Two-phase galaxy evolution: the cosmic star-formation histories of spheroids and discs

0.47 MB·English

by  Simon P. Driver

Mon.Not.R.Astron.Soc.000,1–??(2012) Printed11December 2013 (MNLATEXstylefilev2.2)
Two-phase galaxy evolution: the cosmic star-formation
histories of spheroids and discs
3 S.P. Driver1,2⋆, A.S.G. Robotham1,2, J. Bland-Hawthorn3 M. Brown4, A. Hopkins5,
1
0 J. Liske6, S. Phillipps7, S. Wilkins8
2 1 International Centre forRadio Astronomy Research (ICRAR), Universityof WesternAustralia, Crawley, WA 6009, Australia
2 School of Physics& Astronomy, Universityof St Andrews, North Haugh, St Andrews, KY16 9SS, UK; SUPA
n 3Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW 2006, Australia
a 4School of Physics, Monash University,Clayton, Victoria3800, Australia
J 5 Australian Astronomical Observatory, PO Box 296, Epping, NSW1710, Australia
6 6 European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
7 HH Wills PhysicsLaboratory, University of Bristol, TyndallAvenue, Bristol, BS8 1TL, UK

8 School of Physicsand Astronomy, Oxford University, Keeble Road, Oxford, UK
]
O
C
h. 11December 2013
p
-
o ABSTRACT
r From two very simple axioms: (1) that AGN activity traces spheroid formation, and
t
s (2)thatthecosmicstar-formationhistoryisdominatedbyspheroidformationathigh
a
redshift,wederivesimpleexpressionsforthestar-formationhistoriesofspheroidsand
[
discs, and their implied metal enrichment histories.

1 Adopting a Baldry-Glazebrook initial mass function we use these relations and
v apply pegase.2 to predict the z =0 cosmic spectralenergy distributions (CSEDs) of
9 spheroids and discs. The model predictions compare favourablyto the dust-corrected
7 CSED recently reported by the Galaxy And Mass Assembly (GAMA) team from the
9 FUV through to the K band. The model also provides a reasonable fit to the total
0
stellar mass contained within spheroid and disc structures as recently reported by
.
1 the Millennium Galaxy Catalogue team. Three interesting inferences can be made
0 following our axioms: (1) there is a transition redshift at z ≈ 1.7 at which point
3 the Universe switches from what we refer to as “hot mode evolution” (i.e., spheroid
1
formation/growthviamergersand/orcollapse)towhatweterm“coldmodeevolution”
:
v (i.e., disc formation/growthvia gas infall and minor mergers);(2) there is little or no
i need for any pre-enrichment prior to the main phase of star-formation; (3) in the
X
present Universe mass-loss is fairly evenly balanced with star-formation holding the
r integrated stellar mass density close to a constant value.
a
The model provides a simple prediction of the energy output from
spheroid and disc projenitors, the build-up of spheroid and disc mass, and
the mean metallicity enrichment of the Universe.
Keywords: galaxies:formation—galaxies:evolution—galaxies:bulges—galaxies:
ellipticals — galaxies: star formation — galaxies: spiral
1 INTRODUCTION disc. Exceptions exist, most notably the dwarf populations
which, while dominant in terms of number density, actu-
The vast majority of galaxies can be adequately described
allycontributeonlyamodestamounttothebaryonbudget
as consisting of a compact smooth spheroidal component
at the present time (< 16 per cent; Driver 1999; Geller et
containing a predominantly pressure-supported old [α/Fe]-
al.2012).Thisdichotomyofgalaxiesintospheroidsanddiscs
enhanced stellar population, and/or an extended flattened
hasbeenknownforoverahundredyearsstretchingbackto
star-forming disc component containing intermediate and
even before the confirmation that galaxies are external sys-
youngstarswithawiderangeinmetallicities,havingsmooth
tems (e.g., Hubble 1926, 1936; Zwicky 1957 and references
rotation and embedded in an extensive gaseous cold gas
therein). To some extent this dichotomy has been recently
“rediscovered”, through the statistical studies of large pop-
ulationsasagalaxybimodality(Stratevaetal.2001;Baldry
⋆ SUPA,ScottishUniversitiesPhysicsAlliance
2 Driver et al.
etal.2004).Driveretal.(2006)arguedthatthisbimodality crucial processes, whose motivation is as much driven by
is better interpreted in terms of the earlier bulge-disc di- the requirement to produce realistic looking images as by
chotomyandadvocatedroutinestructuraldecompositionas fundamental physics, and which are both more effective if
vital (e.g., Allen et al. 2006; Simard et al. 2011; Lackner & the merger rate is either low or at least confined to earlier
Gunn 2012) to directly trace the independent evolutionary epochs.
histories of the spheroidal and disc components. As argued in the opening paragraph, we advocate a
Numerical models of galaxy formation struggle to pro- more heuristic approach where we put aside the issue of
ducerealisticgalaxysystemswithatendencytoformoverly dark-matter assembly and start by asking whether the di-
cuspy cores and difficulty in maintaining extended disc chotomyofgalaxystructureisbestexplainedbytwodistinct
structures with a high axis ratio (White & Navarro 1993; formation mechanisms. Following theearlier discussion and
Navarro & Steinmetz 2000; Abadi et al. 2003; House et lessons learnt from the simulations, the obvious two mech-
al. 2011). Both problems are likely to be connected to the anisms can loosely be termed as a hot and cold mode. In
differentfundamentalpropertiesofthedarkmatterandthe the hot mode spheroids are formed early and rapidly via
baryons, and in particular their ability to experience pres- dynamicallyhot (turbulent)processes (collapse, fragmenta-
sureandtheirabilitytodissipateenergy.Inthecoreregions tion,andmerging).Inthecoldmodediscsareformed more
the gravitational coupling of the baryons with the dark- slowly, from an extended quiescent phase of cold gas infall
matter may allow it to exhibit a pseudo-pressure, whereas regulated by internal feedback (i.e., supernova). This basic
in the outer-regions the ability of the baryons to dissi- concept is of course not new (e.g., Larson 1976; Tinsley &
pate energy on a timescale which is faster than the free- Larson 1978) but has laid dormant for sometime overshad-
falltimescalemayallowfortheformation ofathinrotating owedbythedominanceofmerger-drivenevolution.However
baryonic disk. This picture while simple to articulate has the revival is also being championed via a series of semi-
proven extremely hard to simulate, with the need to parti- analyticstudiesbyCooketal.(2009;2010a;2010b,seealso
tionandredistributetheangularmomentuminaquitespe- Dekel et al. 2009) inspired by behaviour seen in numerical
cificmannertoresultingalaxieswithrealisticappearances. simulationsinwhichaninitialrapidhotmergerphaseistyp-
Inparticular mergereventsare extremelydisruptivetothis ically followed by a more quiescent phase of accretion (see
process, imparting both energy and angular momentum to also L’Huillier, Combes & Semelin 2012).
