorder toaccurately reproduce the em- flowsonthestructureofthedarkmatterhalo.Butwenote piricalrelations(solidblacklineswithshadedregions).Thesmall, thatbothoftheseprocesseswillresultinhaloexpansion,or upperright-handpanelshowsthecorrespondinghalomassdepen- at least a reduction of theamount of adiabatic contraction, dence of ǫFB (solid circles) and flost (open circles): reproducing the empirical relations requires both of these parameters to de- i.e.,alowereffectivevalueofν (e.g.,Navarroetal.1996;El- creasestronglywithincreasinghalomass.Seetextforadetailed Zantetal.2001;Mo&Mao2004;Cole,Dehnen&Wilkinson discussion. 2011) In Fig. 7 we show several models with different values of ǫFB and flost. In all models we assume that halo gas is enriched to one tenth solar. The dotted line shows a model with ǫFB =0.0 and flost =0.0: this corresponds to thesim- plecooling model from Fig. 6 and is shown for comparison. The galaxy formation efficiencies are too high for all halo masses. In order to produce realistic galaxy formation effi- ciencies outflows are needed. A model with ǫFB = 0.5 and flost =0.0(shortdashedline)hasǫGF 20%.Inourmodel ∼ outflows increase by preferentially removing gas with j R low specific angular momentum. Since outflows are driven by star formation, regions that have higher star formation rates havehigher mass outflow rates. Star formation is em- pirically more efficient at smaller galactic radii (Kennicutt 1998) aswellasathigherredshifts(e.g.,Noeskeetal.2007; Daddi et al. 2007; Elbaz et al. 2007). Smaller galactic radii correspond to lower specific angular momentum and discs are smaller at higher redshifts (e.g., Dutton et al. 2011a); both of these effects contribute to outflows increasing the specific angular momentum of the material that remains in the galaxy. In our energy driven outflow model the mass loadingfactor( outflowrate/starformationrate)ishigher ≡ in lower mass galaxies, which results in increasing more j R inlower mass haloes (byuptoafactor two).Thisincreases Figure9.SameasFig.8,butherewehaveusedmodelB(stellar the disagreement with the empirically inferred relation be- mass-to-light ratios that are 0.3 dex lower and halo contraction tween and halo mass. We can lower by introducing characterized by ν =0.8) for the empirical relations. Reproduc- j j R R ing the inferred galaxy formation efficiencies now requires even angular momentum loss (i.e., setting f > 0). The long lost dashed-dotted lines in Fig. 7 correspond to a model with strongerfeedback (i.e.,largerǫFB),whilethe fractionofangular momentum that is lost is similar to that for our fiducial model ǫFB =0.5 and flost =0.5. This angular momentum loss re- showninFig.8. duces byafactor 2atallmassscales,bringingitinrea- j R ∼ sonable agreement with the observed value for haloes with (cid:13)c 2012RAS,MNRAS000,1–15

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Preview The angular momentum of disc galaxies: implications for gas accretion, outflows, and dynamical friction

Mon.Not.R.Astron.Soc.000,1–15(2012) Printed10January2012 (MNLATEXstylefilev2.2) The angular momentum of disc galaxies: implications for gas accretion, outflows, and dynamical friction Aaron A. Dutton1⋆ , Frank C. van den Bosch2 1Department of Physics and A†stronomy, University of Victoria, Victoria, BC, V8P 5C2, Canada 2Astronomy Department, Yale University,P.O. Box 208101, New Haven, CT 06520-8101, USA 2 Accepted 2011December 5.Received2011October29;inoriginalform2011June2 1 0 2 ABSTRACT n Wecombineconstraintsonthegalaxy-darkmatterconnectionwithstructuralanddy- a namical scaling relations to investigate the angular momentum content of disc galax- J ies. For haloes with masses in the interval 1011.3M⊙ < Mvir < 1012.7M⊙ we find 7 that the galaxy spin parameters are basically independe∼nt of hal∼o mass with λ′ (J /M )/(√2R V ) = 0.019+0.004(1σ). This is significantly lower thanh fgoarlire≡- ] gal gal vir vir −0.003 O laxed ΛCDM haloes, which have an average spin parameter λ′ = 0.031 0.001. h haloi ± C The average ratio between the specific angular momentum of disc galaxies and their hostdarkmatter haloes is therefore λ′ /λ′ =0.61+0.13.This calls into ques- . Rj ≡ gal halo −0.11 h tion a standard assumption made in the majority of all (semi-analytical) models for p (disc) galaxy formation, namely that = 1. Using simple disc formation models j - R o we show that it is particularly challenging to understand why j is independent of R r halo mass, while the galaxy formation efficiency (ǫGF, proportional to the ratio of st galaxy mass to halo mass) reveals a strong halo mass dependence. We argue that a the empirical scaling relations between ǫ , and halo mass require both feedback GF j [ (i.e., galacticoutflows)andangularmomentuRm transferfrom the baryonsto the dark matter (i.e., dynamical friction). Most importantly, the efficiency of angular momen- 2 v tum loss need to decrease with increasing halo mass. Such a mass dependence may 3 reflecta biasagainstformingstablediscs inhighmass,lowspinhaloesoratransition 6 from cold-mode accretion in low mass haloes to hot-mode accretion at the massive 6 end. However,currenthydrodynamicalsimulations ofgalaxyformation,which should 0 include these processes,seem unable to reproduce the empirical relation between ǫ GF 8. and j. We conclude that the angular momentum build-up of galactic discs remains R 0 poorly understood. 1 Key words: galaxies: formation — galaxies: fundamental parameters — galaxies: 1 : spirals — galaxies: structure v i X r a 1 INTRODUCTION make galaxy discs with the observed sizes (e.g., Dalcanton etal.1997;Mo,Mao,&White1998;Firmani&Avila-Reese In the current paradigm of galaxy formation, galaxy discs 2000; de Jong & Lacey 2000; Navarro & Steinmetz 2000). areconsideredtoformfromtheaccretion ofgasinsidehier- In fact, models in which galaxies have 30% lower spe- archicallygrowingcolddarkmatter(CDM)haloes(seeMo, ∼ cificangularmomentumthantheirdarkmatterhaloesmay vandenBosch&White2010foracomprehensiveoverview). be preferred (Dutton et al. 2007). Another success of this Thedarkmatter(DM)andgasacquireangularmomentum picture is that the large variation in the specific angular viatidaltorquesintheearlyUniverse(Peebles1969).When momentum of dark matter haloes, at fixed halo mass, ex- gasaccretesontothecentralgalaxy,thisangularmomentum plains the large scatter