Molecular Gas at High Redshift

0.14 MB·

by  Fabian Walter

Astron.Nachr./AN999,No.88,789–794(2006)/DOIpleasesetDOI!
Molecular Gas at High Redshift
FabianWalter1,ChrisCarilli2&EmanueleDaddi3
1 MaxPlanckInstitutfu¨rAstronomie,AufdemHu¨gel71,69117Heidelberg,Germany
2 NationalRadioAstronomyObservartory,Socorro,NM87801,USA
3 LaboratoireAIM,CEASaclay,91191Gif-sur-YvetteCedex,France
1
1 Received30May2005,accepted11Nov2005
0 Publishedonlinelater
2

n Keywords earlyuniverse,galaxies:ISM,galaxies:highredshift,radiolines:ISM
a
Inordertounderstandgalaxyevolutionthroughcosmictimesitiscriticaltoderivethepropertiesofthemoleculargascon-
J
tentofgalaxies,i.e.thematerialoutofwhichstarsultimatelyform.Thelastdecadehasseenrapidprogressinthisarea,
0
withthedetectionofmassivemoleculargasreservoirsathighredshiftsinsubmillimeter–selectedgalaxiesandquasars.In
2 thelattercase,moleculargasreservoirshavebeenquantifiedouttoredshiftsz>6,i.e.towardstheendofcosmicreion-

izationwhentheuniversewaslessthanoneGyrold.Therecentdiscoveryofmoleculargasinmorenormalgalaxieshave
O] extendedthesestudiesfromthemostextremeobjectsintheunviverse(SFR∼1000M⊙yr−1;quasarsandsubmillimeter
galaxies)tomore‘normal’ starformingsystemsat redshifts1.5–2.5(withSFR∼100M yr−1).However, detectingthe
C ⊙
moleculargasreservoirsofhigh–redshiftgalaxiesthatonlyhavemoderatestarformationrates(∼10M yr−1,similarto
. ⊙
h thefaintgalaxiesseenintheHubbleUltraDeepField)willlikelyhavetoawaitthecompletionofALMA.
p
- (cid:13)c 2006WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
o
r
t
s 1 Recent Progress inunderstanding Galaxy rate galaxies (LFIR ≥ 1011 L⊙) at z ∼ 2 (‘downsizing’,
a
Formation e.g.,LeFloc’hetal.2005,Smolcˇic´etal.2009).Anotherkey
[
findingis thetightrelationbetweenthe starformationrate

