Resolution-independent modeling of environmental effects in semi-analytic models of galaxy formation that include ram-pressure stripping of both hot and cold gas

0.68 MB·

by  Yu Luo

Mon.Not.R.Astron.Soc.000,000–000 (0000) Printed2February2016 (MNLATEXstylefilev2.2)
Resolution-independent modeling of environmental effects
in semi-analytic models of galaxy formation that include
ram-pressure stripping of both hot and cold gas
6
1
0 Yu Luo1,2,4 ⋆, Xi Kang1, Guinevere Kauffmann2, Jian Fu3
2
1Purple Mountain Observatory, the Partner Group of MPI fu¨r Astronomie, 2 West Beijing Road, Nanjing 210008, China

n 2Max-Planck Institue fu¨r Astrophysik, 85741 Garching, Germany
a 3Key Laboratory for Research inGalaxy and Cosmology, Shanghai Astronomical Observatory, Chinese Academy of Science,
J 80 Nandan Road, Shanghai 200030, China
9 4Graduate School, University of the Chinese Academy of Science, 19A, Yuquan Road, Beijing 100049, China
2

]
A
2February2016
G
.
h ABSTRACT
p
-
o Thequenchingofstarformationinsatellitegalaxiesisobservedoverawiderange
r of dark matter halo masses and galaxy environments. In the recent Guo et al (2011)
t and Fu et al (2013) semi-analytic + N-body models, the gaseous environment of the
s
a satellite galaxy is governed by the properties of the dark matter subhalo in which it
[ resides.ThisquantitydependsoftheresolutionoftheN-bodysimulation,leadingtoa

divergentfractionofquenchedsatellitesinhigh-andlow-resolutionsimulations.Here,
1
v weincorporateananalyticmodeltotracethesubhaloesbelowtheresolutionlimit.We
3 demonstrate that we then obtain better converged results between the Millennium I
0 and II simulations, especially for the satellites in the massive haloes (logMhalo =
0 [14,15]). We also include a new physical model for the ram-pressure stripping of cold
0 gasinsatellitegalaxies.However,wefindverycleardiscrepancieswithobservedtrends
0 in quenched satellite galaxy fractions as a function of stellar mass at fixed halo mass.
.
2 At fixed halo mass, the quenched fraction of satellites does not depend on stellar
0 mass in the models, but increases strongly with mass in the data. In addition to
6 the over-prediction of low-mass passive satellites, the models also predict too few
1 quenched central galaxies with low stellar masses, so the problems in reproducing
: quenched fractions are not purely of environmental origin. Further improvements to
v
i the treatment of the gas-physical processes regulating the star formation histories of
X galaxies are clearly necessary to resolve these problems.
r
a Key words: galaxies: evolution - galaxies: formation - stars: formation - galaxies:
ISM
1 INTRODUCTION gascomponentofgalaxiesandtheirenvironment;unlikethe
stellarcomponent,thegascanbeeasilyaffectedbytheam-
Many of the observed properties of galaxies, such as their
bient pressure in galaxy groups or clusters.
colors, morphologies, star formation rates (SFR) and gas-
When a galaxy moves through a cluster, the ram pres-
to-star fractions, are observed to have strong dependence
sure (hereafter RP, Gunn & Gott 1972) of the intra-cluster
on their environment (e.g. Kauffmann et al. 2004; Bamford
medium (ICM) acts to strip both the hot gas reservoir and
etal.2009;Bosellietal.2014).Galaxiesinclustersorgroups
thecoldinterstellargas,andthisprocessplaysanimportant
tend to have redder colors, bulge-dominated morphologies,
roleinthestarformationhistoryinthegalaxy.Manystudies
lowergas-to-starratiosandlessstarformationthanisolated
haveconcludedthatthisstrippingprocessisthemaincause
galaxies of similar mass (Butcher & Oemler 1978; Dressler
fortheincreaseofS0galaxiesinrichclusters(e.g.Biermann
1980; Balogh et al. 2004; Baldry et al. 2006). This depen-
&Tinsley 1975; Dressler1980; Whitmore,Gilmore&Jones
dence is believed to arise from the interplay between the
1993). In the last decade, observations have revealed direct
evidenceforram-pressurestrippingofgasinclustergalaxies
⋆ E-mail:luoyu,[email protected] in the form of long gaseous tails trailing behind these sys-
(cid:13)c 0000RAS
2 Y. Luo
tems (e.g. Kenney et al. 2004; Crowl et al.2005; Sakelliou will leadtonon-convergentresultsbetween differentresolu-
et al.2005; Machacek et al. 2006). The same process is also tion simulations, as found in recent studies (Fu et al. 2013;
invoked as an explanation of the depletion of the cold gas Lagos et al. 2014; Guo & White 2014), and it also raises
in galaxies in clusters (Boselli & Gavazzi 2006), often re- theconcernthatthehighfractionofpassivegalaxiesinlow-
ferred toas“HIdeficiency”(e.g.Haynes&Giovanelli 1984; resolution simulation is a consequence of resolution effects.