in disc sizes, or equivalently surface mayeventuallyhaltthecollapseandleadtotheformationof brightnesses, at fixed luminosity (Dalcanton et al. 1997). arotationallysupporteddisc(Fall&Efstathiou1980).Ifthe Thispicturealsoaccountsforthetimeevolutionofdiscsizes specificangularmomentumofthebaryonsisconserveddur- (Somerville et al. 2008; Firmani & Avila-Reese 2009; Dut- inggalaxyformation,thereisenoughangularmomentumto ton et al. 2011a), reproducing the observed weak evolution since redshift z 1 (Barden et al. 2005), and the stronger ≃ ⋆ [email protected] † CITANationalFellow (cid:13)c 2012RAS 2 A.A. Dutton & F.C. van den Bosch evolutionobservedfrom z 2toz 1(Trujilloetal.2006; attributed to lack of numerical resolution (e.g. Governato ∼ ∼ Williams et al. 2010). et al. 2004; Kaufmann et al. 2007). However, Piontek & Despitethemanysuccessesofthesemodels,allofwhich Steinmetz(2009) arguethatthedominantcauseisdynami- assumethatspecificangularmomentumisconservedduring calfriction betweenclumpsofbaryonsand thedarkmatter galaxy formation, thereare severalreasons, from aphysical halo. If, based on these findings, we assume that angular pointofview,astowhytheratiobetweenthespecificangu- momentumlossisanaturalfeatureofdiscgalaxyformation lar momentum of the baryons and that of the dark matter in a ΛCDM universe, a relevant question is the following: halo, denoted 1, might bedifferent from unity: If the baryons that make up a discy galaxy carry half the j R specificangularmomentum ofthedarkmatter haloinwhich (i) Selective accretion of baryons. If not all available they reside, is it possible to reproduce in detail the observed baryons make it into the galaxy, the specific angular mo- sizes of disc galaxies? mentumofthegalaxycanbeeitherhigher( >1)orlower Rj This question was addressed by Navarro & Steinmetz ( < 1) than that of all baryons (whether it is higher or Rj (2000). Using scaling relations between disc size, luminos- lowersimplydependsonthespecificangularmomentumdis- ity, and rotation velocity, these authors derived an expres- tribution ofthematerials that areselected). Anexample of sionfor (f intheirnotation)thatdependsonthegalaxy j j a selective accretion process is cooling. Cooling is generally R formation efficiency, ǫGF (Mgal/Mvir)/(Ωb/Ωm) (fbdsk in aninside-outprocess,withtheinnerregionscoolingpriorto ≡ theirnotation),cosmicmatterdensity,Ωm,andstellarmass- the more outer regions. Since the mass of ΛCDM haloes is to-light ratio, Υ . Theyfound that if therotation speedsof I moreconcentratedthanthespecificangularmomentum(see galaxy discs are approximately the same as the circular ve- e.g., Navarro & Steinmetz 1997), one expects that < 1 Rj locitiesoftheirsurroundinghaloes(whichhassubsequently if not all baryonsmanage tocool. been shown to be the case, at least on average, by Dutton (ii) Exchange of angular momentum between the dark etal.2010b),thendiscsmusthaveretainedabouthalfofthe matter and the baryons. An example of this is dynamical availablespecificangularmomentumduringtheirassembly. friction,whichtransfersorbitalangularmomentumfromthe Anotherresultfrom thisanalysis isthat discgalaxies form- baryons to the dark matter, thus resulting in <1 (e.g., Rj ing in low density cosmogonies, such as ΛCDM, need to Navarro&Benz1991;Navarro&White1994).Thestrength have j 2ǫGF, that is, they draw a larger fraction of the ofthisprocessdependsontheefficiencyofstarformationat R ≃ specific angular momentum than the galaxy formation effi- earlytimes.Iftheformationofbaryonicclumpscanbesup- ciency.Thisisatoddswiththeexpectation(see 4.1below) pressed, such as through strong feedback, then the angular § that j ǫGF if the galaxy formation efficiency is mainly momentumtransferisreduced(e.g.,Weil,Eke&Efstathiou R ≃ determined by theinefficiency of cooling. 1998;Sommer-Larsen,Gelato&Vedel1999;Eke,Efstathiou Thus observational measurements of the specific an- & Wright 2000). gular momentum in galaxies place constraints on the way (iii) Feedback.If feedback (from supernova, AGN or any galaxies acquire their mass and angular momentum. The other mechanism) results in the removal of gas, one can specific angular momentum of the dark matter, Jvir/Mvir, achieve values for that are very different from unity Rj where Jvir is the total angular momentum, and Mvir is the by simply removing a specific subset of the baryons. As an total virial mass, is commonly expressed in terms of thedi- example, several studies have argued that supernova feed- mensionless spin parameter (Peebles 1969): backaffectslowangularmomentummaterialmorethanhigh a(Mngaulllearr&moDmeekneltu2m002m;aDteurtitaol,nc&auvsainngdaennBetosinchcre2a0s0e9;oDf uRtj- λhalo= JGvirM|Ev5|i/1r3/2 = √J2viRr/vMirVvivrir fc1/2 =λ′halofc1/2 (1) ton 2009; Governato et al. 2010; Brook et al. 2011). Here E is the halo’s energy, Rvir is the virial radius, Vvir is Cosmological hydrodynamical simulations almost in- the circular velocity at the virial radius, and fc measures variably predict that disc galaxies have significantly less thedeviationofE fromthatofasingularisothermalsphere specific angular momentum than their dark matter haloes truncated at Rvir. It is common to set fc = 1 (Bullock ( < 1), suggesting that mechanisms (i) and (ii) dis- et al. 2001), so that the spin parameter only depends on j R cussed above play an important role. Early simulations the total angular momentum and mass of the halo. In this in a standard cold dark matter (SCDM) cosmology (i.e. casethespinparameterisdenotedλ′ .Theequivalentspin halo Ωm = 1,ΩΛ = 0) resulted in a catastrophic loss of angu- parameter for thegalaxy is defined as lar momentum (Navarro & Benz 1991; Navarro & White (J /M ) 1o9f9m4;agSntietiundmeettzoo&smNaalvla.rMroo1re99r9e)c,enptrosdimucuilnagtiodnisscsinanΛoCrdoeldr λ′gal = √g2aRlvirVgvalir , (2) Dark Matter (ΛCDM) cosmologies haveresulted in less an- (see Section 2.1 for details). gular momentum loss, but typical ratios between the spe- Dark matteronly cosmological simulations haveshown cificangularmomentumofthebaryonsandthatofthedark thatbothλhalo andλ′halo arelog-normally distributed(e.g., matter are 50% (Piontek & Steinmetz 2011; Sales Bullock et al. 2001) with median and scatter independent j R ≃ et al. 2010). This “angular momentum loss” is sometimes of halo mass (e.g., Maccio` et al. 2007; Bett et al. 2007; Mun˜oz-Cuartas et al. 2011) and independent of environ- ment (Lemson & Kauffmann 1999, Maccio` et al. 2007). 