the baryonic disc, which is easily disrupted or ’plumped Thetwo-phasemodel is bothobvious(given thebulge-
up’ (see Barnes & Hernquist 1992 for extensive discussion discnatureofgalaxies) andcontroversial,asit marginalises
and early references on this topic, also Hopkins et al. 2009 themergerraterequiredfordarkmatterassemblytoearlier
for updated simulations on thesurvivability of discs during epochs than simulations typically suggest. This low merger
merger events). In general the greater the merger-rate the rateisarguablycorroboratedbythelocalstudiesofdynami-
more bulge-dominated thefinal galaxy population appears. callyclosepairs(inparticularseePattonetal.2002;DePro-
More contemporary hydrodynamical simulations (e.g., prisetal.2005,2007,2010)—althoughacorrectderivation
Governatoetal.2010;Agertz,Romain&Moore2011;Scan- of the merger rates requires a robust understanding of the
napieco et al. 2011; Domenech-Moral et al. 2012) are now merger timescales, which are currently poorly constrained.
starting to show significant success at producing realistic Perhaps more compelling, however, is the result that only
“looking”bulge-discsystemsbyincorporatingagreaterlevel 40%ofthepresentdaystellarmassresidesinspheroidalsys-
of cold gas infall than previously assumed, as argued ear- tems(Driveretal.2008;Gadotti2009;Tasca&White2011).
lier by Keres et al. (2005) and Dekel et al. (2009). These A key inference is then: If discs are destroyed/thickened
focusedhydrodynamicalstudies,however,areinevitablyex- during mergers, yet the majority of stellar mass resides in
tracted from numerical simulations with particularly qui- discs,thedominantformationmechanismcannotbemerger-
escent merger histories, suggesting such systems should be driven, but presumably the more quiescent process of cold
theexceptionratherthanthenorm.Hence,whilenumerical gasaccretion.Thisstatementbecomesmoreprofoundwhen
simulations of the development of the dark matter haloes one realises that the stellar mass in discs today only mea-
findacontinualprocessofhalomerging,itappearsthatthe suresthatunaffectedbymergers,andthatsomeofthestars
baryonsandwhatweidentifyasgalaxies (baryonicconden- currently in spheroidal systems may have originally formed
sates), might not develop in thesame way. Martig & Bour- within discs via cold accretion prior to a merger event. In
naud (2009) argued that feedback from low and intermedi- somebulgeformation scenarios, star-formation viamerging
atemassstarscancontributesignificantlytotheredistribu- isdispensedwithaltogetherandreplacedbythemigrationof
tion of mass from the bulge to the disc through extensive massivestar-formation clumpsformed withindeeplyturbu-
(or even excessive) mass-loss. This baryonic outflow could lent discs (e.g., Elmegreen, Bournaud & Elmegreen 2008).
help alleviate the problem of excessive bulge-formation by This potentially relegates the stellar-mass build-up driven
allowing some fraction of the collapsed stellar mass (up to by mergers to be in the 0—40 per cent range by mass.
∼50 per cent), to return to the halo and contribute to the Clearly mergersdooccur atall redshifts andsimilarly discs
later growth of a disc — thereby coupling bulge and disc may form, be disrupted, and reform at any redshift. Re-
growth. However this mechanism also relies on a fairly qui- cently L’Huillier, Combes & Semelin (2012) reported that
escentmergerhistoryduringlatertimes,anddoesnoteasily 77 per cent of a galaxy’s mass is formed via gas accretion
explain the broad diversity of bulge-disc ratios seen. As an and 23 percent via direct merging from simulations. Other
aside,simulationshavealsodemonstratedthatbaryonicout- empirical studies also seem to suggest that the bulk (∼ 70
flows from thecore regions can help provide a plausible ex- per cent) of the stellar mass is mostly assembled by z ∼ 1,
planation to the core-cusp problem (Governato et al. 2012; againmarginalisingtheroleoflatetimemajormergers(e.g.,
Zolotov et al. 2012). Clearly feedback and infall are both Bundyet al. 2004; Brown et al. 2007, 2008).
Two-phase galaxy evolution 3
Focused studies of nearby galaxies are also unveiling aperture matched data from GALEX, SDSS, and UKIRT
significant levels of gas accretion in some nearby systems LAS(seeSeibertetal.,inprep.andHilletal.2010).Driver
(Sancisi et al. 2008) and studies of the very rapid evolu- et al. (2012) also provided the CSED subdivided according
tion of galaxy sizes have argued (e.g., Graham et al. 2011) tospheroid-dominatedanddisc-dominatedsystems.Thedi-
thatthecompactellipticalsystemsseenatintermediatered- vision into spheroid and disc dominated was achieved via
shift (1.4 < z < 2.5) by Daddi et al. (2005) and Trujillo et visualclassification,asneitherasimplecolournorS´ersicin-
al. (2006) (see also Bruce et al. 2012) might represent the dexdivisionappearstocleanlyseparatethetwopopulations
naked bulges of present day spiral systems. (see also Kelvin et al., 2012, figure 20).