Solanes et al. 2001; Hughes & Cortese 2009). Inthispaper,we studythequenchedfraction of galax-
TounderstandtheeffectsofRPstrippingonthegalaxy ies in different environments with and without the effects
gascomponentindetail,numericalhydrodynamicalsimula- of RP stripping. We adopt the version of L-Galaxies model
tions are the ideal tools. Abadi et al. (1999) presented the described in Fu et al. (2013, hereafter Fu13), which is a
first study of RP stripping using an idealized SPH simula- recent version of the Munich semi-analytic model that in-
tion,followed bymorerealistichydrodynamicalsimulations cludes the radial distribution of molecular and atomic gas
(e.g., Roediger & Hensler 2005; Roediger & Bru¨ggen 2007; in galaxy disks. This model allows us to model the cold
McCarthy et al. 2008; Tonnesen & Bryan 2009, Tecce et gas stripping as a function of radius in the galaxy . We
al. 2010). These studies showed that RP can strip a signifi- improve the Fu13 model by 1) using a consistent descrip-
cant amount of hot gas and cold gas from galaxies and can tion of physics for satellites whose subhaloes cannot be re-
quicklyreducethetotalSFR(e.g.,Tonnesen&Bryan2012). solved.Ourresolution-independentprescriptionscanalsobe
However,ithasalsobeenalsoargued(Bekki2014)thatstar applied to other SAMs based on merger trees from N-body
formation can also beenhancedbyRP,andthat thereduc- simulations.2)WeaddamodelfortheeffectofRPstripping
tion/enhancementwilldependonmodelparameterssuchas onthecold gas ingalactic disks.Theseimprovementsallow
halo mass, peri-centric distance with respect the the centre us to model ram pressure stripping of cold gas at different
of the cluster etc. radii in disks, and to study how theenvironment can affect
In semi-analytic models (SAMs) of galaxy formation, theHI,H2, and star formation.
the descriptions of the effect of RP on the gas component Thispaperisorganizedasfollows.InSection2,wefirst
are very simplistic. In early versions, satellite galaxies were briefly summarize the L-Galaxies model and then describe
assumed to lose their hot gaseous haloes immediately af- the changes we make to Fu13 model. In Section 3, we an-
ter falling into a bigger halo (e.g., Kauffmann et al. 1993; alyze stellar/gas mass functions and galaxy clustering, and
Somerville et al. 1999; Cole et al. 2000; Kang et al. 2005; we compare our model results with recent observations of
Bower et al. 2006; De Lucia & Blaizot 2007 (hear after thepropertiesofsatellitegalaxies,suchastheirspecificstar
DLB07)). It was then found (e.g., Kang & van den Bosch formation rates and gas fractions. We test the degree to
2008;Kimmetal.2009) thattheinstantaneousstrippingof which our new models give convergent results between two
thehotgascausestheover-predictionofredsatellite galax- N-body simulations of different resolution, and we analyze
ies in the clusters. In later models (e.g., Kang & van den the effect of RP stripping in cluster environments. In Sec-
Bosch 2008; Font et al. 2008; Weinmann et al. 2010; Guo tion4,wesummarizeourresultsanddiscusspossibilitiesfor
et al. 2011, hereafter Guo11)), the stripping of hot gas is futurework.
treated in a more continuous way, i.e the mass loss rate of
hotgashaloisassumedtobethesameasthatofdarkmat-
tersubhaloinwhichthesatelliteresides.Includingonlythe 2 THE MODEL
strippingofhothalogasisnotphysicallyplausible;thestrip-
pingofcoldgasingalaxiesisalsoneededtoaccountforthe Inthissection,webrieflyintroducetheN-bodysimulations
observedcoldgasdepletioninsatellites(Fabelloetal.2012; usedinthisworkaswellastheL-Galaxiesmodels,andthen
Lietal.2012b;Zhangetal.2013).Strippingofcoldgashas describeindetailthemainchangestothephysicsmadewith
beenincludedinsomemodels(Okamoto&Nagashima2003; respect to theprevious models.
Lanzoni et al. 2005), and has been found to have negligible
effect on the colors and SFRsof satellite galaxies.
2.1 N-body simulations and L-Galaxies model
Because thestrippingofgas inthesatellite dependson
the competition between the RP from the hot gas and the OurworkinthispaperisbasedontheMunichsemi-analytic
gravitation of the satellite itself, it is important to model galaxy formation model, L-Galaxies, which has been devel-
the local environment of satellite galaxies accurately. In N- opedovermorethantwodecades(e.g.,White&Frenk1991;
bodysimulations,thelocalenvironmentatthesubhalolevel Kauffmann et al. 1993; Kauffmann et al. 1999; Springel
will be dependent on the resolution of the simulation. In et al. 2001; Croton et al. 2006; De Lucia & Blaizot 2007;
low-resolution simulation, the evolution of subhalo can not Guo et al. 2011&2013; Fu et al. 2010 & 2013; Henriques
be traced to very low mass due to the number threshold et al. 2015). The L-Galaxies model has been implemented
used to identify subhalo, and for given mass a subhalo in on two main N-body cosmological simulations: The Millen-
low-resolution simulation contains less number of particles, nium Simulation (hereafter MS, Springel et al. 2005) and
makingitsidentificationmoredifficult.Inthecentralregion Millennium-II simulation (hereafter MS-II, Boylan-Kolchin
of halo the identification of subhalo is more challanging as et al. 2009). The two simulations have the same number
