Distant early-type galaxies: tracers of the galaxy mass assembly evolution

0.28 MB·English

by  Andrea Cimatti

Mem.S.A.It.Vol.75,282
(cid:13)c SAIt 2004 Memoriedella
Distant early-type galaxies: tra
ers of the galaxy
mass assembly evolution
A. Cimatti1
Istituto Nazionale di Astrofisica – Osservatorio Astrofisicodi Arcetri, Largo E. Fermi 5,
6
I-50125Firenze,Italye-mail:[email protected]
0
0
2
Abstract. Wereviewthemostrecentobservationalresultsontheformationandevolution
n
ofearly-typegalaxiesandtheirmassassemblybyfocusingon:theexistence,propertiesand
a
roleofdistantold,massive,passivesystemstoz ∼ 2,thestellarmassfunctionevolution,
J
the“downsizing”scenario,andthehigh-zprecursorsofmassiveearly-typegalaxies.
5
2
Keywords.Galaxies:evolution–Galaxies:formation

2
v
7 1. Introduction evolution of the stellar population. However,
9 in the modern scenario of CDM cosmology,
4 Early-typegalaxies(ETGs)playacrucialrole theevolutionofthegravitationalperturbations
1 in cosmology. They represent the most mas-
implies that galaxies assembled their mass
0 sive galaxies in the local Universe, contain
more gradually through hierarchical merging
6 most of the stellar mass (M) and are the pri-
of CDM halos and make the monolithic col-
0 mary probes to investigate the cosmic history
lapseunlikelyorevenunphysical.Forinstance,
h/ of galaxy mass assembly. As ETGs are the the most recent ΛCDM hierarchical models
p most clustered galaxies, they are also funda-
predictthatmostoftheETGstellarmassisas-
- mental in tracing the evolution of the large
o sembled at 0 < z < 1 (DeLuciaetal.2006).
scalestructures.Becauseofthecorrelationbe-
r The moderngenerationof galaxysurveyscan
st tweentheblackholeandgalaxybulgemasses, testthedifferentpredictionsontheevolutionof
ETGsarealsoimportanttoinvestigatetheco–
a the number density, colors, merger types and
: evolutionofnormalandactivegalaxies.
v rates,luminosityandmassfunctions,andpro-
Although ETGs at z ∼ 0 are rather “sim-
i videacrucialfeedbacktoimprovethetheoret-
X ple” and homogeneous systems in terms of
icalsimulations.
morphology,colors,stellar populationcontent
r
Despite the intense observational activity
a andscalingrelations(Renzini2006),theirfor-
onETGevolution,ageneralconsensushasnot
mation and evolution is still a debated ques-
been reached yet. For instance, recent results
tion.Inthesocalled”monolithic”collapsesce-
based on optically-selected, moderately deep
nario(Eggenetal.1962;Tinsley1972),ETGs
(R < 24) samples of “red sequence” ETGs
converted most of the gas mass into stars at
(e.g. COMBO-17, DEEP2) (Belletal.2004;
high redshifts (e.g. z > 3) through an intense
Faberetal.2005) suggest a strong evolution
burst of star formation followed by passive
of the number density with φ∗(z) displaying
Sendoffprintrequeststo:A.Cimatti an increase by a factor of ∼6 from z ∼ 1
Cimatti:Early-typegalaxyevolution 283
to z ∼ 0, and a corresponding increase by a spectral energy distribution (SED) selections.
factor of 2 of their stellar mass density since ETGs have strong k–corrections in the opti-
z ∼ 1. Dissipationless ETG–ETG merging cal as they become rapidly very faint for in-
(alsocalled”dry”merging)isadvocatedtoex- creasing redshiftsand can be easily missed in
plain how ETGs assembled at 0 < z < 1 optically–selected samples (e.g. Maoz 1997),
(Belletal.2005;vanDokkum2005).HSTop- while the k–corrections are much smaller in
tical, moderately deep (I < 24) samples of thenear-infrared.Apossibleexampleisgiven
morphologically–selected ETGs also suggest by recent results (Yamadaetal.2005) based
a substantial decrease of the number density on a large sample of ETGs at z ∼ 1 ex-
withn ∝ (1+z)−2.5 (Ferrerasetal.2005),and tracted from a ∼ 1 square degree field in the
spatially resolved color maps show that ∼30- Subaru/XMM-NewtonDeep Survey . Despite
40% of ETGs at 0 < z < 1.2 display varia- the field as large as that coveredin COMBO-
tionsintheirinternalcolorproperties(e.g.blue 17,thenumberdensityofETGsatz ∼ 1turns
coresandinversecolorgradients)suggestiveof outtobesubstantiallylargerthanthatderived
recent star formation activity in a fraction of atz=0.95inCOMBO-17,probablyduetothe
the ETG population (Menanteauetal.2004). deeperandredder–bandselection(z′ < 25vs.