In essence the two-phase model is an attempt to high- Thesampleoriginated from acommon volumeof2.8×
light, conceptually, the possibility of a distinct change in 105 (Mpc/h)3 over the redshift range 0.013 < z < 0.1. Al-
theprimarygalaxyformationmechanismoccurringatsome thoughtheGAMAsurveycurrentlycontainsabout180,000
transition redshift from an era where the dominant mode galaxies with known redshifts, the adopted redshift range
is major mergers leading to spheroid formation, to an era significantly reduces the sample size to around 10,000. It
where the dominant mode is accretion leading to disc for- alsosimplifiesandremovesanyluminositybiasarisingfrom
mation. largescaleclusteringasthesampleispseudo-volumelimited
At early cosmic epochs we see a prevalence of distinct aroundtheL∗ region —i.e., thosegalaxies which dominate
phenomena, in particular highly asymmetrical morphology theluminosity density measurements.
in massive/luminous systems (Driveret al. 1998; Conselice, AstheGAMAsurveyliesentirelywithintheSloanDig-
Blackburn&Papovich.2005;Ravindranathetal.2006)and ital SkySurvey,theGAMA CSED was renormalised to the
significantly increased AGN activity (Fan et al. 2001,2003; fullSDSSsurveyarea.Thisreducesthecosmic/samplevari-
Croometal.2004;Richardsetal.2006).AGNactivityisdi- ancefromaround15percentto5percent(usingtheformula
rectly linked to the formation and growth of theassociated for estimating cosmic variance given by eqn. 4 of Driver &
super-massive black holes (SMBH; Hopkins et al. 2008a) Robotham 2010).
which in turn is linked to spheroid formation via the well
established SMBH-bulgerelations (see, for example,there-
2.1 The CSED of spheroids and discs
viewbyFerrarese&Ford2005ortherecentnear-IRSMBH-
bulgeluminosity relation byVikaetal. 2012). Recentstud- The final CSED values we use here are the spheroid-
ies also argue, from more direct empirical evidence, that dominated and attenuation corrected disc-dominated values
AGNactivityisalmostalwayscoincidentwithmassivestar- taken directly from Table 7 of Driveret al. (2012).
formation and that the two-processes do indeed appear to As dust attenuation is such a crucial issue it is worth
occur hand-in-hand (e.g., Rafferty et al. 2011). This AGN- mentioningthegenesisofthecorrectionsusedbytheGAMA
SMBH-bulge connection therefore implies a clear timescale team. First, the dust correction is only applied to the disc-
for the formation of the spheroid systems (see also Pereira dominated data and the spheroid population is assumed
& Miranda 2011 for a similar argument, albeit applied in dust free (e.g., Rowlands et al. 2012). Second, the correc-
theopposite direction). tions are based on the radiative transfer models of Tuffs
In Section 2 we describe the z = 0 empirical data de- et al. (2002) and Popescu et al. (2011) which have been
scribing the cosmic spectral energy distribution (CSED) of fine-tuned to the multiwavelength (FUV–far-IR) data of
spheroidsanddiscsasrecentlyreportedbytheGalaxyAnd NGC891, and incorporate three distinct dust components;
MassAssemblyteam(Driveretal.2011,2012).InSection3 an extended low opacity disc, a compact high opacity disc
wetaketheaboveargumentstotheirnaturalconclusionand and dust clumps. This fiducial model is calibrated to the
usetheAGN-SMBH-Bulgeconnectiontodefinetheindepen- galaxypopulationatlargebymodifyingtheB-bandcentral
dentstar-formationhistoryofspheroidsandassigntheresid- faceonopacityuntilthepredictedvariationoffluxwith in-
ualstar-formation,impliedbythecosmicstar-formationhis- clination matches the trend of M∗ with inclination seen in
tory, to describe that of the discs. In Section 4 we use our theMillenniumGalaxyCatalogue data(Driveretal.2007).
star-formationhistoriestoproducepredictionsoftheCSED Thiscalibratedmodelwasthenusedtoderivethecombined
ofspheroidsanddiscs,andinSection5comparethepredic- face-on and inclination dependent correction for a popula-
tions to thedata. tion of galaxies averaged over all viewing angles and over
ThroughoutweuseH0=70hkms−1 Mpc−3 andadopt a wavelength range of 0.1 − 2.1µm (Driver et al. 2008).
ΩM =0.27 and ΩΛ =0.73 (Komatsu et al. 2011). This photon escape fraction (varying from 24 per cent in
the FUV to 89 per cent in the K-band) was then used to
correcttheCSEDofdisc-dominatedsystemstotheintrinsic
CSED, which we use here.
2 THE Z =0 COSMIC SPECTRAL ENERGY
The CSED of spheroid-dominated and disc-dominated
DISTRIBUTION
galaxies, however,isnot precisely what werequire,assome
In Driver et al. (2012) we reported the empirical measure- proportion of the CSED flux in the disc-dominated class
ment of the cosmic spectral energy distribution (CSED) in maybecomingfromthecentralbulges.Likewise,somepro-
the nearby Universe, corrected for dust attenuation, and portion of the flux in the spheroid-dominated class may be
spanningthewavelength range from 0.1 to2.1 µm, i.e., the duetofaintdiscs.Inordertoassesshowmuchofaproblem
regimeoverwhichdirectstellar-lightdominates.Thesedata thismightbe,wecancomparetheratiooftheK-bandlumi-
were derived from the combination of the GAMA spectro- nosity densities of the spheroid-dominated to non-spheroid
scopic survey, currently underway on the Anglo-Australian dominated samples, to the ratio of the stellar mass densi-
Telescope(Driveretal.2011),coupledwithreprocessedand ties of bulge+elliptical systems to disc systems from Driver
4 Driver et al.
et al. (2007)1. This test assumes that the K-band lumi- comparison of theAGNactivity shapetotheglobal CSFH.
