A possible solution to the [alpha/Fe]-sigma problem in early type galaxies within a hierarchical galaxy formation model

0.47 MB·English

by  Francesco Calura

Mon.Not.R.Astron.Soc.000,1–6(—-) Printed27January2011 (MNLATEXstylefilev1.4)
α σ
A possible solution to the [ /Fe]- problem in early type
galaxies within a hierarchical galaxy formation model
⋆
F. Calura1,2 , N. Menci3
(1) Jeremiah Horrocks Institute for Astrophysics and Supercomputing, Universityof Central Lancashire, Preston PR1 2HE, UK
1 (2) INAF, Osservatorio Astronomico di Trieste, ViaG.B. Tiepolo 11, 34131 Trieste,Italy
1 (3) INAF, Osservatorio Astronomico di Roma, via Frascati 33, I-00040 Monteporzio, Italy
0
2

n
a Accepted —-.Received—-;inoriginalform—-
J

5
ABSTRACT
2
The most massive elliptical galaxies apparently formed the fastest, because the ratio

] of α elements (such as oxygen) to iron is the smallest. In fact, iron is mainly pro-
O duced from type Ia supernovae on a timescale of ∼ 0.1−1 billion years, while the α
C elements come from massive stars on timescales of a few tens of million years (Mat-
. teucci 1994).Reproducing sucha α/Fe correlationhas long been a severeproblemfor
h
cosmological theories of galaxy formation, which envisage massive galaxies to assem-
p
ble gradually from smaller progenitors, and to be characterized by a star formation
-
o history too much extended towards late cosmic times. While it has recently become
r clearthatfeedback fromActive Galactic Nuclei(AGN) activity playa roleinthe late
t
s quenching of star formation(e.g. Cattaneo et al. 2009),and that early star formation
a history in the galaxy progenitors affect the α/Fe ratio (Calura & Menci 2009), major
[
mergers alone cannot enhance the star formation in the high-redshift progenitors to

1 the levels required to match the steepness of the observed α/Fe correlation (Spolaor
v et al. 2010). Here we report that the inclusion of the effects of fly-by ’harassments’,
8 thattriggerlowerlevelstarbursts,combinedwiththeAGNquenchingofthestarburst
3 activity, considerably enhances the capability to account for the observed α/Fe ratio
9 in ellipticals within cosmologicalgalaxyformationmodels . The criticaldifference be-
4 tweentheearlierworkandthepresentresultistheeffectofstarburstsdrivenbyfly-by
.
1 encountersthat would havebeen very commonamongstthe high-redshiftprogenitors
0 of massive galaxiesand which wouldhave boosted star formationin the first 2 billion
1 years after the Big Bang, combined with quenching of the burst activity within the
1 first 3-4 Gyr.
:
v Key words: Galaxies: formation and evolution; Galaxies: abundances.
i
X
r
a
1 INTRODUCTION (Thomas et al. 2005; Nagashima et al. 2005; Spolaor et al.
2010)inaccountingfortheabovecorrelationisinfactdueto
Thecorrelationbetweenstellarα/Feandvelocitydispersion
such a ”downsizing” aspect. In fact, the precision measure-
observed in local ellipticals (Trager et al. 2000; Thomas et
mentsofthepowerspectrumoftheinitialdensityperturba-
al. 2005; Bernardi et al. 2006; Graves et al. 2007; S´anchez-
tions (Percifal et al. 2007) provide evidence for large-scale
Bla´zquezetal.2006, Spolaoretal.2010; Zhuetal.2010) is
fluctuations to be smaller, on average, compared to small-
generallyreferredtoasoneoftheevidencesofthe“downsiz-
scale ones - in accord to what expected in a Cold Dark
ing”pattern oflocal galaxies, apatternwhich isobservable
Matter (CDM) scenario; this implies a hierarchical build-
up to high redshift (e.g. Cowie et al. 2006; Kodama et al.
up of dark matter (DM) haloes, where small-mass galaxies
2004;Clementsetal.2008)indicatingthatthemoremassive
collapse first, and later assemble to form massive ellipticals
the galaxy, the shorter the star formation timescale (Mat-
(De Lucia et al. 2006). The deep physical origin of the dis-
teucci 1994; Renzini 2006; Pipino et al. 2010; Rogers et al.
agreementbetweentheobservedandthepredictedα/Fe−σ
2010).
correlation has been often considered as an evidence for a
The failure of cosmological galaxy formation models
fundamental flaw of cosmological CDM models of galaxy
formation (Thomas et al. 2005; Baugh 2006).