However, the optically-selected (I < 24) R<24).
VIMOS VLT Deep Survey (VVDS) does not Also the “progenitor bias”
confirm the above findings, and shows that (vanDokkum&Franx2001) plays a role
the rest-frame B-band luminosity function of in the comparison between low and high
ETGs (selected based on the SED) is con- redshiftETGsamples.Asthethemorphology
sistent with passive evolution up to z ∼ ofgalaxiesareexpectedtoevolve,somelow-z
1.1, while the number of bright ETGs seems ETGswerespiralgalaxiesormergersathigher
to decrease only by ∼40% from z ∼ 0.3 redshift. These young ETGs are included
to z ∼ 1.1 (Zuccaetal.2005). The over- in low redshift samples, but drop out of the
all picture is made more controversial by re- samples at high redshift. Therefore, the high
cent results indicating a weak evolution of redshiftsamplesareabiasedsubsetofthelow
the stellar mass function (Fontanaetal.2004; redshift samples, containing only the oldest
Caputietal.2006;Bundyetal.2006)andthat progenitors of low-z ETGs, i.e. the ones with
“drymergers”cannotrepresentthemajorcon- morphological and spectral properties similar
tributorstotheassemblyofETGsat0<z<1 tothenearbyETGs.
(Bundyetal.2006;Masjedietal.2005).
3. HighredshiftETGs
2. Theproblemofselectioneffects
What is the highest redshift at which we still
After the discovery of the strong clustering findETGssimilartothoseobservedatz ∼ 0?
of ETGs at z ∼ 1 (Daddietal.2000), it be- After the isolated case of a spectroscopically
came clear that a major source of uncertainty identified old ETG counterpart of a radio
was the “cosmic variance” (Daddietal.2000; source at z = 1.55 (Dunlopetal.1996),
Somervilleetal.2004), and that its effects studies based on the HDF-South and HDF-
couldexplainthediscrepantresultsoftenfound North suggestedthe existenceof a substantial
by narrow-field surveys, such as the dif- number of photometric/morphological ETG
ferent content of luminous high-z ETGs in candidates up to z ∼ 2 (Treuetal.1998;
the HDF-North and HDF-South (Zepf1997; Benitezetal.1999; Stanfordetal.2004). It
Benitezetal.1999). was with the recent spectroscopic surveys of
Anotherimportantsourceofuncertaintyis near-infrared galaxy samples like the K20
played by the heterogeneous criteria adopted (Cimattietal.2002b; Mignolietal.2005),
to select and classify ETGs: deep vs. shal- GDDS (Abrahametal.2004), TESIS
low surveys, optical vs. near-infrared, color (Saraccoetal.2003), GRAPES
vs. morphological vs. spectrophotometric or (Daddietal.2005a), LCIRS
284 Cimatti:Early-typegalaxyevolution
(Dohertyetal.2005), that high-z ETGs
wereunveileduptoz∼2.
The distant ETGs spectroscopically iden-
tified at 1 < z < 2 are very red (R− K > 5,
s
I −H > 3inVegaphotometricsystem),show
the spectral features of passively evolving
old stars with ages of 1-4 Gyr (Fig.1), and
have large stellar masses with M > 1011M
⊙
(Cimattietal.2002a; Cimattietal.2003;
Cimattietal.2004; McCarthyetal.2004;
Saraccoetal.2003; Saraccoetal.2005;
Daddietal.2005a; YanL.etal.2004;
Fuetal.2005). Their spectral properties
ETGs suggest a weak evolution in the range
of 0 < z < 1 (Mignolietal.2005) (Fig.2).