thebackgrounddensityishigh(Onionsetal.2012).Wealso of particles and cosmology parameters, but the MS-II has
note that the physical descriptions of processes such as gas 1/125 the volume of MS, but 125 times higher in mass res-
cooling, feedback, stripping etc, are very often tied to the olution. Angulo & White (2010) developed a method to
properties of the subhalo (Springel et al. 2001; Kang et al. rescale the cosmological parameters from the WMAP1 to
2005; Bower et al. 2006; Guo et al. 2011;), and these pre- the WMAP7 cosmology. For MS, the box size is rescaled
scriptions will then also be dependent on resolution. This from 500 Mpc h−1 to 521.555 Mpc h−1, and the particle
(cid:13)c 0000RAS,MNRAS000,000–000
Satellite galaxy quenching in SAMs 3
massischangedfrom8.6 108M⊙ h−1to1.06 109M⊙ h−1; dark matter subhaloes are disrupted by tidal effects or the
for MS-II,thebox size is×rescaled to104.311 M×pc h−1, and subhaloesarenotwellresolvedinthelow-resolutionsimula-
8.50 106M⊙ h−1 for the particle mass. In this paper, we tion.Type1and2galaxiesmaylatermergeintothecentral
×
follow Angulo & White (2010) and use the runs appropri- galaxy of their host (sub)halo. In Guo11 & Fu13, the hot
atefor theWMAP7cosmology, with parametersas follows: gaseoushaloand“ejected reservoir”ofaType1galaxycan
Ω =0.728, Ω =0.272, Ω =0.045, σ =0.807 and bestrippedbyRPwhen theforce of RPdominates overits
Λ m baryon 8
h=0.704. self-gravity. A Type 2 galaxy can be disrupted entirely by
In SAMs, galaxies are assumed to form at the centres tidal forces exerted by the central object, if the density of
ofthedarkmatterhaloes.Theevolutionofthehaloesisfol- the main halo through which the satellite travels at peri-
lowedusingmergertreesfromtheN-bodycosmologicalsim- centre is larger than the average baryonic mass density of
ulations,andthemodelsdescribethephysicalprocessesrel- theType2.
evanttothebaryonicmatter,e.gre-ionization,hotgascool- In this paper, we update the semi-analytic models of
ingandcoldgasinfall,starformationandmetalproduction, Fu et al. (2013), which is a branch of therecent L-Galaxies
SNfeedback,hotgasstrippingandtidaldisruptioninsatel- model. Compared with the previous L-Galaxies models,
lites, galaxy mergers, bulge formation, black hole growth, Fu13contains thefollowing two main improvements:
andAGNfeedback.Thedetaileddescriptionsofthesephys- (i)Eachgalaxy diskisdividedintomultiplerings,andthus
ical processes can befound in Section 3 of Guo11. theevolutionoftheradialdistributionofcoldgasandstars
In the L-Galaxies model, galaxies are classified into can betraced.
threetypes.Type0 galaxies arethose located at thecentre (ii) A prescription for the conversion of atomic gas into
of the main haloes found with a Friends-of-Friends (FOF) moleculargasisincluded,andthestarformationisassumed
algorithm in the simulation outputs. A Type 0 galaxy is a to be directly proportional to the local surface density of
“central”galaxywithitsownhotgaseous halo,andthehot molecular gas Σ Σ .
SFR ∝ H2
gasdistributesisothermallyinthedarkmatterhalo.Thehot Theseimprovementsenableustocalculatewhetherthe
gas can cool onto the central galaxy disk through a “cool- RP at a certain radius in the galaxy disk is sufficient to
ing flow” or a “cold flow”, and the cold gas is the source of remove the gas, and thus to trace the depletion of atomic
star formation. An instantaneous recycling approximation and molecular gas in cluster galaxies.
is adopted for mass return from evolved stars and for the Inthispaper,Wemaketwofurthermainchangestothe
injection of metals into the ISM; this implies that the mas- Fu13model:
sivestarsexplodeasSNat thetimewhen theyform. 1 The (1) weintroducean analyticmethod totracethemass evo-
SN feedback energy reheats part of disk cold gas into the lutionofunresolvedsubhaloesoncetheyarenotresolvedby
hot gaseous halo of the central galaxy. If the SN energy is the simulation. This enables us to model the evolution of
largeenough,partofthehotgaseoushalocanbeejectedout low-mass satellite galaxies in a resolution-independent way.
of the dark matter potential and become ejected gas. With (2) we include new prescriptions for ram pressure stripping
the growth of the dark matter halo, the ejected gas will be of thecold gas;
reincorporated back to thecentral halo. Inthefollowing sections, wedescribeourmodifications
BothType1andType2galaxiesareregardedassatel- in detail.
litegalaxiesinthemodel.AType1galaxyislocatedatthe
centerof asubhalo, which is an overdensitywithin theFoF
2.2 Tracing galaxies in unresolved subhaloes
halo(Springeletal.2001).Thehaloes/subhaloes containat
least 20 bound particles for both MS and MS-II (Springel As discussed in Section 1, the properties of low-mass satel-
et al. 2005, Boylan-Kolchin et al. 2009). Boylan-Kolchin et lite galaxies predicted by L-galaxies will be resolution de-
al.(2009)havepointedthatabovethisparticlenumber(20) pendent, because the treatment of the physics depends on
theabundanceofsubhaloesbetween twosimulations differs whetherornotthesubhaloofthesatelliteisresolvedbythe
only about 30%, and with more than 50-100 particles the simulation (i.e. whether the galaxy is Type 1 or Type 2).