⋆ E-mail:[email protected] To produce downsizing within standard galaxy forma-
(cid:13)c —-RAS
2 F. Calura, N. Menci
tion models, some physical mechanisms are required which 2 THE MODEL
can enhance star formation in massive galaxies at early
times, possibly quenching it at late times. In cosmological To study the integrated abundances of local ellipticals, we
galaxy formation, environmental effects are likely to play a usethesamemodelasdescribedinCalura&Menci(2009).
major role in producingsucha scenario. Theprogenitors of We defer the reader to that paper for further details. Here
massive galaxies formed from biased regions of the density we summarize the main features of ourmodel.
field,andfly-byeventsandmerginginsuchregionsmaytrig- Galaxy formation is driven by the collapse and growth of
gerstarburstsatearlycosmictimes(t≤2.5Gyr),whenthe DM halos, which originate by gravitational instability of
Universe was denser and the rate of galaxy encounters was overdense regions in the primordial DM density field, a
at least 5-10 times higher than the present. A fundamen- random, Gaussian density field with CDM power spectrum
talmechanism toquenchstarformation inmassivegalaxies within the ”concordance cosmology” (Spergel et al. 2007)
is linked to their Active Galactic Nuclei (AGN). Feedback for which we adopt round parameters ΩΛ = 0.7, Ω0 = 0.3,
frompowerfulAGNmayswitchoffstarformationwhenthe baryonic density Ω = 0.04 and Hubble constant (in units
b
progenitors assembled into the main progenitor of the final of100km/s/Mpc) h=0.7. Ascosmictimeincreases, larger
elliptical (Cattaneo et al. 2009). and larger regions of the density field collapse, and eventu-
These processes may concur in yielding stellar popu- allyleadtotheformationofgroupsandclustersofgalaxies,
lations in massive galaxies characterized by a prompt star which grow by merging with mass and redshift dependent
formationatearlyepochsfollowedbyaquenching,inagree- rates provided by the Extended Press & Schechter formal-
mentwithwhatsuggestedbytheobservedα/Fe−σcorrela- ism (Press & Schechter1974).
tion. Thus,testing whetherthelatter processes may in fact The clumps included into larger DM halos may survive as
explaintheabovecorrelationiscrucialforassessingthecon- satellites,mergetoformlargergalaxiesduetobinaryaggre-
sistency of CDM models, and requires developed modelling gations,orcoalesce intothecentraldominantgalaxydueto
which includesall thephysical processed detailed above. dynamicalfriction.Theabovecomplexdynamicalevolution
of galaxies is followed through a Monte Carlo simulation of
thecollapseandsubsequentmerginghistoryofthepeaksof
In a previous paper (Calura & Menci 2009, hereinafter
theprimordialdensityfield,whichenablestogenerateasyn-
CM09), we investigated the chemical properties of local
theticcatalogueofmodelgalaxiesandoftheirpastmerging
galaxies within a cosmological hierarchical clustering sce-
history.
nario through a semi-analytic model (SAM) of galaxy for-
For each model galaxy, we follow the evolution of its gas
mation. We used a hierarchical semi-analytic model where
and stellar content; the gas which has radiatively cooled in
chemicalevolutionwascomputedbytakingintoaccountthe
the galactic haloes (with mass Mg) partly condenses into
stellar lifetimes, a significant step forward with respect to
stars at a rate ψ ∝ Mg/td on long timescales, td ≈ 2−10
theinstantaneousrecyclingapproximation.However,chem-
GyraccordingtotheKennicuttlaw(Kennicutt1998).Star
ical abundances were computed by considering the total
formationcanalsooccurthroughinteraction-triggeredstar-
(summedoverallprogenitors)starformationhistoryofeach
bursts(ITS),drivenbymergingorbyfly-byeventsbetween
modelgalaxyandassumedanaverageinterstellarα/Fe.This
galaxies.Suchastarformationmodecanconvertlargefrac-
treatmentwassuitedtothestudyofinterstellarabundances,
tions(upto100%inmajor mergingevents)ofthecoldgas
comparable tothe ones observed today in stars in thesolar on short timescales (≈106 yrs), and provides an important
neighbourhood, but in many cases it lead us to underesti-
contributiontotheearlyformationofstarsinmassivegalax-
mate the integrated stellar α/Fe abundances measured in
ies (Menci et al. 2004a).
local early typegalaxies.