The deepest optical images of ETGs at
0.5 < z < 1.3 in the Hubble Ultra Deep
Field coupled with the GRAPES ACS slitless Fig.1. The observed average spectrum (top) of
spectra show that their stellar populations four massive ETGsat 1.6 < z < 2compared with
are rather homogeneous in age and metal- asynthetic spectrumof asimplestellar population
licity and formed at redshifts z ∼ 2 − 5, (bottom)(Cimattietal.2004).
f
and that the isophotal structure of z ∼ 1
ETGsobeysthe correlationsalreadyobserved
among nearby elliptical galaxies, implying
no significant structural differences between
z ∼ 0andz ∼ 1(Pasqualietal.2006).Distant
ETGs are also strongly clustered, with a
comoving r ∼ 10 Mpc at z ∼ 1 measured
0
for the Extremely Red Object (ERO) popu-
lation (Daddietal.2002; Firthetal.2002;
Rocheetal.2003; Brownetal.2004;
Georgakakisetal.2005), and similar to
thatofpresent-dayluminousETGs.
Thestellarmassesofthesegalaxiesarees-
timatedbyfittingtheirmulti–bandphotometric
SEDswithstellarpopulationsynthesismodels Fig.2.CompositespectraoftheK20surveyETGs
(Fontanaetal.2004; Saraccoetal.2005; divided in three redshift bins (arbitrary flux scale)
Longhettietal.2005; Bundyetal.2006). (Mignolietal.2005).Frombottomtotop:0 < z <
0.6, 0.6 < z < 0.75, 0.75 < z < 1.25. The upper
Such “photometric” stellar masses are in rea-
and lower panels also show the difference spectra
sonableagreementwiththedynamicalmasses
betweentheintermediate-redshiftcompositeandthe
estimated from the absorption line velocity
high-zandlow-zcompositespectrarespectively.
dispersion (diSeregoAlighierietal.2005;
Retturaetal.2006). ETGs dominatethe high-
luminosity andhigh-masstails of the totallu- data at 1 < z < 2 are consistent with a
minosityandstellarmassfunctionsuptoz∼1 range from 10% to 100% of the local den-
(Pozzettietal.2003; Fontanaetal.2004; sity of luminous ETGs (Cimattietal.2004;
Glazebrooketal.2004;Bundyetal.2006). Daddietal.2005a; Saraccoetal.2005). Old,
However, due to the strong cosmic vari- passive,massiveETGsmayexistevenatz>2
ance, the number density of high-z ETGs is (Labbe’etal.2005), but their fluxes are typi-
still so poorly constrained that the current callybeyondthecurrentspectroscopiclimits.
Cimatti:Early-typegalaxyevolution 285
4. Downsizing
Distant ETGs have a higher stellar mass-
to-optical light ratio (M/L ) than late-type
R
star-forming galaxies at the same redshift
(Fontanaetal.2004) (Fig.3). Moreover, the
M/L showsanoveralltrenddecreasingfrom
R
low to high redshifts, but with a substantial
scatterthatcanbeinterpretedasduetoawide
rangeofformationredshiftsand/orstarforma-
tion histories (see also McCarthy et al. 2001; Fig.3. M /L ratio as a function of redshift
stars R
Cimattietal.2003).ThetypicalM/LRofmore forthespectroscopicallyidentifiedETGsinthethe
luminous objects (M < −22) is significantly K20 sample. Large hollow and small filled circles
R
largerthanthatofthefainterones(MR >−22) indicate MR < −22 and MR > −22 respectively.
atlowandintermediateredshifts(Fig.3).This Lines show Mstars/LR values predicted for a set of
impliesthat, while the M/L of the luminous exponentially–decaying star formation rate models
R
withz = 3(thinlines)orz =20(thicklines)and
populationis consistentwith either very short f f
time-scalesofτ=0.1Gyr(dashedlines)andτ=3
star-formation time-scales (τ) or high forma-
Gyr(solidlines)(SalpeterIMFandnodustextinc-
tion redshifts (z ≥ 3) (and some objects ap-
f tion)(Fontanaetal.2004).