results agree much better. Type 1 galaxies have their own The detailed issues are thefollowing:
hotgaseoushaloes,sogascancoolontothesegalaxies.Cold
(i) Forsatellitegalaxies,M ,V ,R andV arefixed
gasreheatedbytheSNexplosionsinaType1willbeadded vir vir vir max
atthemomenttheyfirstfallintoalargerhalo,whereasM
to the hot gaseous halo of its own subhalo or the halo of sub
is measured in each simulation output until the subhaloes
the central Type 0 galaxy, depending on the distance be-
aredisrupted.M foraType2willbefixedatthelasttime
tween the Type 1 and the central object. A Type 2 galaxy sub
when it was a Type 1, which will be strongly dependenton
is an “orphan galaxy”, which no longer has an associated
theresolution of thesimulation.
dark matter subhalo. A Type 2 galaxy does not have a hot
(ii) Thehotgasinasubhaloisassumedtobedistributed
gaseous halo and thus has no gas cooling and infall. The
the same way as the dark matter. When a subhalo is dis-
supernova reheated cold gas from Type 2 is added to its
rupted and the galaxy becomes a Type 2, it is assumed to
centralgalaxy,i.e., theassociated Type0orType1object.
loseitshotgasandejectedgasreservoirsimmediately.Thus,
Both Type 1 and Type 2 galaxies are initially born as
gas cooling, infall and reincorporation no longer take place
Type 0 objects. They become Type 1 when they fall into
if thegalaxy is a Type2.
a group or cluster and may later become type 2 after their
(iii) Only Type 2 galaxies can be disrupted by the tidal
force of central galaxies.
1 Yates et al. 2014 considered morerealistic model for chemical These three assumptions will cause inconsistencies between
enrichmentofdifferentelements. modelsbasedondarkmattersimulationswithdifferentres-
(cid:13)c 0000RAS,MNRAS000,000–000
4 Y. Luo
olutions,becausemanyType1galaxies inahighresolution continuously through stripping processes in the same way
simulation will beType2galaxies in low resolution simula- as Type 1s. The hot gas is assumed to have a isothermal
tions (e.g. Fig.A1 in Guo11). distribution:
In the following section, we incorporate the model of M
Jiang & van den Bosch (2014, hereafter JB14) to trace the ρhot(r)= 4πRhotr2 (4)
vir
evolution of a subhalo after it is no longer resolved by the
When Type 2s fall within the virial radius of the cen-
N-body simulation. In this way we can estimate the key
tral galaxy, we calculate the stripping radius as R =
propertiesofunresolvedsubhaloes,suchasM ,V ,and strip
sub max min(R ,R ).WeuseEq.25and26inGuoetal.(2011)
treat Type 2s in the same way as Type 1s. We will then tidal r.p.
to calculate R and R :
show that this procedure helps to alleviate the resolution- tidal r.p.
dependentproperties of satellite galaxies in the models. R =( Msub )R (5)
tidal M vir,infall
sub,infall
2.2.1 An analytic model for subhalo evolution where Msub is the subhalo mass given by Eq.1, Msub,infall
andR arethedarkmattermass andvirialradiusof
sub,infall
According to JB14, the average mass loss rate of a subhalo
thesubhalo at thelast time when it was Type0.
dependsonlyontheinstantaneousmassratioofthesubhalo
mass m and parent halo M,
ρ (R )V2 =ρ (R)V2 (6)
m m ζ sat r.p sat par orbit
m˙ = ϕ (1)
− τdyn M whereρsat(Rr.p)isthehotgasdensityofthesatelliteatRr.p;
(cid:16) (cid:17) V is the virial velocity of the subhalo; ρ (R) is the hot
whereϕandζ arefreeparameters,τ isthehalo’sdynam- sat par
dyn gasdensityofthemainhaloatthedistanceR;andV is
ical time orbit
theorbital velocity of thesatellite (we simply usethevirial
3π velocity of themain halo).
τ (z)= , (2)
dyn s16Gρcrit(z) All the hot gas beyond Rstrip is removed and added to
the hot gas component of the parent central galaxy. Then
whereρcrit(z)isthecriticaldensityatredshift z.Sowecan we set the hot gas radius to be rhot = Rstrip. The hot gas
get the subhalo mass m(t+∆t) in a static parent halo at in Type 2s will cool and fall onto the galaxy disk with an
t+∆t as: exponentialsurface density distribution
m(t+∆t)=m(t)[1+ζ(Mm)ζ(∆τt)]−1/ζ (3) Σgas(r)=Σ(g0a)sexp(−r/rinfall) (7)
where the infall scale length r = (λ/√2)r . We keep
where τ =τ(z)/ϕ is the characteristic mass loss time scale infall vir
theoriginal spin parameter λ and r when thegalaxy was
at redshift z. We adjust the parameters ζ and ϕ so that vir
last a Type 0 unchanged.
thedistributionofthepredicteddistributionofM forall
sub
Inthemodels ofGuo11 andFu13,SNfeedbackreheats
Type1galaxiesatz =0matchesthedistributionofM for
sub
the cold gas in the disk, and if there is remaining energy,
Type1galaxiesmeasureddirectlyfromthez=0simulation
hot gas will be ejected out of the halo. In the Guo11 code,
output. The best-fit values we find are ζ = 0.07, ϕ = 9.5.
thesupernovareheating efficiency ǫ is written as
NotethatourϕismuchlargerthanthevalueofJB14(their disk
best value is 1.34) and the difference is mainly due to the ǫ =ǫ [0.5+( Vmax )−β1] (8)
definition of ρ. If we use the same definition as JB14, our disk × V
reheat
best fittedϕ is about 40% lower than that of JB14.