Themodel also includesa detailed treatment of thegrowth
of supermassive black holes at the centre of galaxies, of
In this paper, by means of the same SAM, we perform thecorrespondingAGNactivitypoweredbytheinteraction-
a thorough study of the the average stellar α/Fe in early triggered inflow of cold gas onto theblack holes, and of the
typegalaxies, taking into account thecontribution from all AGNfeedbackontothegalacticgas.Duetothecrucialrole
the single progenitors. Although computationally more ex- ofAGNfeedbackin theaffectingthestarformation history
pensive, this constitutes a step forward with respect to our of massive galaxies, this sector of the model has been ex-
earlier work (CM09), since thanks to the inclusion of the tensive tested; the predicted QSO luminosity function and
contribution of all the progenitors, our estimates of the in- X-rayluminosityfunctionandemissivityhavebeenchecked
tegrated abundances are more accurate. A second enhance- against observation over a wide range of redshift 0≤z ≤5
ment with respect to our previous work is the calculation andbolometricluminosities1043 ≤L/ergs−1 ≤1046(Menci
of the luminosity-weighted abundances (against the mass- et al. 2003; 2004a); thedescription of theAGN feedback as
weightedonesusedinthefirstpaper),anotheraspectwhich resultingfromthesweepingofgalacticgasbytheblastwave
improvesthecomparisonofourresultstolocalobservations. produced by the injection of the AGN radiative energy has
Theaimofthispaperistoshowtherelativerolesofthevar- beentestedbycomparingtheabsorptionpropertiesofAGNs
ious processes in shaping the α/Fe−σ correlation, stress- (as a function of both luminosity and redshift) with recent
ing the importance of some processes which have been ne- observation(Mencietal.2008);theconsistencyofthemod-
glectedinpreviousstudies,suchasinteraction-triggeredfly- elling of the AGN feedback with the color distribution of
by eventsand merging-triggered starbursts. galaxies has been discussed in Menci et al. (2006).
This paper is organized as follows. In Sect. 2, our model As shown in Fontana et al. (2004), the inclusion of pro-
is briefly described. In Sec. 3, we present our main results, cesses such as ITS and AGN feedback within the SAM af-
which are then discussed in a broader context in Sect. 4. fectsmostlythemodelrenditionofthestellarmassfunction
(cid:13)c —-RAS,MNRAS000,1–6
Apossiblesolution to the [α/Fe]-σ probleminearlytype galaxieswithina hierarchicalgalaxyformationmodel 3
athighredshift.Allmodelsdiscussedinthispaperhaveba- essential tounderstandthemajor role offly-byinteractions
sically thesame stellar mass function at z∼0. in driving thecorrelation between α/Fe and σ.
Galaxyluminositiesindifferentbandsarecomputedbycon-
volving the computed star formation history (SFH) with
synthetic spectral libraries of stellar populations (Bruzual
4 DISCUSSION
& Charlot 2003).
Tocompareourpredictiontotheobservedα/Fe-σ relation, Our results show that a standard CDM galaxy formation
asperformedinourpreviouspaper,weselectellipticalgalax- model including ITS, consisting in both merger-triggered
ies from the SAM synthetic catalogue on the basis of their andfly-bytriggered starbursts,andAGNfeedbackcannat-
present-day (B-V) colour. In specific, we consider elliptical urally lead to shorter star formation timescales in larger
galaxies all the systems with present (B-V)≥0.85 (Roberts galaxies, i.e. to star formation histories reversed with re-
& Haynes 1994). For any selected galaxy, we compute the spect totheir mass-assembly histories.