pear to requireboth),the less luminouspopu-
lationexperiencedamorerecenthistoryofas-
sembly,asindicatedbythelargerτandlower
low-mass ETGs respectively. Although, the
z required to reproduce the typical M/L
f R relation betweenthe measured M/L evolution
(Fig.3).Thus,themostluminousandmassive
and mass is partially due to selection effects
ETGs reach the completion of star formation
because the galaxies are selected by luminos-
first,whilelessmassiveoneshaveamorepro-
ity, not mass, the M/L – mass relation still
longed star formation activity till later times.
holdsevenif thisselectioneffectistakeninto
This downsizing scenario was first noted by
account.Theseresultsindicatethatthefraction
(Cowieetal.1996),anditisnowsupportedby
of stellar mass formed at recent times ranges
severalresults which suggestthat galaxyevo- from <1% for M > 1011.5M to 20%-40%
⊙
lution is strongly driven by the galaxy mass. below M ∼ 1011M , and that there is no
⊙
It is interesting to note that recent studies of
significantdifferencebetweentheevolutionof
ETGsatz∼0havereachedconsistentconclu-
massivefieldandclusterETGs(Fig.4).
sions(Thomasetal.2005).
Recently it has been found that while the
Additional clues on downsizing come bright end of the cluster colour-magnitude
from the most recent results on the evo- relation is already built at z ∼ 0.8, the
lution of the Fundamental Plane (FP) faint end is still in the process of build-up
both in the field and in distant clusters (Tanakaetal.2005) (see also De Lucia et al.
(vanderWeletal.2004; Treuetal.2005; 2004). In contrast to this, the bright end of
diSeregoAlighierietal.2005; the field colour-magnitude relation has been
Jorgensenetal.2006). The FP at z ∼ 1 built all the way down to the present-day,but
showsalreadya remarkablesmallscatter and, the build-up at the faint end has not started
with respect to the local FP, an offset and a yet. This suggests again the downsizing evo-
possible rotation.The evolutionof the overall lutionary pattern: massive galaxies complete
FP can be represented by a mean change in their star formation first and the truncation of
the effective mass-to-light ratio, but with a starformationispropagatedtosmallerobjects
rate depending on the dynamical mass, being as time progresses. This trend is likely to de-
slower for larger masses. This differential pendonenvironmentsincethebuild-upofthe
evolutionisconsistentwithstellarpopulations colour-magnituderelation is delayed in lower
thatformedatz > 2andz ∼ 1forhigh-and densityenvironments.Theevolutionofgalax-
f f
286 Cimatti:Early-typegalaxyevolution
Fig.5. Stellar mass functions in the K20 sam-
ple. Different symbols correspond to different
methods adopted to estimate the stellar mass
(Fontanaetal.2004). In the highest redshift bin,
largehollowcirclescorrespondtoobjectswithonly
Fig.4. ThedifferentialevolutionoftheM/L ratio
B spectroscopicredshift.Thethicksolidlineisthelo-
withrespect tothat ofoldclustermassivegalaxies
calgalaxymassfunction(Coleetal.2001).
atthesameredshiftforETGsatz∼1fromtheK20
survey(diSeregoAlighierietal.2005).
iestookplaceearliestinmassivegalaxiesand
inhigh-densityregions,anditisdelayedinless
massivegalaxiesandinlowerdensityregions.
Other evidences of the downsizing sce-
nario come from the evolution of the stellar
mass function (Fig.5-6). Several surveys
consistentlyindicatethatthehigh-masstailof
the stellar mass function (which is populated
mostly by ETGs; Fig.6) evolves weakly from
z∼0.8−1.0toz∼0comparedtothelocalstel-
larmassfunctionandslowerthanthelow-mass
tail (Fontanaetal.2004; Droryetal.2005;
Caputietal.2006; Bundyetal.2006;
Pannellaetal.2006;Franceschinietal.2006).