1 Notethat inthe notation of Mo,Mao,& White (1998), Rj = For relaxed haloes identified with a spherical overdensity (jd/md), where jd is the ratio between the angular momentum (SO) algorithm Maccio` et al. (2008) find a median value of ofthediscanddarkmatterhalo,andmdistheratiobetweenthe λ′halo = 0.031±0.001. Using a similar halo definition Bett massofthediscanddarkmatterhalo. et al. (2007) find a median λhalo = 0.0367. For the cosmol- (cid:13)c 2012RAS,MNRAS000,1–15 Angular momentum of disc galaxies 3 ogy adopted by Bett et al. , fc 1.3 for a 1012M⊙ halo, galaxy-galaxy lensing (e.g, Mandelbaum et al. 2006; Cac- ′ ≃ andthusλ 0.032,whichisinexcellentagreementwith ciatoetal.2009),abundancematching(e.g.,Conroy,Wech- halo ≃ the results of Maccio` et al. (2008). Simulations with gas in sler & Kravtsov 2006), and satellite kinematics (e.g., More whichthereisnocoolingfindsimilarspinparametersforthe et al. 2009). baryonsanddarkmatter(vandenBoschetal.2002;Sharma In this paper we compute the average specific angu- &Steinmetz2005).Thusinwhatfollowsweassumethatthe lar momentum and spin parameters of disc galaxies as a median spin parameter of thehalo is function of their halo mass. We improve on previous stud- ies in a number of ways. We constrain halo masses us- ′ λhalo=0.031±0.001. (3) ing recent results for the halo mass - stellar mass relation for disc galaxies obtained from weak gravitational lensing Themaingoalofthispaperistodeterminetheaverage and satellite kinematics (Dutton et al. 2010b), rather than valueof for alarge sample of discgalaxies as afunction j R abundance matching results as in Tonini et al. (2006), or of their halo mass, and to use semi-analytical models to the highly questionable assumption of a constant galaxy explore implications for how disc galaxies form. Since it is massfractionmadebyHernandez&Cervantes-Sodi(2006), impossible to measure the specific angular momentum (or Hernandez et al. (2007), and Berta et al. (2008). We also spin parameter) of a real (as opposed to simulated) dark improve upon these studies by taking account of realis- matterhalo,onecanonlyobtainanestimatefortheaverage tic uncertainties on halo masses. We consider both stel- value of by comparing the distribution of galaxy spin j parameteRrs, λ′ , obtained from a sample of galaxies to the lar and gaseous discs, rather than just stellar discs, which distribution ofgahlalo spin parameters, λ′ , obtained from is important for low mass galaxies which tend to be gas halo rich (Tonini et al. 2006). We calculate disc angular mo- numerical simulations of structure formation in a ΛCDM mentum by integrating a model that is constructed to re- cosmology.Thisistheapproachwewillfollowinthispaper. Measuring λ′ for an individual galaxy necessarily in- produce the size-mass and velocity-mass (i.e., Tully-Fisher, gal Tully & Fisher 1977) relations, rather than simple scaling volves thefollowing steps: arguments for exponential discs in isothermal haloes (i.e., (i) Determine the specific angular momentum of the Jgal/Mgal =2RdVrot),asusedbye.g.,Navarro&Steinmetz baryons. (2000) and Hernandez et al. (2007). We also discuss how (ii) Determine the mass (and virial radius) of the dark uncertainties related to stellar mass-to-light ratios and adi- matter halo in which thegalaxy resides. abaticcontractionimpacttheinferredvaluesof j,anduse R simple models of disc galaxy formation to discuss the im- Step (i) is relatively straightforward. Using that, for a disc plications of our findings for cooling, outflows and angular galaxy, momentum loss. Rvir We adopt a flat ΛCDM cosmology with matter den- Jgal =2πZ Σ(R)Vrot(R)RdR, (4) sity parameter Ωm =0.27, baryon density Ωb =0.044, and 0 Hubble parameter h = H0/100kms−1Mpc−1 = 0.7. Dark itisclearthatonecandeterminetheangularmomentumof matter halo masses are defined so that the mean density of adiscgalaxyfrommeasurementsofitssurfacemassdensity, the halo (assumed to be spherical) is 200 times the critical Σ(R), (accounting for both gas and stars), and its rotation density. curve Vrot(R) (see e.g., van den Bosch, Burkert & Swaters 2001). Alternative, since disc galaxies in general have ex- ponential surface density profiles and flat rotation curves, one can use simple scaling relations between mass (or lu- minosity), size and characteristic rotation velocity in order 2 DEFINITIONS AND MASS MODELS to obtain an estimate of J without having to measure gal This section gives a brief overview of the main parameters Σ(R) or Vrot(R) (e.g., Navarro & Steinmetz 2000; Tonini we discuss in this paper, and the constrained mass models et al. 2006; Hernandez & Cervantes-Sodi 2006; Hernandez we use toconstrain them. et al. 2007; Berta et al. 2008). Although less accurate than using Eq. (4), this method has the advantage that it can beapplied to much larger samples of galaxies. Step (ii) is a little bit more involved. Typically, accurate measurements 2.1 Mass and Angular Momentum Parameters of halo mass require probes on scales of the virial radius. Examples of such ‘probes’ are gravitational lensing, X-ray There are four main parameters that are relevant for what dataandsatellitekinematics.However,unlessthegalaxyin follows:galaxymass,Mgal,halovirialmass,Mvir,galaxyan- question is the central galaxy of a cluster-sized halo, such gular momentum, Jgal, and the angular momentum of the measurements are rarely available for individual systems, dark matter halo, Jvir. The galaxy mass is the sum of the andonehastorely onuncertainestimates ofthehalomass massinstarsandcoldgas.Thevirialmassisthetotalmass from thegalaxy’s rotation curve.Alternatively,onecan use (stars, cold gas, hot gas, dark matter) within the virial ra- variousstatisticaltechniquestoassigntoeachgalaxyanav- dius. The galaxy angular momentum is the sum of the an- erage halo mass. Recent years have seen dramatic progress gular momenta of the stars and of the cold gas. The virial inusingavarietyoftechniquestoconstraintheaveragerela- angular momentum is the total angular momentum within tionbetweengalaxyproperties(typicallyluminosityofstel- thevirial radius. larmass)andhalomass,inparticularfromgalaxyclustering Our main goal is to constrain the angular momentum (e.g., Yang, Mo & van den Bosch 2003; Tinker et al. 2005), ratio, , defined as the ratio between the galaxy specific j R (cid:13)c 2012RAS,MNRAS000,1–15 4 A.A. Dutton & F.C. van den Bosch angular momentum and the total specific angular momen- lensing, satellite kinematics, galaxy clustering, and abun- tum2: dancematching.Wewillusethescatteramongthesediffer- (J /M ) λ′ entstudies/techniques,asanestimatefortheuncertaintyin Rj ≡ (Jgvairl/Mgvairl) = λ′hgaallo , (5) tthieesMtovitrhvoss.eMonstaǫrGrFelaantidon,ja.nFdinparollpy,agtahteertehlaetsieonunbceetrwtaeienn- where the spin parameters λ′halo and λ′gal are defined by halo mass, Mvir, and haloRconcentration, c, is taken from Eqs. (1) and (2), respectively. Using that for disc galaxies Maccio` etal.