The supernovaejection efficiency is written as
JB14 also provides a formula to estimate the V of
max
V
smuobrheaslloowdluyritnhganthteheevmoalusstieovno.luTthioeny,fibnecdauthseatVmVmaxaxisemvaoilnvelys ǫhalo =η×[0.5+(Vemjeacxt)−β2], (9)
determind by the inner region of the subhalo which is not The efficiency of the SN feedback in Type 2s is assumed to
strongly afftected by the tidal force of the host halo. For scalewiththemaximumcircularvelocityV ofitscentral
max
simplicity and consistency with Guo11, we keep the Vmax galaxy and reheated gas from Type 2s is added to the hot
for the Type 2 satellite also fixed at the value when it was gascomponentofthecentralgalaxy.WenotethatinGuo11,
lastaType0object.Thismeansthatthemainchangewith Type2shavenohotgascomponent.Here,weassumeSNin
respecttotheGuo11modeloccursafterthesubhaloeshave Type 2s will reheat cold gas to its own hot gas component
been fully tidally disrupted. So we only apply the above and allow for an ejected gas reservoir in Type 2s in the
model for subhalo mass evolution after it is not resolved in same way as for Type 0s and 1s. Due to the fact that hot
thesimulation. gas is stripped from satellites, only a fraction R /R of
hot vir
reheatedgasremainsinthesubhaloandtherestisreturned
to the main halo. We replace V of the Type 2s’ central
2.2.2 A consistent treatment of the physics of satellite max
galaxy with the V for the Type 2 galaxy is taken as the
galaxies max
valueofV whenitwaslastaType0.Notethatinournew
max
IntheGuo11andFu13models,Type2slosealltheirhotgas model, the SN heating efficiency in a Type 2 is determined
after their subhaloes are disrupted. Using the methodology by its own V , which has a lower value than the V
max max
outlined in Sec.2.2.1, we can now estimate the evolution of of the central galaxy that is used to scale the SN reheating
subhaloestoarbitrarilylowmasses,andthusType2galax- efficiencyinasatellitegalaxyintheGuo11andFu13models.
ies will retain their own hot gas haloes and lose hot gas Thischoiceisalsomotivedbysomerecentwork(e.g.,Lagos
(cid:13)c 0000RAS,MNRAS000,000–000
Satellite galaxy quenching in SAMs 5
et al. 2013; Kang 2014) which shows that SN feedback is RP stripping for a satellite galaxy at the distance R to the
morelikelydeterminedbythelocalgalaxypotential.Usually centreof its parent halo is written as:
theV ofsatelliteislowerthanV ofthecentral,sothe
SN hmeaatxing efficiency is higher inmsaaxtellite galaxies in our 2πG[Σ∗(r)+Σgas(r)]Σgas(r)6ρICM(R)v2 (14)
model and we will later show in Section 3.2 that it leads to From the radial distribution of cold gas Σ (r) and stellar
gas
a slightly better agreement with the observed galaxy two- surface density Σ∗(r), we evaluate the stripping radius r in
pointcorrelationfunctiononsmallscalesinlowstellarmass Eq. (14) and assume that cold gas exterior to this radius r
bins. will bestripped .
ThehotgasinthesubhaloesofType2swillcoolandfall The above description is very simplistic. It is not clear
onto the gas disks of Type 2s later on. With the growth of ifallthecoldgaswillbestrippedimmediatelywhentheRP
thedarkmatterhalo, theejected gas in thesubhalowill be forceislargerthanthegravityofthesatelliteitself.Totake
reincorporated into the hot gas again. The reincorporation account of this uncertainty, we define a stripping fraction
efficiency is f asthefraction ofthecoldgasstrippedbyrampressure
rps
in the region of a satellite galaxy where P (R)>P (r).
V M r.p ISM
M˙ejec =−γ(22vi0r)(t ejec), (10) In thesimplest case, frps is 100%, which means all thecold
dyn,h gas in the region where P (R)> P (r) will be stripped
r.p ISM
where γ is free parameter, t = R /V is the halo byRP.Thissimpleassumptioncausesasuddencutoffofthe
dyn,h vir vir
dynamical time. coldgasradialprofile,i.ethecoldgasradialprofilebecomes
Finally, we note that in the Guo11 model, only Type a step function at the radius r. To ensure a continuously
2 galaxies are disrupted by tidal forces (see Sec. 3.6.2 in decreasing cold gas radial profile, we modify the stripping
Guo11). In our new model, we follow the evolution of sub- law as follows : if Pr.p > PISM, the stripping ratio frps is
haloesanalytically,sowetreatthetidaldisruptionforType related to thedifference between P and P as:
r.p ISM
1s and 2s in the same way. A satellite galaxy (Type 1 or
0, P <P
Type 2) will be disrupted by the tidal force of the main r.p ISM
f = (15)
halo,when1)ithasmorebaryonicmatterthandarkmatter rps (Pr.pP−r.PpISM, Pr.p >PISM
(M >M );2)itsaveragebaryonicmassdensityislower
bar sub
The guarantees that only part of the cold gas M =
than main halo density at the peri-centreof its orbit. stripped
f M (r) in the region where P (R) > P (r) will
rps coldgas r.p ISM
bestripped.