nosity is a suitable single-band proxy for stellar mass. We For our second axiom we elect to maximise the spheroid
find reasonable agreement (within 12 per cent), suggesting CSFH by setting theamplitudeas high as possible without
that acomparable amount of fluxneedstoberedistributed exceedingtheglobalCSFH(i.e.,amaximalspheroidforma-
in either direction. In detail, the K-band luminosity densi- tionscenario).Conceptuallythen,theheartofthetwo-phase
ties are 1.2 and 2.2 (×1034 h W Mpc−3) for the spheroid- model can bedefined empirically from two axioms:
dominatedandnon-spheroid-dominatedpopulationsrespec-
tively(takenfromTable7ofDriveretal.2012).Meanwhile 1) AGN activity traces spheroid growth
the stellar mass densities for spheroids (bulge+ellipticals)
and discs are 2.9 and 4.7 (×108h M⊙ Mpc−3) respectively 2) Spheroid formation dominates at high-z
(takenfrom Table1ofDriveretal.2007). Thisgivesagree-
mentto∼10percentandsuggeststhatforthemomentwe Astheabovetwoaxiomsarealreadyconstrainedbyem-
can adopt the following approximation: piricaldata,thisprovidesazero-parameterstartingpointfor
the two-phase model — bypassing the need for any initial
elliptical+bulge CSED ≈ Spheroid-dominated CSED conditionsordetailednumericalsimulations. Fig.1(upper)
shows thefit (solid curve)to thecosmic star-formation his-
disc CSED ≈ non-Spheroid dominated CSED tory data (grey data points) taken from Hopkins & Bea-
com (2006; see their figure 2a and table 1 column 2). This
In due course all galaxies at z < 0.1 in the GAMA adopts the parametric form defined by Cole et al. (2001)
survey will be decomposed into bulge and disc components and where the UV data has been calibrated to a modified-
to enable a direct derivation of the true spheroid and disc Salpeter(1955)IMF.ThedatadescribingtheAGNluminos-
CSEDs. itydensityaretakenfromRichardsetal.(2006)andrescaled
such that the peak of the AGN luminosity density lies on
theCSFH curve(requiringan arbitrary multiplication bya
3 THE STAR-FORMATION HISTORY OF factor of 3.51×106M⊙/yr−1Li⊙).Immediately noticeable is
SPHEROIDS AND DISCS theapparentdiscrepancy/uncertaintyat veryhighredshift.
Particularly as the axioms above require that at the very
ThelocalCSEDshouldbeapredictablequantityifthecos- highest redshift the CSFH and AGN activity curvesshould
micstar-formationhistory(CSFH)isknown,theinitialmass havethesameform.Tosomeextenttheevidentdiscrepancy
function(IMF)isuniversalandknown,andaplausiblestel- simply reflects data uncertainty, as the dust corrections on
larevolutioncodeapplied.Ofcoursethisisnotquitesosim- the CSFH at high-z are poorly constrained with significant
pleandinparticularthemetallicityenrichmentadoptedwill ongoing debate as to the true shape of the CSFH at the
significantly modify the predicted CSED shape. Upcoming highest redshifts. For example, measurements of the star-
papers will explore these issues in more detail, but here we formation history based on gamma-ray bursts (Yuksel et
wish to construct a basic first-look model and focus on the al.2008;Kistleretal.2009)oftenfindhigherstar-formation
viabilityofthehypothesisthatgalaxyformationprogressed rates. Likewise the incidence of dust-obscured AGN is an
in two fairly distinct phases: rapid spheroid formation fol- equally hotly debated topic (e.g., Polletta et al. 2006; Tri-
lowed by more quiescent disc growth. ester et al. 2010). Most recently Behroozi, Wechsler
In order to construct a model of the present day & Conroy (2012) argue that the compendium by
spheroid and disc CSEDs we need not just the CSFH, but Hopkins & Beacom (2006) potentially leads to an
theCSFHsub-dividedintospheroidsanddiscs.TheseCSED over-estimate of the cosmic star-formation history
predictions can then be compared to the data from Section at very high redshifts as the pre-2006 UV luminos-
2. ity densities may have been over-estimatedand find
Theexistenceofvarioussuper-massiveblack-holebulge a modified CSFH which agrees very well with the
relations(e.g.,Ferrarese&Ford2005),providestheobvious high-z AGN data (see their figure 2).
smoking gun, as it couples SMBH growth to bulge growth. Whenpresentedasafunctionoftime(Fig.1,lower),it
This is because SMBHs are believed to grow via mergers, isclearthatthisdiscrepancyisnotactuallythatsignificant,
resulting in an active-galactic nucleus phase (Hopkins et and the accumulated stellar mass able to be formed during
al. 2006). The growth of spheroids is therefore, arguably, this high-z interval is small compared to subsequent mass
mirrored via themore readily observable AGN activity his- growth.Wecannowtriviallyfitthemodified-Richardsdata
tory.Thislogicalconnection,fromacorrelationtocausality, to derive the star-formation history of spheroids. This fit
isthekeyassumptionunderpinningourmodelandformsthe canthenbesubtractfrom theglobal star-formation history
first of our two axioms. In the recent study by Richards et andinturnfittorecovertheimplieddiscformationhistory.
al. (2006), the integrated AGN activity versus redshift was Theresultingexpressionsaregivenbelowandrepresentthe
reportedand,ignoringanysignificantlag(ineitherspheroid star-formation rate(ρ˙)versustimeinGyrs(tGyrs)sincethe
formationorAGNactivity),canbeusedasaproxyinshape Big Bang for the spheroidal (S) and disc (D) populations:
for the spheroid cosmic star-formation history.