chemical evolution a posteriori by means of detailed chem- The absolute novel ingredient of our model consists in the
ical evolution equations taking into account the stellar life- simultaneous inclusion of both the starbursts triggered by
times, fundamental for studying the chemical evolution of fy-by events, absent in any previous investigation of this
Fe. Thus, the SFH, the gas content, and the gas inflow fundamental property of early-type galaxies, and the AGN
and outflow histories of all the progenitors of the consid- feedback related tothe active Quasar phaseof AGNs.
ered galaxy provided by the SAM constitute the input for The general problem of quenching the star formation his-
calculating the chemical evolution. This constitutes a step tories in massive galaxies have been investigated in several
forward with respect to our earlier work (CM09), where we studies(e.g.Gaboretal.2010)andisofgreatinterest.Fig.2
consideredonlythetotal(summedoverallprogenitors)star isusefultobetterunderstandhowthefeaturesofourmodel
formation historyofeachmodelgalaxy andassumedanav- act directly on the star formation histories of single galax-
erage interstellar α/Fe. ies. In each panel of this figure, we show the SFH of three
galaxies drawn from the sample including ITS and AGN,
includingAGNbutnotITSandfromtheoneincludingITS
3 RESULTS but not the effects of AGNs. The three galaxies shown in
each panelpresentthesame mergerhistories. Itis worth to
The average stellar α/Fe may be calculated as:
stress that in each case, the final stellar masses are not the
<α/Fe>∗≃ R(α/Fe)(t)ψ(t)LSVSP(t0−t)dt (1) saanmdeA,GsinNcecatnhecionncsliudseiroanbolyf pahltyesrictahleinsgtarerdfioernmtastuicohnahsisIoTrSy
R ψ(t)LV(t0−t)dt
and the feedback history of the systems. The final stellar
i.e. it is the time integral of the interstellar (α/Fe) abun- mass of each model is reported in Fig. 2 (see caption for
dance over the star formation history, represented by the furtherdetails).
star formation rate ψ(t), multiplied by the luminosity ItisimportanttonotethatthesuppressionofITShaseffects
LSVSP(t0−t),whichistheluminosityatepocht0 ofasimple alsoontheefficiencyoftheAGNfeedback,since,according
stellar population of unitary mass with an age of (t0−t), tothemodelusedhereanddescribedinMencietal.(2008),
andisgivenbySSPphotometricmodelsasafunctionofage the cold gas which causes the interact-triggered starbursts
and metallicity (BC03), divided by the present total lumi- feeds also the nuclear activity. In fact, in every panel, both
nosity. models without ITS and AGN present higher initial star
InFig. 1,we show thepredictedα/Fe vsσ relation for our fromation rates with respect to the standard case. The ex-
selected elliptical galaxies, considering four different cases. planationofthisisthatinbothcases(noITSandnoAGN)
In Fig. 1, we report two different sets of data (Arrigoni et the AGN is inefficient in heating and ejecting gas from the
al. 2009; Spolaor et al. 2010), in order to have an idea on galaxies, resulting in higher amount of cold gas when the
the uncertainty of the slope of the observed α/Fe-σ rela- star formation has already turned on.
tion. In the standard case, we consider a model including Atthepresenttime,themodelswhichincludeITSbutwith-
interaction-triggeredstarburstsinthemostmassivegalaxies outAGNfeedbackshowSFRvalueslargerthanthoseshown
and Active Galactic Nuclei (AGN) feedback. In the second bytheonesdrawnfromthestandardsample.Globally,their
case, we suppressed the ITS and consider AGN feedback SFHs are less skewed towards early times with respect to
alone. In the third case, we suppressed AGN feedback and theones of themodel with both AGN and ITS. The effects
considerITSalone.SincetheITSconsistoftwocomponents of AGNs in quenching the star formation activity within
(merging and fly-by events), it may be interesting to sepa- the first 3-4 Gyr is evident from this figure. Finally, the
rate these two processes in order to appreciate the impor- model with AGN but without ITS presents very extended
tanceoffly-byevents.Inthefourthcase,weshowourresults SFHs,withratherflatandsmoothbehavioursandveryhigh
obtainedhavingsuppressedAGNfeedbackandfly-byinter- present-dayvalues.WithouttheeffectofITS,gasconsump-
actions,butconsideringonlymerger-triggeredstarbursts.In tion at early times is not very efficient and at later times,
thestandardcase,theslopeoftheobservedα/Fe-σrelation afterafewGyrs,thelargeamountsofcoldstar-forminggas
is satisfactorily reproduced. In the second and third cases, prevent AGNs from being effective and quenching the star
althoughwithourmodelacorrelation betweenα/Feandσ formation.Itisthereforeclearthat,inordertohaverealistic
is obtained, thepredicted relation is considerably shallower star formation histories for early typegalaxies, both effects
than the observed one. In the fourth case, the predicted of ITS and AGN feedback are necessary.