2.3 The physical prescriptions for ram pressure
stripping of cold gas
3 PERFORMANCE OF THE NEW MODEL
In the L-Galaxies model, ram pressure strips only the hot
gas component in the satellite galaxies, while the cold gas In this section, we compare our new model with the Fu13
component in the ISM is not affected. Following the pre- modelandexploreifweobtainmoreconvergentand/orim-
scriptions of Gunn & Gott (1972), we consider a satellite proved results for the following galaxy properties: the stel-
galaxy moving through its host halo. The RP force can be lar/gas mass function and the two-point correlation func-
written as, tion. These two quantities are often used as basic tests of
the overall model. We then study the fraction of quenched
Pr.p(R)=ρICM(R)v2 (11) galaxiesasafunctionofhalomassandcomparewithobser-
vational data. In the following subsections, we abbreviate
wherevistheorbitalvelocityofthesatellite,whichwetake
our model as “new” if the option for cold gas stripping is
to be the virial velocity of its parent halo. ρ is the den-
ICM switched off, and it islabeled as “new +rps”if thecold gas
sity of the hot gas of the parent halo as Eq.(4). When the
stripping is turnedon.
rampressureexceedstheinterstellarpressureP ,coldgas
ISM
will be stripped. We adopt the Eq.(12) given by Tecce et
al.(2010): 3.1 The stellar and cold gas mass functions
PISM=2πGΣdisc(r)Σgas(r) (12) In Fig. 1, we plot the model mass functions of stars, HI
and H at z = 0 compared with the observations. The
2
where r is the radius to the centre of satellite. Σ is the
disc “new” modelresults(with RPstrippingof cold gas off) are
surfacedensityofthegalacticdisc,whichequalstothesum
shown as black and blue lines for the MS and MS-II simu-
of the cold gas and stellar surface densities:
lations, respectively. All the free parameters of the models
are fixed as in Fu13 (see the Table 1. in that paper), with
Σdisc =Σ∗(r)+Σgas(r) (13)
the exception of the hot gas accretion efficiency onto black
WecalculatePISMineachradialconcentricringinsatel- holes κAGN which is changed from 1.5 10−5M⊙yr−1 to
lite galaxies based on the division of the disk into multiple 1.5 10−6M⊙yr−1. Tuning this paramet×er is necessary in
×
rings introduced in the Fu13 model. In our model, the cold ordertobetterfitthestellarmassfunctionatthehighmass
gas in a satellite galaxy can be stripped by RP only when end.Ascan beseen, with thisminorchange, ournewmod-
the satellite falls within the virial radius R of the cen- els are in very good agreement with the observational data
vir
tralgalaxyandP (R)>P (r).Thestrippedcoldgasof at z=0 for both simulations.
r.p ISM
the satellite is added to the hot gas component of its cen- Fig. 1 also shows that there is difference in the pre-
tral galaxy. According to Eq. (11) & (12), the criterion for dicted mass functionsat thelow-mass end between thetwo
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6 Y. Luo
Figure 1. The stellar, HI and H2 mass functions at z = 0 from the new model compared with observations. The solid curves show
results for the new model without the process of ram pressure stripping of cold gas. Blue and black curves show results for generated
from the MS and MS-II simulations, respectively. For the observations, stellar mass functions are from Li & White (2009) and Baldry
et al. (2008); the HI mass functions are from Zwaan et al. (2005) and Martinet al. (2010)a the; H2 mass function is from Keres et al.
(2003).
Figure 2.Convergence check: the difference of the predicted stellar, coldgas, HI and H2 mass functions between MSand MS-II. The
black/redlinesareforournewmodelwithout/withrampressurestrippingofcoldgas.Thegreen/bluelinesarefortheFu13andGuo13
models.
simulations. In Fig. 2, we check the divergence between produces more convergent results for the cold gas mass. In
two simulations in detail, where the difference defined as the Guo11 model, there is a threshold in cold gas mass for
∆MF=|log10MFMS−log10MFMS−II|.Theleftpanelshows star formation which implies that all satellite galaxies con-
that for low-mass galaxies (log M < 8.5) there is obvious tain cold gas even when they stop forming stars. In a low-
10
differenceinthemassfunction,andallthemodelshavesimi- resolution simulation, the satellite will soon become type 2
larpredictions(alsoseeFig.7inGuo11).Suchadifferenceis andgascooling will stopasthehothalogas isimmedaitely
expectedanditisbasicallyduetolow-masscentralgalaxies stripped.Sotherewillbemorecoldgasinthelow-resolution
not being resolved in the low-resolution simulation. In fact run for the Guo11 model. Compared to the Fu13 model on
thesimulation resolution canhavemorecomplicated effects which our model is based, it is seen that our new model
on the galaxy properties. For example, if the merger trees does not producea more convergent results on thecold gas
of some massive halos in the MS simulation are not well- and HI gas mass, butwe obtain slightly betterconvergence
resolved at higher redshifts, this will lead to divergence in on the H mass (right panel), and the convergence is more
2
the properties of both the central and the satellite galaxies obviousin our new+rpsmodel.
inthosehalos.Overallwefindthatthestellarmassfunction
is more convergent for galaxies with log10M∗ > 8.5 and in In Fig. 3 we further check the convergence in different
our later analysis we only focus on these galaxies. halomassbins.Itisclearlyseenthattheconvergenceinthe
new model is better at 107M⊙ <MH2 <1010M⊙ than the
TherightpanelsofFig.2showtheconvergenceteston Fu13 model. The new+rps will mostly affects the low-mass
the cold gas mass functions, also divided into HI and H2 end (< 107M⊙) as rps is efficient to strip the cold gas if
components. The sample is selected with log10M∗ >9. It is there is little of it. In the Fu13+rps model, the H2 mass
found that, compared to the Guo11 model, our new model function is converged only at the low-mass end, but not at
(cid:13)c 0000RAS,MNRAS000,000–000
Satellite galaxy quenching in SAMs 7
Figure 3. Convergence check: the difference of the predicted H2 mass functions of the satellites (logM∗ > 9.5) in different halo mass
binsbetweenMSandMS-II.Theblack/redlinesareforournewmodelwithout/withrampressurestrippingofcoldgas.Thegreen/blue
linesarefortheFu13andFu13+rpsmodels.Moreobviousimprovementinconvergence isseeninmassivehaloes.