(2008),andiscalibrated usinghigh-resolution Jgal RdMgalVrot, where Rd is the disc scale length and numerical simulations. ∝ Vrot isa characteristic rotation velocity of thedisc, onecan The usage of all these empirical and theoretical scal- use Eq.(5) to show that ingrelationsimpliesthattheninemodelparameterscanbe Rd ∝Rjλ′halo (cid:16)VVrvoirt(cid:17)−1 Rvir, (6) rfinuefldnuuctecienodcnets(oItMhtweFos)t,praurnicmdtuaarraylprpuernsokcprnieoprwttiinoessn:oftfohrtehhesotdwelalargkralmianxaiyttitagelrromhwaatslhos whichisthestandardapproachusedinanalyticalandsemi- (hereafter“thehalocontractionmodel”).Itiscommonprac- analytical models to predict the sizes of disc galaxies (e.g., tice to assume that disc formation is an adiabatic process Kauffmann1996;Mo,Mao&White1998;Somerville&Pri- and that the halo responds by contracting using the adi- mack 1999; van denBosch 2000; Cole et al. 2000). In many abatic contraction (hereafter AC) prescription of Blumen- casesthesestudiessimplifyEq.(5)byassumingthat j =1 thaletal.(1986;hereafterBFFP).However,numericalsim- R and that Vrot/Vvir is a constant. In this paper we examine ulations have shown that this prescription may not be suf- whether there is empirical support for the former assump- ficiently accurate, and several alternatives have been pro- tionandwhatthisimpliesfortheformation ofdiscgalaxies posed(e.g.,Gnedinetal.2004;Abadietal.2010).Wefollow in general. Aswe will demonstrate, it isinstructivetocom- Dutton et al. (2007) and use a generalized halo contraction pare j to thegalaxy formation efficiency parameter model, according to which a shell of dark matter initially R ǫGF ≡ (M(Ωgabl//ΩMmv)ir). (7) m(baetfioorne,daitscafroardmiuatsion) at radius ri ends up, after disc for- whichdescribeswhatfractionofthecosmologicallyavailable rf =ΓνBFFPri. (8) baryonshaveended up in a galaxy. HereΓBFFP istheamountofcontractionaspredictedbythe AC-model of BFFP, and ν is a free parameter. Note that 2.2 Mass Models ν = 1 corresponds to the standard adiabatic contraction model of BFFP, while the Gnedin et al. (2004) and Abadi Wecomputetheaveragevaluefor fordiscgalaxiesusing Rj et al. (2010) models can be well approximated by ν = 0.8 the mass models of Dutton et al. (2011b), which have been and ν =0.4, respectively. A model without contraction has constructed to reproduce the observed structural and dy- ν = 0, while ν < 0 indicates that the dark matter halo namical scaling relations of late-type(i.e., star-forming and responds todisc formation byexpanding. disc-dominated) galaxies. They consist of four components: AsshowninDuttonetal.(2011b),reproducingtheob- a stellar bulge, a stellar disc, a gas disc, and a dark matter servedTully-Fisherrelationimpliesdifferentvaluesforν for halo. Boththebulgeandthedarkmatterhaloareassumed adifferentchoiceof theIMF.Inthispaperwe will consider to be spherical and to follow a density distribution that is two different models that fit the Tully-Fisher relation (and given by a Hernquist profile (Hernquist 1990) and a NFW all other scaling relations mentioned above) equally well: profile (Navarro, Frenk, & White 1997), respectively. The modelAwhichadoptsaChabrier(2003)IMFandν = 0.5 density profile of the halo, however, is modified to account − (i.e., halo expansion), and model B, in which we adopt the forthegravitational responsetogalaxy formation (mostof- AC model of Gnedin et al. (2004), i.e., we set ν =0.8, and tenmodelledasadiabaticcontraction,seeBlumenthaletal. we assume an IMF that yields stellar mass-to-light ratios 1986). The disc is assumed to have an exponential surface that are 0.3 dex lower than for the Chabrier (2003) IMF. density and to be infinitesimally thin. AsshowninDuttonetal.(2011b) boththesemodelsarein Each halo-galaxy system is described by nine parame- excellent agreement with the data. As we will show below, ters: three for the halo (virial mass, Mvir, concentration, c, model B results in galaxy spin parameters that are 30 andaparameterthatdescribestheimpactofgalaxyforma- ∼ percent ( 0.12 dex) higher than those of model A. This tion on the halo density profile); and six for the baryons ∼ uncertainty is comparable to that arising from the uncer- (stellar mass, Mstar, gas mass, Mgas, bulge fraction, fb, tainty in the Mvir vs. Mstar relation. Throughout we will bulge size, R , stellar disc size, R , and gas disc size, b d mainly present the results for model A, while occasionally Rg). The 6 parameters for the baryons are constrained by showing, for comparison, the results for model B. the observed scaling relations: Rb vs. Mstar; Rd vs. Mstar; Foragiven choiceof IMFandν,themodel completely Rg/Rd vs.Mstar;fbvs.Mstar,andMgas/Mstar vs.Mstar (see specifiesthemassdistributionsofdisc-plus-bulgegalaxiesas Dutton et al. 2011b for details). For the relation between a function of halo mass. From these mass models we com- Mvir and Mstar we use the empirical relation from Dutton pute the angular momentum of the baryons by evaluating et al. (2010b), which is inferred from a large number of in- Eq. 4, where the surface density is the sum of the surface dependent studies and techniques, including galaxy-galaxy densities of the gas and stars: Σ(R) = Σgas(R)+Σstar(R). We assume that the bulge has no net angular momentum. 