α/Fe-σ relation is overly flat with respect to the observa- Both effects of AGN feedback and ITS seem to play an im-
tions.Thecomparisonbetweenthethirdandfourthcasesis portant role in determining galactic downsizing. However,
(cid:13)c —-RAS,MNRAS000,1–6
4 F. Calura, N. Menci
the effect of the former seems slightly dominant, and this single progenitor, instead of a total SFR and an abundance
issupportedbythefact thatthepredictedα/Fe-σ relation averagedoveralltheprogenitors,wecansuccessfullyrepro-
withoutITSisslightlyshallowerthantheonewithoutAGN. ducetheobservedα/Fe-σrelationwithoutanymodification
Ourstudyoftheα/Fe-σrelationtellsusthatourgalaxyfor- of themain chemical evolution parameters.
mationmodelcannaturallyaccountforoneofthemostim-
portant aspects of the “downsizing” character of early type
galaxies. Downsizing (i.e. oldest stars are in most massive
galaxies,starformationshiftstolowermassesatlatetimes) ACKNOWLEDGMENTS
in hierarchical models will arise if thereisa minimummass
requiredtohavestarformationandamaximummassabove FCwishtoacknowledgeseveralinterestingdiscussionswith
whichfeedbackfromAGNsuppressesfurtherstarformation F. Matteucci, B. K. Gibson and P. Sanchez-Blazquez. We
(Sheth 2003; Croton & Farrar 2008). Together, these will alsoacknowledgeusefulcommentsbyananonymousreferee.
also lead to an [α/Fe]-sigma relation, since in principle, in
suchascenario,thedistributionoftheformation timescales
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Apossiblesolution to the [α/Fe]-σ probleminearlytype galaxieswithina hierarchicalgalaxyformationmodel 5
Figure 1. Stellar average α/Fe ratio vs velocity dispersion compared to local observations. The colour code represents the predicted
numberofgalaxieswithagivenα/Feandavelocitydispersionσ,normalisedtothetotalnumberofgalaxieswiththatvelocitydispersion.
The solidlines represent linear-regressionfits to the model. Thedashed lineand dash-dotted linerepresent linear-regressionfits to the
observationalα/Fe-σrelations,ascompiledbySpolaoretal.(2010;opendiamonds)andArrigonietal.(2009;solidcircles)respectively.
Top-left panel: our results have been computed by means of our standard assumptions, i.e. by taking into account both interaction-
triggered starbusts at high redshift and AGN feedback. Top-right panel: no interaction-triggered starbursts and fly-by interactions.
Bottom-left panel:noAGNfeedback. Bottom-right panel:noAGNfeedback, nofly-byinteractions.
1000
100
10
1
0.1
0.01
100
10
1
0.1
0.01
0.1 1 10 0.1 1 10
time (Gyr) time (Gyr)
Figure 2.Star Formationhistories of afew early-typegalaxies of our model.In each panel, we show the SFH of three galaxies drawn
fromthesampleincludingITSandAGN(solidblacklines),includingAGNbutnotITS(cyan dotted lines)andincludingITSbutnot
AGNs(dashedredslines).Ineachpanel,thethreesystemspresentthesamemerginghistory.Thefinalstellarmassesofthemodelsare
indicatedineachpanel.Fromtoptobottom,theindicatedmassesrefertothestandardcase,theNoITScaseandtheNoAGNcase.
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