Recently, Bundy et al. (2006) confirmed
Fig.6.StellarmassfunctionsintheK20samplefor
this downsizing evolution of the stellar mass
differentspectraltypes.Emptypointscorrespondto
functionpergalaxytype uptoz ∼ 1 byusing
latespectraltypes,filledtoETGspectraltypes.The
the DEEP2 spectroscopicsample.In addition,
solidlinesshowtheSchechterfitstothetotalmass
theyidentifiedamasslimit,M ,abovewhich
Q function of the K20 sample at the corresponding
starformationappearstobe“quenched”.This redshifts(Fontanaetal.2004).
threshold mass evolves as ∝ (1+ z)4.5, i.e. it
decreases with time by a factor of 5 across
the redshift range of 0.4 < z < 1.4, while wherethedownsizingseemstobeaccelerated.
no significant correlation has been found be- AlsothespectralpropertiesofETGssupporta
tween downsizing and environment, with the downsizingscenario:Fig.7showsthat,atfixed
exception of the most extreme environments redshift,theD4000continuumbreakofETGs
(i.e.theoneshostinghighernumbersofETGs), (taken as an age indicator of the stellar popu-
Cimatti:Early-typegalaxyevolution 287
5. Comparisonwithmodel
predictions
These high-z old and massive ETGs are
very rare objects in most N-body + semi-
analytic simulations published to date. The
predicted number densities are much lower
than the observed ones (up to a factor
of 10 in some cases), with this discrep-
ancy becoming larger for increasing redshift
(Somervilleetal.2004).However,asthereare
enough dark matter halos to host these unex-
pected massive galaxies (Fontanaetal.2004),
what becomes crucial is to understand how
the physics and evolution of baryons within
the halos can explain the properties of high-z
ETGs,andhowitispossibletobuild-upmas-
siveETGsathigherredshiftsthanexpected.
Fig.7. The 4000 Å break (D4000) in K20 ETG
The models incorporating a new treat-
composite spectra(Mignolietal.2005).TheETGs
ment of the feedback processes (includ-
ineachredshiftbinhavebeendividedintwoequally
populated groups, according to their luminosity. ing AGN), or the ones based on hydro-
Circlesandtrianglesindicatethebrighterandfainter dynamic simulations, seem to obtain a
subsamples respectively. The dashed line indicates better agreement with the observations
theD4000valuemeasuredintheaveragespectrum (Granatoetal.2004; Mencietal.2004;
of all ETGs, whereas the dashed-dotted line is the Nagamineetal.2005; Silvaetal.2005). The
D4000 measured in the SDSS composite of ETGs most recent simulations (Springeletal.2005)
(Eisensteinetal.2003).
are also capable to predict a downsizing
evolutionarypatternforthestarformation,but
mostofthemassassemblystilloccursatmod-
eratelylowredshifts(0<z<1),andaspecific
lation) is stronger in more luminous systems,
comparison with the observed properties and
suggesting a higher formation redshift of the
evolutionof theETGshasnotbeenpublished
mostmassiveETGs(Mignolietal.2005).
yet (DeLuciaetal.2006; Boweretal.2005).
Additionalevidence supportingthe down-
It is interesting to mention that very recent
sizing scenario comes from the results on the
hydrodynamic simulations showed that it is
star formation history properties. Feulner et
possibletobuildarealisticmassive(M∼1011
al. (2005) found that, at all redshifts, lower
M ) giant elliptical galaxy with a plausible
mass galaxies show higher specific star for- ⊙
formation history without requiring neither
mation rates (SSFR = SFR/M) than higher
recent major mergers nor feedback from
massgalaxies,andthatthehighestmassgalax-
supernovae or AGN, but simply starting
ies containthe oldeststellar populationsat all
from appropriate ΛCDM initial conditions
redshifts. With increasing redshift, the SSFR
(Naabetal.2005).
for massive galaxies increases by a factor of
10, reaching the era of their formation at z ∼
2 and beyond. These findings can be inter-
6. Searchingfortheprogenitors
preted as evidence for an early epoch of star
formation in the most massive galaxies and The spectral and color properties of the
for ongoing star formation activity in lower distant, old ETGs discussed in Section
massgalaxies.Similarresultshavebeenfound 3 imply unambiguously a star formation
also inotherrecentworks(Juneauetal.2005; history with: strong (> 100 M yr−1) and
⊙
Perez-Gonzalezetal.2005). short-lived (τ ∼0.1-0.3 Gyr) starburst (where
288 Cimatti:Early-typegalaxyevolution
SFR∝ exp(t/τ)), the onset of star formation the same redshift range of 1.4 < z < 2.5
occurring at high redshift (z > 2 − 3), and (Daddietal.2005b; McCarthyetal.2004;
f
a passive–like evolution after the major star- Kongetal.2005).
burst (Cimattietal.2004; Daddietal.2005a;
McCarthyetal.2004; Longhettietal.2005).