2 NotethatRj isnotanefficiencybecauseitcanbelargerthan Ifweassumedthatbulgesfollow thesamerotation curveas unity. the disc, then this would increase the specific angular mo- (cid:13)c 2012RAS,MNRAS000,1–15 Angular momentum of disc galaxies 5 Figure 1. Left: Galaxy specific angular momentum vs. virial mass for disc galaxies. The shaded regions indicate the one and two σ uncertainties on virial mass. The dashed line shows the relation between virial specific angular momentum and virial mass from cosmologicalN-bodysimulations,correspondingtoaspinparameterλ′=0.031.ThepentagonshowsestimatesfortheMilkyWay(MW) with the shaded region showing one and two σ uncertainties. Right: Specific angular momentum ratio (Rj) vs. virial mass. The solid line and shaded regions indicate the mean and the one and two σ uncertainties for our fiducial model A, which assumes a Chabrier (2003)IMFandahalocontractionmodelcharacterizedbyν=−0.5.ThedashedlineshowsthemeanformodelB,inwhichthestellar mass-to-lightratios are0.3dex smallerand ν =0.8. Note that, for both models, Rj isless than unity and approximately independent ofvirialmass. mentumofthegalaxyby<20%.Theeffectislargestforthe uncertaintiesinvirialmassesatfixedgalaxymass3,whichis most massive galaxies, bu∼tsince massivebulges, in general, the dominant source of uncertainty. The dashed line shows are not rotationally supported, we believe our assumption the relation between virial specific angular momentum and that bulges have no angular momentum to be of no major virial mass from cosmological N-body simulations (Maccio` concern for what follows. et al. 2008), which corresponds to a spin parameter of AshownbyToninietal.(2006)asignificantfractionof λ′ = 0.031. The galaxy specific angular momentum scales theangularmomentumofagalaxyisinthecoldgas.Inour withvirialmassasJ /M M0.70,whichisverysimilar modelsthecoldgasinhighmassgalaxies(Mgal ≃1011M⊙) tothevirialscalingogfaJlvir/gMalv∝ir ∝vMirv0i.r67.ThustheratioRj contains 30% of the angular momentum. This fraction betweenthegalaxyandvirialspecificangularmomentumis rises to ov≃er 50% for galaxy masses below Mgal 1010M⊙. roughly constant (right-handpanelof Fig. 1).However, the ≃ Thus if cold gas fractions increase at higher redshifts, as is galaxyspecificangularmomentumissignificantlylowerthan commonly thought (but see Dutton et al. 2010a), measur- the virial specific angular momentum with = 0.61+0.13 ingangularmomentumofhighredshiftgalaxieswillrequire (1σ)foravirialmassofMvir =1012M⊙.TheRdjashedli−ne0.1in1 measurements of the distribution of cold gas, and not just theright-hand panel of Fig. 1 corresponds to model B, and thestars. isshownforcomparison. Notethatthecombination ofhalo contractionandlowerstellarmass-to-lightratiosresultin j R values that are 0.1 higher than for our fiducial model A. ∼ 3 RESULTS This difference is comparable to the 1σ uncertainty on j R duetotheuncertaintiesintherelationbetweenstellarmass Usingourconstrainedmassmodelswenowdiscussanumber and halo mass. ofcorrelationsbetweenmassandangularmomentumofdisc The left-hand panel of Fig. 2 shows the correlation be- galaxiesanddarkmatterhaloes,andcomparewithprevious tweengalaxymassandvirialmassforthediscgalaxiescon- results. These correlations are presented in Figs. 1-3 and sideredhere.Thedashedlineshowstherelationforthecos- Table 1. mically available baryonic mass, i.e., Mgal =(Ωb/Ωm)Mvir, corresponding to ǫGF = 1. The right-hand panel of Fig. 2 3.1 Galaxy Mass and Specific Angular shows the galaxy formation efficiency, ǫGF, which is the ratio between the solid and dashed lines in the left-hand Momentum The left-hand panel of Fig. 1 shows the correlation be- tween galaxy specific angular momentum and virial mass 3 Notethatthesearetheuncertaintiesonthemean;theydonot for model A. The shaded regions show the one and two σ reflectthescatter. (cid:13)c 2012RAS,MNRAS000,1–15 6 A.A. Dutton & F.C. van den Bosch Figure2.Left:Galaxymassvs.virialmassfordiscgalaxies.Theshadedregionsreflecttheoneandtwoσuncertaintiesonvirialmasses. Thedashedlinecorrespondstotheuniversalbaryonfraction(Ωb/Ωm=0.163).ThepentagonshowsestimatesfortheMilkyWay(MW), withthe shaded regionshowing one andtwo σ uncertainties. Right:Galaxy formationefficiency (ǫGF)vs. virialmass.As inFig.1, we show the results for both models A and B. For clarity, we only show the impact of virial mass uncertainties for our fiducial model A. NotethatunlikeRj,whichrevealsnosignificantdependenceonhalomass(seeFig.1),thegalaxyformationefficiencyvariesbyafactor of∼3overthemassrangeprobed. panel. Unlike the angular momentum ratio, the galaxy for- An interpretation advocated by Tonini et al. (2006) is mationefficiencyisastrongfunctionofvirialmass,varying that if one restricts attention to haloes that havenot expe- by a factor of 3 from ǫGF 0.1 at the low mass end riencedamajormergersincez 3,theaveragespinparam- (Mvir 2 10∼11M⊙) to ǫGF ≃0.3 at the high mass end etersλ′ turnsout to bearound∼0.023 (D’Onghia& Burkert (Mvir ∼ 5×1012M⊙). Once a≃gain, the dashed line in the 2004). This is motivated by the idea that bulge-less disc ∼ × right-handpanelcorrespondstomodelB,whichyieldslower galaxies require a quiescent merger history. However, this galaxy formation efficiencies dueto thelighter IMF. simple expectation has been cast into doubt by numerical Aswedemonstrateanddiscussbelow,(semi)-analytical simulations which haveshown that (gas rich) discscan sur- modelsandhydrodynamicalsimulationsofgalaxyformation vive major mergers (e.g., Springel & Hernquist 2005; Hop- do generally not predict values of that are independent kinsetal.2009).Whilewecannotruleoutamergerhistory j R ofhalomass,andiftheydo,thecorrespondingvaluesofǫGF bias for massive disc galaxies, we argue that such a bias are inconsistent with theempirical results inferred here. cannot occur for low mass galaxies (Mgal < 3 1010M⊙, Mvir < 1012M⊙) because typical low ma∼ss g×alaxies are bulge-∼less,andthusmustforminhaloeswithtypicalmerger 3.2 Galaxy Spin Parameter histories (and hencetypical halo spins). Wenowdiscussthegalaxyspinparameter,asthisisaquan- tity that has been measured and interpreted by previous studies. Fig. 3 shows the average galaxy spin parameter as 3.3 Comparison with the Milky Way afunctionofhalovirialmass.Thisisthesameastheright- handpanelofFig.1,butwithazeropointshift.Asdiscussed As a reference, the black pentagons and light blue shaded above, there is a slight mass dependence to λ′ , but given regions in Figs. 1 & 2 show estimates for the specific an- gal the uncertainties in halo masses, λ′ is consistent with be- gular momentum and mass of the Milky Way (MW). Here gal ing independent of halo mass. The normalization is 60% we have estimated the specific angular momentum of the ≃ lowerthanpredictedforΛCDMhaloes(dashedline).There MW assuming Jgal/Mgal = 2RdVrot, which is appropriate aretwosimpleinterpretationsofthisresult.Oneisthatthe foranexponentialdiscwith aflatrotationcurve.Weadopt baryonsthatformdiscgalaxieshavelostabout40%oftheir R = 2.8 0.23kpc(1σ) based on the dynamical models d angular momentum during galaxy formation, which could of Widrow±et al. (2008) and Vrot = 219 20 kms−1(1σ) ± occur via dynamical friction. Another is that the baryons fromtheobservationsofReidetal.