The amplitude of the spheroid SFH can be set from ρ˙S =ξ1.03×10−5h30.7(tG2y1rs.8h60.7)8.57exp(−tG2y1rs.8h60.7) (1)
1 TheDriveretal.(2007)studyisbasedonbulge-discdecomposi- ρ˙D =ξ1.80×10−3h30.7(tG2y9rs.3h90.7)5.50exp(−tG2y9rs.3h90.7) (2)
tionsoftheMillenniumGalaxyCataloguedata(Liskeetal.2003;
Driveretal.2005)describedinfullinAllenetal.(2006) whereξistheIMFmultiplierasgiveninTable1.Theguide-
Two-phase galaxy evolution 5
Figure1.(upperpanel)thecosmicstar-formationhistoryfromHopkins&Beacom(2006),Table2(blackline)andtheactualdata(grey
points)calibratedtoamodified-Salpeter IMF(seeTable1).OverlainaretheQSOluminositydensitydatafromRichardsetal.(2006),
Figure20.TheQSOluminositydensityisscaleduntilthepeakalignswiththepeakoftheCSFH.(lower panel)thesamedatabutnow
shownwiththeordinateinunitsofage.OverlainareourparametricfitstothesedatawhichprovideourinferredCSFHsfortheSpheroid
andDiscsystems.OnbothpanelswealsoshowtheSFRderivedfromtheSDSS/GALEXFUVluminosityfunctiongiveninRobotham
&Driver(2006) convertedtoamodified-Salpeter AIMF(seeTable1orHopkins&Beacom2006).
lineerroratanyparticulartimeshouldbetakenas∼±25% asweshallseeinSection.4willrequireamodestdownward
based on the scatter of the original data shown on Fig. 1 correctionof25%tomatchtheobservedCSED(i.e.,within
(lower) i.e., ∼ 70%of the grey data points lie on the grey thegrey shaded error bounds).
shaded regions. At this point it is also worth highlighting
thatwhiletheAGNdata(mauvepoints)placeastrongcon- Asanalternativeforthespheroidpopulation,onecould
strain on the shape of the spheroid star-formation history insteadusethetotalCSFHfort<3Gyrscombinedwiththe
the normalisation is very uncertain and based on the rela- AGN luminosity density data for t>3Gyrs, which is given
tivelylimitedhigh-zcosmicstar-formationhistorydataand by:
6 Driver et al.
Table 1.CSFHmultiplicationfactors(ξ)forvariousIMFs.
0.04
IMF Multiplier
ξ
Salpeter(1955) ×1.3
modified-Salpeter(Hopkins &Beacom 2006) ×1.0
Baldry&Glazebrook(2003) ×0.7
Kroupa(1993) ×1.7 0.03
Kroupa(2001) ×0.85 +10%
Chabrier(2003) ×0.85
ρ˙S2=ξ2.72×10−4h30.7(tG1y6rs.8h20.7)6.97exp(−tG1y6rs.8h20.7) (3) 0.02
As stated these CSFHs are calibrated, via the UV data,
to the modified-Salpeter IMF used by Hopkins & Beacom
(2006)whichwasfirstlaiddownasSalAbyBaldry&Glaze-
0.01
brook(2003).ToconverttootherIMFsoneneedstomultiply
by the factor (ξ) shown in Table 1. The simple expressions
above are shown in Fig. 1 (lower) in red (spheroid), blue
(disc),andgreen(spheroid+disc)andprovideagoodfitto
the data given the accuracy to which the data are known. 0
13 12 11 10 9 8 7 6 5 4 3 2 1 0
These equations now provide a blueprint for the formation
Time since Big Bang (Gyrs)
ofthepresentdayspheroidsanddiscsoverthefullageofthe
Universe, leading to a clear prediction of the stellar energy
Figure 2. The adopted (gas-phase) metallicity for spheroids
output and stellar mass growth.
(red solid line) and discs (cyan solid line) as a function of time.
Of particular interest should be the transition point
Soliddatapointsshowtheacceptedmeanz=0valuestakenfrom
around 4.2Gyrs (z ≈ 1.6) from which point star-formation
Tremonti et al. (2004). Also shown are approximate data values
resulting in disc growth dominates over star-formation re- from Erb et al., (2006) and Zahid et al., (2011). Note that all
sulting in spheroid growth. This suggests a key epoch at datahaveanarbitraryerrorof±10%.Thegas-phasemetallic-
which the Universe switches from merger dominated evolu- ity assumes no lag between star-formation and enrichment. The
tion toaccretion dominatedevolution andmeshesverywell dotted lines show the implied metallicity of the stellar popula-
with the evident change in the morphological appearance tions for the spheroids and discs. The grey line shows the mean
of galaxies from highly disturbed to more ordered systems bymassoftheintegratedstellarmetallicityandtheinstantaneous
at z ∼ 1.5 (see Driver et al. 1998; and also van den Bergh integratedgas-phasemetallicity.
2002).
define our gas-phase metallicity at redshift zero
to be Z = 0.030 and Z = 0.010 for the spheroids
4 CONSTRUCTING THE MODEL and discs. These values were determined by noting
themetallicityat1011M⊙ (predominantlyspheroids)
WiththeCSFHofspheroidsanddiscsdefined,wenowbuild and at 109M⊙ (predominantly discs). Toconvertthe
ourempiricaltwo-phasemodeladopting“vanilla”choicesat
given 12+log (O/H) values to those shown on Fig.2
10
everyopportunity.Insummary thekeyinputsandassump-
we adopt a solar metallicity of Z⊙ = 0.019 with
tions to themodel are:
12+log10(O/H)⊙ = 8.9. We then argue that in the ab-
senceof otherfactors themean metallicity will rise approx-
1) The star-formation history shown in Fig. 1 as discussed
imately linearly with the cumulative cosmic star-formation
in section 3.
history normalised to the present day values. This ignores
2) The adoption of a Universal IMF, in this case Baldry &
theprospectofeitherpre-enrichmentvia,forexample,Pop-
Glazebrook (2003, henceforth BG03).
ulation IIIstars, oranylag between thestar-formation and
3) The adoption of pegase.2 (Fioc & Rocca-
theincrease in metallicity (i.e., instantaneous enrichment).