In addition, the ETG precursors should also 6.2.Otherprecursorcandidates
have the strong clustering expected in the
ΛCDM models for the populations located Other massive galaxy progenitor candi-
dates at z > 2 characterized by old stellar
in massive dark matter halos and strongly
populations and/or strong star formation,
biased environments. Recent observations
large stellar and/or dynamical masses and
have indeed uncovered galaxies matching the
high metallicity include the submm/mm-
aboverequirements.
selected galaxies (Chapmanetal.2005;
Dannerbaueretal.2004;
6.1.BzK–selectedgalaxies Swinbanketal.2004; Greveetal.2005),
the “Distant Red Galaxies” se-
The K20 survey unveiled a population of
lected with J − K > 2.3
star-forming galaxies at z ∼ 2 which have s
(Franxetal.2003; vanDokkumetal.2004;
strong star formation rates of ∼ 200 − 300
Papovichetal.2006), the optically-selected
M yr−1, substantial dust extinction with
⊙ BM/BX/LBG systems with bright K-
E(B − V) ∼ 0.3 − 0.6, stellar masses up
band fluxes (Adelbergeretal.2005;
to 1011 M , merging-like rest-frame UV
⊙ Reddyetal.2005; Shapleyetal.2004), the
morphology becoming more compact in the
IRAC Extremely Red Objects (IEROs)
rest-frame optical, strong clustering with
(YanH.etal.2004), the HyperEROs
r ∼8–12 Mpc depending on the adopted
0 (Totanietal.2001; Saraccoetal.2004),
redshift distribution, and possibly high metal-
andafractionofdustyEROs(Yanetal.2005).
licity (Daddietal.2004a; Daddietal.2004b;
Daddietal.2005b; deMelloetal.2004;
Kongetal.2005). These starburst galaxies 6.3.TheGMASSproject
can be efficiently selected at 1.4 < z < 2.5
with a “reddening-free” color–color dia- GMASS (“Galaxy Mass Assembly ultra-deep
gram (z-K vs. B−z; the BzK selection) SpectroscopicSurvey”)isanewongoingspec-
s
(Daddietal.2004b). The BzK–selected star- troscopic project based on an ESO Large
bursts with K < 20 are on average more Program (PI A. Cimatti) aimed at doing ul-
s
reddened, massive and star-forming than tradeep spectroscopy with VLT+FORS2 of
the optically-selected star-forming galaxies, galaxiesselectedat4.5µmwithSpitzer+IRAC
but they show a significant overlap with adoptingtwosimplecriteria:m(4.5µm)< 23.0
other star-forming galaxy populations in the (AB) and z > 1.4. The integration times
phot
same redshift range selected based on other are very deep (15-30 hours per spectroscopic
optical/near-IR criteria (Reddyetal.2005) mask). The main scientific driver of GMASS
or on mid–infrared - to - radio selections isthespectroscopicidentificationandstudyof
(Daddietal.2005b; Dannerbaueretal.2006). thehigh-zprogenitorsofthemassiveETGsina
The properties of the BzK–selected starbursts waycomplementarytoothersurveysandwith
suggest that these galaxiesare massive galax- a selection more sensitive to the rest-frame
ies caughtduring their major activity of mass near-IRemission(i.e.stellarmass)uptoz∼3.
assembly and star-formation, possibly being
theprogenitorsofthepresent-daymassiveand
metal-richETGs.Atthestellarmassthreshold Acknowledgements. I am grateful to Emanuele
of M > 1011M⊙, the numberdensity of these DaddiforhiscommentsandtotheK20andGMASS
BzK–selected starbursts is comparable with collaborators for their invaluable contributions to
that of old passively evolving galaxies in theprojects.
Cimatti:Early-typegalaxyevolution 289
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