(1999).Forthebaryonic thatform discgalaxies have40% lowerspecificangularmo- mass of the MW we use the bulge and disc masses, again mentumthanthecosmically availablebaryons,whichcould fromthedynamicalmodelsofWidrowetal.(2008)toobtain occurduetofeedbackand/orinefficientcooling. Wediscuss log10(Mgal/M⊙)=10.71±0.075 (1σ).Forthevirialmassof these interpretations in more detail in 4. the MW we average the results of Smith et al. (2007) and § (cid:13)c 2012RAS,MNRAS000,1–15 Angular momentum of disc galaxies 7 Table 1. Relations between mass and angular momentum of galaxies and dark matter haloes for model A, as presented in Figs. 1-3. Uncertainties onhalomassesare1and2σ,andarepropagatedtootherparametersasappropriate. log10(Mgal) log10(Mvir) ǫGF log10(Jgal/Mgal) Rj λ′gal [M⊙] [M⊙] [kpckms−1] 9.484 11.318+0.125 +0.250 0.09−0.03 −0.07 2.417 0.57−0.10 −0.18 0.018−0.003 −0.006 −0.125 −0.250 +0.02 +0.04 +0.12 +0.27 +0.004 +0.008 9.631 11.414+0.125 +0.250 0.10−0.03 −0.08 2.482 0.57−0.10 −0.18 0.018−0.003 −0.006 −0.125 −0.250 +0.03 +0.04 +0.12 +0.27 +0.004 +0.008 9.784 11.513+0.125 +0.250 0.11−0.04 −0.09 2.550 0.57−0.10 −0.18 0.018−0.003 −0.006 −0.125 −0.250 +0.03 +0.05 +0.12 +0.27 +0.004 +0.008 9.941 11.615+0.125 +0.250 0.13−0.04 −0.10 2.621 0.58−0.10 −0.18 0.018−0.003 −0.006 −0.125 −0.250 +0.03 +0.06 +0.12 +0.27 +0.004 +0.008 10.103 11.720+0.125 +0.250 0.15−0.05 −0.12 2.697 0.59−0.10 −0.19 0.018−0.003 −0.006 −0.125 −0.250 +0.04 +0.06 +0.12 +0.27 +0.004 +0.008 10.270 11.832+0.125 +0.250 0.17−0.06 −0.13 2.779 0.60−0.10 −0.19 0.018−0.003 −0.006 −0.125 −0.250 +0.04 +0.07 +0.13 +0.28 +0.004 +0.009 10.442 11.950+0.125 +0.250 0.19−0.06 −0.15 2.868 0.61−0.11 −0.19 0.019−0.003 −0.006 −0.125 −0.250 +0.05 +0.08 +0.13 +0.29 +0.004 +0.009 10.618 12.078+0.125 +0.250 0.21−0.07 −0.17 2.960 0.62−0.11 −0.20 0.019−0.003 −0.006 −0.125 −0.250 +0.05 +0.09 +0.13 +0.29 +0.004 +0.009 10.796 12.215+0.125 +0.250 0.23−0.08 −0.18 3.050 0.62−0.11 −0.20 0.019−0.003 −0.006 −0.125 −0.250 +0.06 +0.10 +0.13 +0.29 +0.004 +0.009 10.979 12.361+0.125 +0.250 0.25−0.08 −0.20 3.154 0.63−0.11 −0.20 0.019−0.003 −0.006 −0.125 −0.250 +0.06 +0.11 +0.13 +0.29 +0.004 +0.009 11.164 12.517+0.125 +0.250 0.27−0.09 −0.21 3.261 0.63−0.11 −0.20 0.020−0.003 −0.006 −0.125 −0.250 +0.07 +0.12 +0.13 +0.30 +0.004 +0.009 11.352 12.680+0.125 +0.250 0.29−0.10 −0.22 3.391 0.66−0.12 −0.21 0.021−0.004 −0.007 −0.125 −0.250 +0.07 +0.13 +0.14 +0.31 +0.004 +0.010 3.4 Comparison with Previous Studies Several studies in the past used similar methods to deter- mine thespin parameters of disc galaxies. These studies all find higher spin parameters than we find here, but as we show the differences can be ascribed to differences in the estimation of halo masses, which is the dominant source of systematic uncertainty in measuring thespin parameter. Van den Bosch, Burkert & Swaters (2001; hereafter BBS01) used Eq. (4) and halo masses inferred from rota- tion curve modelling to determine the galaxy spin parame- tersforasampleof14disc-dominated dwarfgalaxies. They found that λ′ = 0.048 (see Fig. 4)4, albeit with a rel- h gali atively large uncertainty due to the small sample size and theuncertaintiesinthestellar mass-to-light ratios andhalo masses. At first sight, this is much larger than the average galaxy spin parameters inferred here; λ′ = 0.019. How- h gali ever,thereareseveralfactorsthatcanaccountforthislarge difference. BBS01 used dark halo masses and sizes derived from fitstorotation curvesassumingadiabatic contraction. Since therotation curves only extend to 10 20% of the ∼ − halo virial radius, this involves a significant extrapolation, and thus the potential for systematic errors. As discussed Figure 3. Average spin parameter vs. halo virial mass, for disc in detail in Dutton et al. (2007), the assumption that disc galaxies(solidlineandgreyshadedregion),andΛCDMdarkmat- formationresultsinadiabaticcontractionofitsdarkmatter ter haloes (dashed line, red shaded region). For the galaxy spin halomaynotberealistic.ThegalaxiesinBBS01haveame- parametertheshadedregionsshowtheoneandtwoσuncertain- tiesintroducedbythesystematicuncertaintyinhalomasses.The dianVlast/Vvir =1.55, whereVlast istherotation velocityat pentagon shows an estimate for the Milky Way (MW) with the the last measured point (i.e., at the largest radius) . Since shadedregionshowingoneandtwoσ uncertainties. dark matter haloes in the ΛCDM concordance cosmology have Vmax/Vvir <1.2, where Vmax is the maximum circular velocity, the der∼ived values from BBS01 are strongly influ- enced by the assumption of adiabatic contraction. Dutton Xue et al. (2008) yielding log10(Mvir/M⊙) = 11.97±0.22 etal.(2010b)haveshownthatthehalovirialvelocityis,on (2σ). average, approximately equal to the optical rotation veloc- The resulting galaxy spin parameter (see Eq.2) and ity. Using this assumption to derive halo masses results in g0Ba.o0lta3hx1+−yλ00′gf..ao00l11ra61mna(d2tiǫσoG)nF,eaaffinredciheǫingGchFyer(=stehe0a.nE03qfo3.7r+−)0t0y..o21pf85itc(ha2elσlMa),tWer-etsyappreeecλgtia′gvaleallx=y-. l1λo0′gga%1l0=u(Mn0cve.i0rr2t/aM3in(⊙try)ig=ohnt-1Vh1ra.o2ntd/±Vpv0ia.r3n(eaDlnuodtftaoFnmige.etd4ai)al..nI2fs0p1wi0neb)cpoatnhraseimndeetrhteiasr ies. This could signify that the MW is atypical, or it could simplybeareflection ofthemeasurementerrors(especially in halo masses) both for the MW and for samples of late- 4 Here we have converted the results from BBS01 to the spin typegalaxies. parameterdefinitionofBullocketal.