the high-mass end. This plot clearly shows that without a ofthesubhalo,andnotthecentralhalo,asinpreviousmod-
sub-resolutiontreatmentofthestarformationphysics(such els.Sothefeedbackefficiencyinsatellites inournewmodel
asFu13model),itisdifficulttoachieveconvergenceinhigh- is larger, and this decreases the mass growth of satellites
mass H2 galaxies and rps is effective in convergence in low afterinfallwith respecttotheresultsofFu13.Asshownby
gas-mass galaxies. As in the Fu13 and our model, the star Kang(2014),strongfeedbackinsatellites flattensthesatel-
formation is determined by local H gas density, it is ex- lite galaxy mass function in massive haloes, thus reducing
2
pectedthatourmodelwillproducemoreconvergentresults theclustering amplitude on small scales.
on the fraction of passive/star forming galaxies, and this is The panel inserts show that the disparity in clustering
shown in detail in Sec.3.3.2. amplitudebetweentheMSandMS-IIsimulationsisslightly
lower in the new model compared to Fu13 model. The new
model shows a little “better” convergence at small scales,
3.2 The projected two-point correlation functions but in some mass bins new model get “worse” at Mpc
∼
(2PCF) scales.However,thesedifferenceon2PCFsbetweenthetwo
simulations isalwayssmall (often lowerthan0.2 dex),indi-
The two-point correlation function (2PCF) is another im-
cating that resolution is not of primary importance in the
portant galaxy statistic, because it describes how galaxies
prediction of galaxy clustering.
aredistributedinandbetweendarkmatterhaloes.Onsmall
In Fig. 5 we show 2PCFs for galaxies classified into
scales, theclustering dependsstrongly on howsatellites are
red and blue according to their g r colors as described
distributed in massive haloes, so it is interesting to ask if −
in Guo11. The red/blue lines are model predictions for
the model results are dependent on simulation resolution.
red/blue galaxies, and the red/blue points are data points
Another reason to check the 2PCF is that, as shown by
from Li et al. (2006). As shown in the previous figure, the
Kang (2014), strong SN feedback in satellites can decrease
difference between theMS and MS-II simulations are small
the 2PCF on small scales, and this results in better agree-
for 2PCFs, so we only show results from the MS. The new
ment with theobservations. Ournew model also includes a
modelfitsbetterthecolor-dependentclusteringinlow-mass
feedbackmodelsimilartoKang(2014)forsatellitegalaxies,
binsi, particularly for blue galaxies in the low mass bins.
in which the feedback efficiency depends on the local grav-
In intermediate mass bins (logM∗ =[9.77,10.77]) the clus-
itational potential of the satellite. It is thus worthwhile to
tering of red galaxies is too strong, and too low for blue
check if our new model can give better agreement with the
galaxiesforbothmodels.ThisisbecauseboththeFu13and
observed 2PCFs.
our models over-predict the fraction of red satellite galax-
In Fig. 4, we plot the projected galaxy 2PCFs for the
iesandunder-predictthefraction ofredcentrals, asseenin
Fu13 model in green and for our new model in black. Re-
Fig.8.Bothmodelsalsofitwellatthehigheststellarmasses.
sultsareplottedforboththeMSandMS-IIsimulationsand
the difference between the two is shown as in inset at the
bottom of each panel. The different panels show results for
galaxiesindifferentbinsofstellarmass.Thedatapointsare
3.3 Satellite quenching and cold gas depletion
fromLietal.(2006)andarecomputedusingthelarge-scale
structuresamplefrom theSDSSDR7.Herewedonotshow Inthissubsection,wewillinvestigatetheeffectsofRPstrip-
theresultsfromthe“new+rps”–theresultsareverysimilar pingofcoldgasingalaxies.Wewillstudyhowram-pressure
to themodel without cold gas stripping. strippingchangesthequenchedfractionofgalaxiesasafunc-
Fig.4showsthatournewmodelproducesbetteragree- tion of galaxy mass, halo mass and cluster-centric radius,
mentwiththeobserved2PCFscomparedtotheFu13model, andwewillalsoshowcomparisonswithrecentobservations.
especially for low-mass galaxies. As discussed in the previ- Asintheprevioussubsections,wewillalsoaddresstheissue
ous section, our new model employees a feedback prescrip- ofconvergencebyshowingresultsfromboththeMSandthe
tion where the SN ejection efficiency is dependent on V MS-II simulations.
max
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8 Y. Luo
Figure 4. The projected 2PCFs for galaxies in different stellar mass bins (top graph in each panel) and absolute values of difference
of the amplitude of the 2pcfs between MS and MS-II, ∆ = |log10wMS−log10wMS−II| (lower graph in each panel). The results from
MS are shown with solidlines, and those from MS-II are shown using dashed lines. Black curves show results for our new models and
greencurvesshowresultsfortheFu13models.TheorangedotsaredatafromSDSS.Hereweonlyshowtheresultsfromournewmodel
withoutRPstrippingofcoldgas.