Volmerange 1997; 1999) to model the spectral output
Conceptuallythesearelooselyconsistentwithaclosed-
of the evolving stellar population (using default options
box model for spheroids (i.e., rising to a metallicity close
throughout).
to typical yields), and an infall model for discs or one in
4)Theassumptionthatthegas-phasemetallicityincreases
which thedisc isgradually growing from alarge gas “reser-
linearly with star-formation from Z =0.0 to Z =0.030 for
voir” (e.g., perhaps analogous to the “equilibrium model”
spheroids and to Z =0.010 for discs, with no time lag (i.e.,
putforwardbyDav´e,Finlator&Oppenheimer2012).Toex-
instantaneous enrichment).
plore thebounds and importance of this metal enrichment,
however,wealsoshowtheCSEDpredictionsusingoursim-
pleevolving metallicity history and for constant metallicity
at the highest and lowest values. Fig. 2 shows the implied
4.1 Metal/chemical enrichment history
metallicity histories derived from Eqns. 1&2, for the two
Perhaps the most uncertain of the above list is the ap- populations (as indicated by the red and cyan solid lines).
propriate metallicity history to adopt. Here we have been Note that the grey lines on Fig. 2 show the com-
guided by the study of Tremonti et al. (2004) to bined metallicity of all stars formed (grey dotted
Two-phase galaxy evolution 7
[h]
0.1 1
evolving Z, BG03 (S -25%, D +0%)
constant Z, BG03 (S +0%, D +0%)
zero Z, BG03 (S -25%, D +0%)
0.1 1
Wavelength (microns)
Figure 3. The zero-parameter output, assuming a BG03 IMF for various metallicity histories as indicated and adopting the star-
formation histories given by Eqns. 1 & 2. Note that the star-formation rates have be multiplied by a factor of 0.55 to convert from a
Salpeter(1955)IMFtothatofBG03.ThedatapointsaretranscribeddirectlyfromtheCSEDreportedinDriveretal.(2012)wherethe
redpoints represent spheroid-dominated, the blue disc-dominated andthe black the sum of the two. Themodel linesare forspheroids
(orange), discs(cyan) andthesum(black).
line) and the integrated gas-phase metallicity (grey systems have comparable gas-phase metallicity at z =0.8
solid line). This shows interesting behaviour in that tolocal systems, whilelowmass systemshaveagas-phase
the average gas-phase metallicity (of the gas about metallicity reduced by 0.15dex. However Erb et al., (2006)
to form stars) peaked at z∼2. find that the implied gas-phase metallicity, for massive,
i.e., spheroidal-like systems, at z ∼ 2 is approximately
Constraints from the literature on the mean metallic-
half that at z =0. Both of these results are crudely consis-
ity at intermediate and high redshift are minimal, however
tent with our inferred metallicity history if one equates (as
we note that Zahid, Kewley & Bresolin (2011) find from a
weexplicitly do), themassive systems tospheroidsand low
sample of 1350 galaxies drawn from DEEP2 that massive
8 Driver et al.
mass systems to discs. Note that one natural byprod-
0.1 1
uctofthisisthatasintermediatemasssystemshave
bothaspheroidandadisccomponenttheirsystemic
metallicity will lie somewhere between the two ex-
tremes and exhibit strong radial gradients as one
evolving Z, Salpeter (1955) (S -20%, D +10%)
moves from the central spheroid component to the
outer disc component. As the mean bulge-to-total
ratioincreasesfairlysmoothly withstellarmassthis
naturally gives rise to the mass-metallicity relation
(Tremonti et al.2004). Note that the±10% error ranges
shown on Fig. 2 are purely indicative as the actual ranges
are poorly constrained. evolving Z, Kroupa et al (1993) (S -50%, D -25%)
4.2 Stellar population synthesis evolving Z, Kroupa (2001) (S -35%, D -10%)
To construct the redshift zero CSED the pegase.2 code
(Fioc&Rocca-Volmerange1997;1999)wasusedtoproduce
0.1 1
a series of single-stellar population (SSP) templates with Wavelength (microns)
an appropriate range of metallicities (Z = 0.000 to 0.025 in
0.001intervals)andwiththepegase.2defaultstepsinages, Figure4.Thezero-parameteroutputforalternativeIMFsusing
(i.e., roughly logarithmic from 0-20Gyrs). For all SSP tem- the evolving metallicity shown in Fig.2 and adopting the star-
plates the star formation is set to a short continuous burst formationhistoriesgivenbyEqns.1&2.Notethatingenerating
overa1Myrperiodwithconstantmetallicity(leadingtothe themodelswemodifytheinputstar-formationbyfactorsof×1.0,
formation of 2.0×10−3 M⊙ in pegase.2-normalisedstellar ×1.3and×0.7respectively.
mass units).TheseSSPspectra werethen combinedto cre-
atealibraryof1Gyrtimeaveragedspectrafrom0-1Gyrto
4.3 Normalisation of the CSEDs
13-14Gyr in 1Gyr intervals, and for each metallicity class.
Note that the 0-1Gyr bin which dominates the FUV and In order to determine the correct normalisation we need
NUVregionisextremelyhardtomodelcorrectlybecauseof to multiply the output pegase.2 SSP spectra which are in
the rapidly changing UV flux and requires more care. Here unitsofergs−1˚A−1by 109λ .Herethefactor109scalesto
we take the rather simplistic approach of combining all the 1Gyr bins, the factor 1100770c.o00n2verts ergs−1 to W, the wave-
spectra provided by pegase.2 in the 0-1Gyr range in the
lengthisinAngstroms,andthefactor0.002scalesthespec-
following manner i.e.,
trato1solar mass. InapplyingEqns.1-3weset ξ to0.7 to
correct theCSFHs to theBG03 IMF.