(2001). (cid:13)c 2012RAS,MNRAS000,1–15 8 A.A. Dutton & F.C. van den Bosch Figure4.Histograms(hatched)ofthedistributionofgalaxyspin parameters (λ′ )forthe 14discdominated dwarfgalaxies from gal van den Bosch, Burkert, & Swaters (2001; hereafter BBS01). In the left-hand panel λ′ has been obtained using the halo virial gal velocitiesfromBBS01.Intheright-handpanel,thehalovirialve- locitieshavebeencalculatedassumingVvir=Vlast,whereVlastis therotationvelocityatthelastmeasuredpoint(i.e.,atthelargest radius).Thelatter resultsinspinparameters ingoodagreement withourresults(cf.Fig.3). resultsinarangeof0.019 0.028forλ′ ,whichisconsistent Figure 5. The cumulative, normalized fractions of mass (blue − gal with our measured values(see Fig. 3). line)andangularmomentum(dashedredline)forΛCDMhaloes Tonini et al. (2006) used average scaling relations be- (Navarro et al. 1997; Bullock et al. 2001). The shaded region tween stellar mass, gas mass, halo mass and disc size to shows the uncertainty on J(r)/Jvir assuming s = 1.3±0.3 in Eq. 9. This clearly demonstrates that mass is more centrally compute an average galaxy spin parameter as function of concentrated than angular momentum. For example, the inner halo mass, under the assumption that dark matter haloes 50%of themasscontains just20+5%oftheangular momentum follow a Burkert (1995) profile. They found that λ′ −4 h gali ≃ (blackdotted lines).Thusiftheefficiency ofgalaxyformationis 0.025 0.030 with no significant dependenceon halo mass. − regulated by the efficiency of cooling, we would expect a strong Acomparison between thehalomass -stellar massrelation correlationbetweengalaxyspinparameterandgalaxyformation usedbyTonini etal. (2006) and byourselvesindicates that efficiency,whichisnotobserved. their halo masses are a factor of 2 lower than ours. This ∼ differenceinhalomassesfullyaccountsfortheirhigherspin λ′ F, and thus their assumption of F = 0.04 accounts parameters. gal ∝ for at least half the discrepancy between our result and Finally, Hernandezet al. (2007) determined thegalaxy theirs. spin parameters for a large sample of galaxies taken from the Sloan Digital Sky Survey (SDSS). Rather than Tonini et al. (2006), they obtained estimates for λ for individ- gal ual galaxies using the method proposed by Hernandez & 4 COMPARISON WITH GALAXY Cervantes-Sodi (2006), which is based on the following as- FORMATION MODELS sumptions: (i) dark matter haloes are singular isothermal 4.1 Inside-Out Cooling Models spheres, (ii) the self-gravity of the disc can be ignored, and (iii) all galaxies havethesameratio ofdark mattermassto If the galaxy formation efficiency is determined by the ef- discmass.Inaddition,theyinferredtherotationvelocityof ficiency of cooling, then we expect that will be higher j R eachindividualgalaxyinastatisticalsense,usinganempir- for higher galaxy formation efficiencies. This is because in ical Tully-Fisherrelation. They found that for disc galaxies ΛCDMhaloesmassismorecentrallyconcentratedthanan- λ′ follows a log-normal distribution with λ′ 0.059, gularmomentum.ThisisillustratedinFig.5.Thebluesolid gal h gali ∼ whichisafactorof 3higherthanourresult.Themethod line shows the cumulative mass fraction, M(<r)/Mvir as a ∼ used by Hernandezet al. (2007) makes several highly ques- functionofradiusforaNFWhalowithconcentrationc=10. tionable assumptions, and thus we do not consider their The red dashed line shows the cumulative angular momen- result reliable. For example, Hernandez et al. (2007) as- tum fraction, J(<r)/Jvir, as a function of radius assuming sume a constant galaxy mass to dark matter mass ratio of F =0.04,whichcorrespondstoagalaxyformationefficiency J(<r)/Jvir =[M(<r)/Mvir]s, (9) ofǫGF 0.25.ComparisonwithFig.2showsthatwhilethis M(<r)/Mvir isagoo≃d assumption fora2 1012M⊙ halo,it significantly wheres=1.3 0.3(Bullocketal.2001).Undertheassump- × ± over-estimates the galaxy formation efficiency, and hence tionsthat (i) thebaryonsinitially acquirethesame specific spin parameters, of lower mass galaxies. Since Hernandez angularmomentumasthedarkmatter,and(ii)thebaryons et al. (2007) use a volume limited sample, their measure- accreteontothecentralgalaxyinside-out,thenEq.9implies ment of the spin parameter is biased towards lower mass =ǫs . Rj GF galaxies, where their assumption of F = 0.04 is high by a Wenow extend thissimple inside-out cooling model to factorof 2.InthemethodusedbyHernandezetal.(2007) cosmologically evolvingdarkmatterhaloes.Theredlinesin ≃ (cid:13)c 2012RAS,MNRAS000,1–15 Angular momentum of disc galaxies 9 Figure 6.Correlations between halo mass, Mvir, galaxy forma- Figure7.SameasFig.6,butthistimewecomparetheempirical tionefficiency,ǫGF,andspecificangularmomentumratio,Rj,for relations(solidblacklineswithshadedregions)withmodelsthat modelsandobservationsatredshiftz=0.Theempiricalrelations include feedback and/or angular momentum loss (red lines). All fordiscgalaxies areshownas solidblack lines,withthe shading models shown adopt a hot gas metallicity of Zhot =0.1Z⊙, but reflecting the one and two σ uncertainties on halo masses. Note theydifferinfeedbackefficiency,ǫFB,andthefractionofangular thatthegalaxyformationefficiencyincreaseswithincreasinghalo momentum lost by the baryons during galaxy formation, flost. mass,whilethespecific angularmomentum ratioisindependent Thedottedline(ǫFB=flost=0)isidenticaltothesimplecooling of halo mass. The corresponding relations for a simple cooling modelfromFig.6,andisshownforcomparison.Notethatnone model are shown with red lines, where the different line types of the models is able to simultaneously match the low galaxy correspondtodifferentmetallicities,asindicated. Inthiscooling formationefficienciesandtheconstantangularmomentumratio. modelRj ∼ǫGF (independent ofthemetallicityofthehotgas), Seetextforadetaileddiscussion. inconsistentwiththeempiricalfindings. pirical result. As for the parameter , the simple cooling j R modelspredictthat shoulddecreasewithincreasinghalo Fig.6showtherelationsbetween j,ǫGFandMvirforasce- Rj R mass, which is a consequenceof cooling beingan inside-out narioinwhichthegalaxyformationefficiencyisdetermined process combined with the fact that mass is more centrally bytheefficiencyofcooling.Thismodelisasimplifiedversion concentratedthanangularmomentum(cf.Fig.5).Hence,in ofthegalaxyformationmodelfromDutton&vandenBosch (2009).BrieflythismodelconsistsofNFWhaloesthatgrow massivehaloeswhereǫGF 1duetoinefficientcooling,the ≪ galaxyendsuphavinglessspecificangularmomentumthan bysmoothaccretionofbaryonsanddarkmatter.Theangu- its dark matter halo. This strong mass dependence of larmomentumofeachshellofaccretedgasisdeterminedby j R isagain inconsistent with ourempirical results. Finally, the assuming that the angular momentum