3.3.1 Where is ram-pressure stripping most effective? both MS and MS-II haloes and are seen to agree to within
10%. We find that about 50% galaxies in massive haloes
To understand which galaxies have been affected most by ∼
have f > 0.1. We then define a galaxy with f > 0.1
ram-pressure stripping, we define the cumulative stripped sp sp
as having had significant cold gas stripping, and we plot
cold gas fraction as
thefractionofthesegalaxiesN /N asfunctionsof
stripping total
M stellarmassandhalomassinFig.7.Ascanbeseen,thefrac-
f = asp (16)
sp M +M tion of galaxies with significant stripping increases steeply
asp coldgas
with halo mass, but decreases with stellar mass.
where M is the cumulative mass of stripped cold gas
asp
throughouttheformationhistoryofagalaxy(evaluatedby If we increase the significant stripping threshold from
summingupthestrippedcoldgasmassinthemainprogen- 0.1 to 0.5, we find that the stripped fraction in haloes of
itor),andMcoldgas isitscurrentcoldgasmass. Wefocuson 1013M⊙ decreases from 0.6 to 0.2. Bru¨ggen & De Lucia
satellitegalaxiesinrichgroupsandclusters,andselectthose (2008) found that about one quarter of galaxies in massive
with M∗ >109M⊙ in haloes with mass Mhalo >1013M⊙. clusters(Mhalo >1014M⊙)aresubjectedtostrongrampres-
Fig.6showsthefractionofgalaxieswithf largerthan surethatcauseslossofallgasandmorethan64%ofgalaxies
sp
a certain value x (0.1 < x < 1.0). Results are shown for thatresidein aclustertoday havelost substantial gas. Our
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Satellite galaxy quenching in SAMs 9
Figure 5.AsinFig.4,butseparatelyforredandbluegalaxies.
new model agrees well with these results. We also conclude etal.(2015)showedthatthelatestversionofL-Galaxieswas
thatmostlow-masssatellitegalaxiesinmassiveclusterswill able to reproduce the overall red galaxy fraction, but they
lose significant fraction of their cold gas by RP stripping, didnotanalyzecentral/satellitegalaxiesseparately,nordid
and some will lose all their interstellar cold gas. they examine the dependence of red fraction on halo mass
andradial distancefrom thecentresof groupsand clusters.
Wetzeletal.(2012)re-analyzedthefractionofquenched
3.3.2 Effect of ram-pressure stripping on the quenched
galaxiesingroupsandclustersusingSDSSDR7data.They
fraction of satellite galaxies
classified galaxies as quenched based on their specific star
The fraction of quenched galaxies is known to depend on formationratesratherthantheircolors.Thispermitsamore
environment.It hasalso beenshown (e.g., Weinmannet al. direct comparison with models, because the galaxy color is
2006) thatearly versionsof SAMs(e.g., Croton etal. 2006) a complicated function of star formation history, metallic-
over-predicted the fraction of red satellites in all environ- ityanddustextinction.TheanalysisofWetzeletal.(2012)
ments. It was suggested that instantaneous strippingof the made use of global star formation rates with fibre aperture
hot halo gas of satellites was to blame. By introducing a correction asgiven in Salim et al. (2007). These corrections
non-instantaneousstrippingmodel, laterSAMS(e.g., Kang may be prone to systematic effects, because there can be
&vandenBosch2008;Fontetal.2008)wereabletoproduce a large spread in broad-band colors at a given specific star
morebluegalaxies,buttheagreementwasstillnotsatisfac- formation rate and because the corrections are calibrated
tory. In the Guo11 and Fu13 models, the stripping of hot using average relations between these two quantities. The
halogasismodeledasagradualprocess.RecentlyHenriques corrections also account for two thirds of the total star for-
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10 Y. Luo
Figure 8. The quenched fraction of galaxies as a function of stellar mass: the left three columns are for satellite galaxies in different
halomassbins,andtherightcolumnisforcentralgalaxiesinallhaloeswithmassesgreaterthan1011.4M⊙.Thetoppanelsshowresults
whenquenchedgalaxiesaredefinedusingsSFRsmeasuredwithintheSDSSfiber,andthelowerpanelsshowresultswhencorrectedtotal
starformationratesareusedinstead.Ineachpanel,thetrianglesaretheSDSSdataandthecoloredlinesaredifferentmodelpredictions
(solidforMS,dashedforMS-IIresults).
Figure6.Thefractionofgalaxieswithstrippingfractionfsp>x Figure7.Thenumberfractionofgalaxieswithfsp>0.1(black
as a function of x. Solid and dashed lines are based on MS and curves) and fsp > 0.5 (red curves) as a function of stellar mass
MS-IIhaloes respectively. Heregalaxies areselected with stellar andhalomass.SolidanddashedlinesarebasedonMSandMS-II
massM∗>109M⊙ inhaloeswithMhalo>1013M⊙. haloesrespectively.
mation rateonaverage.Incontrast,thestarformation rate dependence of the quenched fraction in different halo mass
measured inside the fiber aperture is derived directly from bins.
the dust-corrected Hα luminosity and should be more re- Following the same procedure described in Wetzel et
liable. In the following comparison, we present the results al. (2012), we extract galaxies from the MPA-JHU SDSS
on quenched fractions using both total and fiber aperture DR7 catalogue with M∗ > 109.5M⊙ at z < 0.04 and M∗ >
specific star formation rates. We also examine the radial 109.5M⊙ at z =0.04 0.06 that are included in the group
∼
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