0−1Gyr= 1+2+31+0...+101+020+30+...+100+200+300+...+1000Myr Finally we allow the normalisations to float by ±25%
10 to account for the uncertainty highlighted in Fig. 1 by the
grey shading, along with uncertaintiesin themultiplication
TocreatetheCSEDatanyredshiftwethensumallpre-
factorsinTable1,thecosmicvarianceintheGAMACSED
viouslyformedpopulations,agedappropriately,drawnfrom
data,and theimpact of metallicity on star-formation rates.
theappropriatemetallicityclass,andscaledbytherequired
These 1 values are shown in brackets in Figs. 3 & 4.
star-formation rate. The modelling approach we adopt is
thereforerelativelysimplisticandeffectivelyassumesallval-
ues (star-formation rate, metallicity etc) are held constant
over a 1Gyr time period. At this stage we feel this is suf- 5 MODELS V DATA
ficient time resolution given the inherent uncertainties in
5.1 The CSED and adopted metallicity
the initial assumptions (i.e., the input CSFH and AGN ac-
tivity data). CSEDs were then derived at all 13 time steps Fig.3showsthedirectcomparisonofourz=0CSEDmod-
and combined to producesimple evolution movies available els against the recent GAMA data (Driver et al. 2012), for
from: the three assumed metallicity histories (as indicated). The
toppanel,whichadoptstheevolvingmetallicity,showsare-
http://star-www.st-and.ac.uk/∼spd3/model1.gif — evolv- markableagreementacrossthefullwavelengthrangeandfor
ing metallicity both the spheroid and disc systems. Note that in achieving
these fits the spheroid data have been renormalised down-
http://star-www.st-and.ac.uk/∼spd3/model2.gif — con- wards by 25% which is within the specified range of uncer-
stant high metallicity tainty.ThecentralandlowerpanelsofFig.3showthesame
models except for a constant high or low metallicity. This
http://star-www.st-and.ac.uk/∼spd3/model3.gif — con- has a negligible impact on the disc-CSED, suggest-
stant zero metallicity ing very little dependency on the assumed metal-
licity evolution for discs (perhaps in part due to
Two-phase galaxy evolution 9
5.3 Stellar mass history
Fig.5showstheimpliedbuild-upofstellarmassinspheroids
and discs (and combined), as indicated. Note that we show
boththetotalcumulativestellarmassformed(dashedlines),
along with that remaining based on default pegase.2 as-
sumptions as to mass-loss (solid lines). Also shown are the
direct empirical stellar mass measurements from the Mil-
lennium Galaxy Catalogue (Liske et al. 2003; Driver et
al. 2005), which includes corrections for dust attenuation
(Driver et al. 2008). The agreement is reasonable with the
discs agreeing with the MGC data to within the error and
thespheroidmassover-predictingtheMGCvaluebyamod-
est amount. It is worth noting from Fig. 5 that the stellar
mass of spheroids is actually declining, with mass-loss hav-
ingexceedingmass-gain forthepast9billion years.Forthe
discs, the two almost exactly balance, such that the over-
all stellar mass density appears to asymptote to a constant
valuearound thepresent day.
Also shown in Fig. 5 as grey shaded data are the com-
14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 pendium of total stellar mass estimates given in Wilkins,
Age (Gyrs) Trentham&Hopkins(2008a).Thesedataclearlyfallsignif-
icantlybelowtheblackshadedline,highlightingasignificant
Figure 5.Theimpliedbuild-upofstellarmassinspheroidsand discrepancybetweenthetotalstellarmassinferredfromthe
discs versus recent measurements from the Millennium Galaxy cosmic star-formation history and that derived from direct
Catalogue. Shown in grey are the compendium of data from empiricalconstraints.Thisoffsetiswellknownanddiscussed
Wilkins,Trentham&Hopkins.2008a.
in detailed in Wilkins, Trentham & Hopkins (2008a), here
wemaketwoadditionalcomments:(1)theshapeofthedata
andtheblackcurvedobroadlyagreewitha×2offsetatal-
most all ages, (2) the z = 0 data from the MGC includes
the low value adopted). Conversely the impact on the
detaileddustcorrectionsforboththeopticallythinandop-
spheroid CSED is quite marked with the CSED tilting ei- tically thick regions and is typically a factor of ×2 higher
therredward orblueward for constant high orconstant low
than most local measurements. It is possible then that the
metallicity respectively. This perhaps lends some argument
valuesfromtheliteraturearemissingmassembeddedinop-
against any very strong pre-enrichment phase as interme-
ticallythickregions.Perhapsamorelikelyexplanation,also
diate and low metallicity stars in spheroids are required to
put forward by Wilkins et al. (2008) is that the IMF was
produce a plausible CSED. The obvious caveat is whether
simply lighter at earlier times. This would reconcile quite
theshapeofthecurrentlyadoptedspheroidCSEDissignif-
nicely as the low-z CSED is fairly impervious to the very
icantly modified/contaminated by youngdisc light.
low mass-end of the IMF.
5.4 Discussion
5.2 Dependency on assumed IMF
Atthispointwehaveasimpleheuristicmodelwhichadopts
We now briefly explore the impact of the adopted IMF. two simple axioms motivated by the physically distinct ap-
Fig. 4 shows the CSED predictions for the evolving metal- pearance of spheroids and discs in the nearby Universe.
licity scenario using either a (top) Salpeter (1955), (centre) These axioms combined with the empirical compendium of
Kroupaetal.(1993),(bottom)orKroupa(2001)IMF.Note AGNandcosmicstar-formation activity/historyareableto
that the Kroupa (2001) IMF is extremely close in form to reproduce the z = 0 CSEDs of spheroids and predict the
the Chabrier (2003) IMF and therefore the Kroupa (2001) mean mass and metallicity evolution of present day discs
prediction can betaken as representative of both. andspheroids.Themodelalso providesacompletedescrip-
Essentially all IMFs provide an equally good fit to the tion of the energy output from stars within those systems
CSEDexceptintheUVregime.Atwavelengthslongerthan whichwilleventuallymakeupthelocalspheroidpopulation
u-band (> 0.4µm) the resulting shape is not particularly (projenitors) asafunctionofredshift,themetallicity build-
sensitive to the detailed shape of the IMF. This is mainly up, and suggests key cross-over epoch at z ∼ 1.6 between
because at z = 0 stars close to solar luminosity, where the the hot and cold mode evolution. This later transition red-
IMFisleastcontentious,aredominatingmostoftheCSED. shift is consistent with the obvious change in morphologies
Systems at the very high-mass end which formed at high seeninHSTimagesatthisredshift(e.g.,Driveretal.,1998,
redshiftwillnolongerbecontributingtotheCSEDwhereas figure3).