distribution (AMD) coolingmodelspredictsanalmostlinearrelationbetween of a given halo is independent of time. The AMD is spec- j R ified by two parameters: the normalization (λ) and shape and ǫGF, which is independent of the gas metallicity (lower right-hand panel of Fig. 6). Clearly, this linear relation is (α).Whenthebaryonsenterthehalotheyareshockheated very different from its empirical equivalent, indicating that to the virial temperature. The cooling time depends on the additional mechanisms for accreting and/or expelling gas temperature,densityandmetallicity of thehotgas. Weuse are required. the metallicity dependent collisional ionization equilibrium coolingfunctionsofSutherland&Dopita(1993).Resultsfor four different metallicities from primordial to one third so- 4.2 Outflows and Angular Momentum Loss lar are shown. For low mass haloes (Mvir 1011M⊙) cool- ∼ ing is very efficient, so that 90% of the baryonic mass We now consider two modifications to the simple cooling is accreted (left-hand panels)≃5. For more massive haloes, model: outflows and angular momentum loss. We assume the galaxy formation efficiency, ǫGF, depends strongly on that outflows, which move at the local escape velocity, are the metallicity of the gas, with higher metallicities yielding driven by energy from supernovae (SNe). The fraction of larger values of ǫGF. Clearly, the trend that ǫGF decreases the SNe energy that goes into an outflow is specified by withincreasinghalomassiscompletelyoppositetotheem- the free parameter ǫFB (see van den Bosch 2002; Dutton & van den Bosch 2009). Other than inefficient cooling in massive haloes, and UV heating in low mass haloes, galac- 5 It is not 100% because it takes a free fall time for the most tic outflows are the primary mechanism for explaining the recentlyaccretedbaryonstoreachthegalaxy. observed low galaxy formation efficiencies (e.g., Dekel & (cid:13)c 2012RAS,MNRAS000,1–15 10 A.A. Dutton & F.C. van den Bosch Silk 1986; Cole et al. 1994). For simplicity, we assume that outflow gas leaves the disc and halo, and does not return. However, recent cosmological hydrodynamical simulations haveshown that ejected gas may returnto thegalaxy (Op- penheimer et al. 2010). Some of the ejected gas may mix with the halo gas and thus re-accrete with higher specific angular momentum (Brook et al. 2012). Since this process is a re-distribution (i.e., ejected gas gains angular momen- tum,halogaslosesangularmomentum),itdoesnotchange the specific angular momentum of the baryons. If the halo gas accretes onto the central galaxy (as is expected for low mass, Mvir < 1012M⊙, haloes) then there is no net change intheangul∼armomentumofthegalaxy.Ifthehalogasdoes not accrete (which is expected for high mass haloes) then the galaxy can gain angular momentum via this galactic fountain effect. Thus if we were to implement re-accretion intoourmodelwewouldneedhigherfeedbackefficienciesto achieve the same galaxy formation efficiencies, but to first order there would be no change to the specific angular mo- mentum. We assume that during the accretion process, a frac- tion f of theangular momentum of thegas is lost to the lost darkmatterhalo.Themechanismresponsibleforthistrans- Figure8.SameasFig.7,butthistimewehavetunedthefeed- fer is dynamical friction. In our models we donot takeinto back efficiency, ǫFB, and angular momentum loss, flost, of the account the effects of angular momentum transfer and out- model (red, soliddots) inorder toaccurately reproduce the em- flowsonthestructureofthedarkmatterhalo.Butwenote piricalrelations(solidblacklineswithshadedregions).Thesmall, thatbothoftheseprocesseswillresultinhaloexpansion,or upperright-handpanelshowsthecorrespondinghalomassdepen- at least a reduction of theamount of adiabatic contraction, dence of ǫFB (solid circles) and flost (open circles): reproducing the empirical relations requires both of these parameters to de- i.e.,alowereffectivevalueofν (e.g.,Navarroetal.1996;El- creasestronglywithincreasinghalomass.Seetextforadetailed Zantetal.2001;Mo&Mao2004;Cole,Dehnen&Wilkinson discussion. 2011) In Fig. 7 we show several models with different values of ǫFB and flost. In all models we assume that halo gas is enriched to one tenth solar. The dotted line shows a model with ǫFB =0.0 and flost =0.0: this corresponds to thesim- plecooling model from Fig. 6 and is shown for comparison. The galaxy formation efficiencies are too high for all halo masses. In order to produce realistic galaxy formation effi- ciencies outflows are needed. A model with ǫFB = 0.5 and flost =0.0(shortdashedline)hasǫGF 20%.Inourmodel ∼ outflows increase by preferentially removing gas with j R low specific angular momentum. Since outflows are driven by star formation, regions that have higher star formation rates havehigher mass outflow rates. Star formation is em- pirically more efficient at smaller galactic radii (Kennicutt 1998) aswellasathigherredshifts(e.g.,Noeskeetal.2007; Daddi et al. 2007; Elbaz et al. 2007). Smaller galactic radii correspond to lower specific angular momentum and discs are smaller at higher redshifts (e.g., Dutton et al. 2011a); both of these effects contribute to outflows increasing the specific angular momentum of the material that remains in the galaxy. In our energy driven outflow model the mass loadingfactor( outflowrate/starformationrate)ishigher ≡ in lower mass galaxies, which results in increasing more j R inlower mass haloes (byuptoafactor two).Thisincreases Figure9.SameasFig.8,butherewehaveusedmodelB(stellar the disagreement with the empirically inferred relation be- mass-to-light ratios that are 0.3 dex lower and halo contraction tween and halo mass. We can lower by introducing characterized by ν =0.8) for the empirical relations. Reproduc- j j R R ing the inferred galaxy formation efficiencies now requires even angular momentum loss (i.e., setting f > 0). The long lost dashed-dotted lines in Fig. 7 correspond to a model with strongerfeedback (i.e.,largerǫFB),whilethe fractionofangular momentum that is lost is similar to that for our fiducial model ǫFB =0.5 and flost =0.5. This angular momentum loss re- showninFig.8. duces byafactor 2atallmassscales,bringingitinrea- j R ∼ sonable agreement with the observed value for haloes with (cid:13)c 2012RAS,MNRAS000,1–15

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