very low-mass stars are yet to dominate the near-IR flux. Howeveranobviousweaknessisthatthemodelprovides
The CSED is therefore unable to constrain the IMF other no clear prediction of the morphological, size- and shape-
than the normalisations required for these IMFs are gener- evolution,mergerrates,ortheclusteringofthegalaxypop-
ally higher than for themodels based on BG03. ulation. Furthermore the model does not stipulate the ac-
10 Driver et al.
tualmechanismbywhichstar-formationisoccurringandfor
thehot modecould besome combination ofmonolithic col-
lapse, major merging, and/or clump migration. Within the
recent literature the exact status of the z ∼ 2 population
is also unclear. Deep IFU studies (e.g., Forster Schreiberet
al., 2009,2011) find that the majority of star-formation at
z ∼ 2 appears to be taking place in rotating clumpy disc
structures with no obvious central bulge component. Simi-
larly Chevance et al., (2012, see also Weinzirl et al., 2011)
from a study of 31 high-z galaxies, find that the S´ersic in-
dicesaresignificantlyflatterthanonewouldexpectforlow-z
spheroids and obvious disc structures are present in many
cases. From our Fig. 1 we can see that at z ∼2 we are still
withintheepochwherespheroidformationshouldbedomi-
nating.Howeverspheroidformationdoesnotnecessarilyim-
plyspheroidmorphologiesuntilaftersomeunspecifiedtime-
laginwhichthesystemsettles.Infactviolentlystar-forming
systemswillinevitablyappearblue,asymmetrical,gas-rich,
and dusty,i.e., quite disc-like in many aspects. Otherstud-
ies, e.g., van Dokkum (2008), find that 45% of massive
galaxies at z ∼ 2.3 do indeed have evolved stellar popula-
tions,littleornoongoingstar-formation,andcompactearly-
typemorphologies.Henceapictureofaspheroidpopulation
Figure 6. As for Fig. 1 (lower) except with the spheroid star-
emerging from a highly turbulent progenitor phase around
formation history down-weighted by 25% by incorporating the
z∼2appearstobequalitativelyconsistentwithourmodel.
CSEDconstraints fromFig.3(upper).
Alternatively our model may need to be adjusted to allow
for gas infall and disc-formation from the outset with some
fraction of thedisc-formed stars merging into bulges, i.e., a its dependency on environment should enable an investiga-
relaxation of the maximal spheroid formation axiom. This tion into dependencies on clustering, and to assess whether
wouldhavetheneteffectedofalsoincreasing thecross-over star-formationproceedsmorerapidlyorwhetheritismerely
redshift to >1.6 therelativemixofspheroidvdiscformationwhichischang-
A further intriguing observations is that high-z ing.Similarly,usingobservationsatintermediateredshift,it
spheroids are significantly more compact than nearby ellip- should be possible to compare data from high-z studies to
ticals byfactors of ×3−4 at fixedstellar mass (e.g., Daddi the predictions of our two-phase model. Both of these av-
et al., 2005; van Dokkum et al., 2008 etc). Within our sce- enueswill be explored in futurepapers.
nario this could be consistent with thehigh-z sample being
“naked”-bulges yet to grow discs or yet to be “puffed-up”
through successive minor merger interactions or adiabatic
6 CONCLUSIONS
expansion. These two pathways, disc-growth verses “puff-
ing”, are likely to be strongly environmentally dependent From twoverysimpleaxioms: (1) that AGNactivitytraces
with minor mergers more frequent in high density environ- spheroidformation,and(2)thattheCSFHisdominatedby
ments,andgasinfallmoreprevalentinlow-densityenviron- spheroid formation at high redshift, we are able to derive
ments. A particularly interesting comparison might there- simpleexpressionsforthecosmicstar-formation historiesof
fore be the mass-size relation of high-z spheroids to low-z spheroids and discs. Following comparisons to the z = 0
bulges. CSED for spheroids and discs we find a modest downward
Withthecaveatthatthemorphologyandsizeevolution adjustment of 25% provides theoptimal fit resulting in our
within our model is unspecified we nevertheless appear to finalrecommended star-formation histories of:
haveapredictionoftheenergyoutputofspheroidanddiscs
pprhoajesneitmorestaolvliecritayllheipstoocrhysf(oFrige.a3chtoppoppuanlaetli)o,nth(eFmig.ea2n),gaansd- ρ˙S =ξ0.77×10−5h30.7(tG2y1rs.8h60.7)8.57exp(−tG2y1rs.8h60.7) (4)
dthisetibnugiuldis-huptohfessteelplaorpmulaastsio(nFsigo.b5s)e.rWvahteiotnhaerlohnoewceavnerreiasdialny ρ˙D =ξ1.80×10−3h30.7(tG2y9rs.3h90.7)5.50exp(−tG2y9rs.3h90.7) (5)
open question. where ξ is the IMF multiplier as given in Table 1. Fig. 6
Finallyitisworthwhilereiteratingthatthismodelcon- shows these final relations compared to thecompendium of
tains no tunable parameters nor any dependency on initial data provided by Hopkins & Beacom (2006) and despite
conditions beyond the underlying cosmology. The model is the renomalisation of the spheroid star-formation history
built entirely from empirical data and provides a fully con- stillprovideaperfectlysatisfactorydescriptionoftheglobal
sistentempiricalscaffoldinguponwhichmorephysicallymo- CSFH.
tivated models can be built. Our conclusion is that the ini- AdoptingaBaldry&Glazebrook(2003)IMFandusing
tial axioms on which the model is based are viable and the theseexpressionstopredictthez =0CSED,weareableto
star-formation histories defined are tenable. provideasatisfactoryexplanationoftheobservedCSEDsof
Furtherstudiesofthevariationofthez=0CSEDand spheroids and discs from theFUV to theK-band.