1
andthestellarmassforstarforminggalaxiesuptoredshifts
The last decade has seen dramatic advances in our under-
v
z∼3(e.g.Noeskeetal.2007;Elbazetal.2007;Daddietal.
2 standingofgalaxyformation.Cosmic geometry,the mass-
2 energycontentoftheUniverse,andtheinitialdensityfluc- 2007;Magdisetal.2010).High–redshiftstarforminggalax-
0 tuation spectrum, have been constrained to high accuracy ies are typically selected through optical/IR studies (e.g.,
4 Steidel et al. 1999, Daddi et al. 2004, van Dokkum et al.
(e.g., Spergel et al. 2007). Given the underlying distribu-
.
1 tion of dark matter, the main challenge of galaxy forma- 2006)
0 tionstudiesistounderstandhowthebaryonsassembleinto Besides the star forminggalaxies, a populationof pas-
1 the observedUniverse.In this context,structure formation sivelyevolving,relativelymassive(stellarmasses∼1010to
1
: throughgravitationalinstabilitieshasbeencalculatedinde- 1011 M⊙) galaxies(‘redanddead’),havebeendetectedat
v tailthroughnumericalstudies,andobservationallyverified z > 1,comprisingroughly50%ofthegalaxiesselectedin
i
X throughstudiesofgalaxydistributions(e.g.,Springeletal. near-IRsurveys(see,e.g.,reviewbyRenzini2006).These
r 2006). The history of star formation, and the build up of galaxiesmustformthemajorityoftheirstarsquicklyateven
a stellarmass,asafunctionofgalaxytypeandmass,arenow earlierepochs(e.g.,Wiklindetal.2008).
constrainedouttothetail-endofcosmicreionization,within Lastly,substantialpopulationsofsupermassiveblackholes
1GyroftheBigBang,andpushingtowardfirstlightinthe andgalaxiesare nowbeingdetectedroutinelybackto first
Universe(e.g.,Bouwensetal.2010). light and cosmic reionization (z ∼ 6.5). Populations in-
Theprincipleresultsofthesestudiescanbesummarized clude normal star forming galaxies, such as the Ly-α se-
asfollows.Thecomovingcosmicstarformationratedensity lected galaxies, with star formation rates of order 10M⊙
increasesbymorethananorderofmagnitudefromz ∼ 0 year−1(Taniguchietal.2005,Ouchietal.2009)aswellas
to 1, peaks around z ∼ 2 to 3, and likely drops gradually quasarhostgalaxies,oftenwithstarformationratesthatare
outtoz ∼8(e.g.,Hopkins&Beacom2006,Bouwensetal. 100 times higher (e.g., Wang et al 2008). The latter likely
2010).Thebuild-upofstellarmassfollowsthisevolution,as represent a major star formation epoch for very massive
doesthetemporalintegral(forz<2,e.g.Ilbertetal.2010). galaxieswithin1GyroftheBigBang.Mostrecently,GRBs
Theredshiftrangez ∼1.5to3hasbeencalledthe‘epochof areshowinggreatpotentialtoprobegalaxyformationoutto
galaxyassembly’,whenabouthalfthestarsintheUniverse z >8(Tanviretal.2009,Salvaterraetal.2009).
forminspheroidalgalaxies.Ithasbeenshownthatstarfor- Theseobservationshaveledtoanewmodelforgalaxy
mationshiftssystematicallyfromlowerluminositygalaxies formation. As opposed to either cooling of virialized, hot
(LFIR ∼ 1010 L⊙) at low redshift, to high star formation halogas,ormajor,gas-richmergers,thedominantmodeof
(cid:13)c 2006WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
790 Walter,Carilli&Daddi:MolecularGasatHighRedshift
starformationduringtheepochofgalaxyassemblymaybe plotwouldjustbeareflectionofthemoleculargasdensity
driven by cold mode accretion, or stream-fed galaxy for- historyoftheUniverse.
mation (e.g., Keresˇ et al. 2005, Dekel et al. 2009). Simu- The stellar to gas mass density ratio: A related, and
lationssuggestthatgasaccretioninearlymassivegalaxies similarlyfundamentalissueistocomparethestellarandgas
occursalongcold streamsfromthe filamentaryintergalac- mass of galaxies versus redshift. Two extremes scenarios
tic medium(IGM) thatnevershock-heat,butstreams onto canbeenvisioned.Atoneextreme,thegasbuildsuptolarge
thegalaxyatclosetothefree-falltime.Thiscoolgasforms values (approaching 1011 M ), and then a dramatic star-
⊙
athick,turbulentrotatingdiskwhichveryefficientlyforms burstistriggeredvia,e.g.,amajorgasrichmerger,whenthe
starsinafewgiantregionsinthedisk(e.g.,Bournaudetal. gasisrapidlyconvertedintostarsondynamicaltimescales
2007).Thestarformingregionsthenmigratetothegalaxy ∼ 108 years.At theotherextreme,a continuoussupplyof
center via dynamical friction and viscosity, forming com- gascomesfromtheIGM,coolingintomolecularcloudsand
pact stellar bulges. The process leads to relatively steady, formingstarsataratecomparabletothefree-fallrateofgas
active (∼ 100 M⊙ year−1) star formationin galaxiesover ontothegalaxyover∼Gyrtimescales.Thislattercasecor-
timescalesoforder1Gyr.Subsequentdrymergersatz <2 responds to the cold mode accretion model. This point is
lead to continued total mass build up, and morphological currentlyaddressedbyanumberofobservationalprograms
evolution,butlittlesubsequentstarformation(e.g.,Skelton thataimatstudyingstarforminggalaxiesatz∼2(Daddiet
etal.2009,VanderWeletal.2009,Robainaetal.2010). al.2010a,2010b,Tacconietal.2010,Genzeletal.2010,as
discussedinSec.4.3and4.4below).
Moregenerally,thestudyofthecoolmoleculargasphase
2 Moleculargas:the key to testing galaxy in‘typical’highhigh–redshiftgalaxies(withSFR∼100M yr−1)
⊙
formationmodels providekeyinsightintotheprocessofgalaxyformationin
anumberofways:
While progress has been impressive, the model above is
– A universal star formation law: The low order transi-
based almost exclusivelyon opticaland near-IRstudiesof
tions of CO provide the most direct means of measur-
the stars, star formation,and ionized gas. There remains a
ing the total molecular gas mass. These measurements
majorgapinourunderstandingofgalaxyformation,namely,
can then be compared to star formation rates to deter-
observationsof the cool gas, the fuel for star formation in
mine the evolution of the fundamental star formation
galaxies. In essence, current studies probe the products of
lawsrelatingcoolgasandstarformation.Isthereauni-
the process of galaxy formation, but miss the source. Nu-
versal relationship that governs the efficiency of con-
merous observational and theoretical papers have pointed
versionof moleculargasto starsin galaxiesof various
outthecrucialneedforobservationsofthecoolmolecular
types through cosmic time? What are the fundamental
gas feeding star formationin galaxies (e.g., Dressler et al.
physicalparametersbehindtherelationship?Forafirst
2009;Obreschkow&Rawlings2009).
attempt to address this topic see Sections 4.3 and 4.4
The molecular gas density history of the Universe: below.
Thekeytofuturestudiesofgalaxyformationistodetermine – Howdoestheratioofmoleculargastostellarmassevolve?
theevolutionofthemoleculargasdensityoftheUniverse. Thisratioistypically<0.1inthenearbyUniverse,even
Overthelasttwodecades,thestarformationhistoryofthe for gas rich galaxies such as the Milky Way. Observa-
Universe(SFHU)plothasbeenperhapsthemostfundamen- tions of a few the sBzK, and BX/BM galaxiesprovide
tal tool for studying galaxy formation. At the same time, evidence that this ratio may increase to > 1 at z ∼ 2
very detailed studies of star formation in nearby galaxies (Daddietal.2010a,Tacconietal.2010).
havereachedtwocriticalconclusions.First, starformation – Galaxydynamicsandstarformation:Molecularlineob-
relates closely with the molecular gas content, but has lit- servationsarethemostdirectmeansofstudyinggalaxy
tlerelationtotheatomicneutralgascontent(e.g.,Bigielet dynamics.Arethesesystemsdominatedbyrotatingdisks,
al.2009).Thisfactisverifiedathighz bythelackofevo- aspredictedbycoldmodeaccretion,ormajormergers?
lution of the cosmic HI mass density, as determined from What is the relative magnitude of ordered motion vs.
damped Lyα systems, over the same redshift range where turbulence?What are the stability criteria, and how do
the star formation rate density is increasing by more than theycomparetothedistributionofstarformation?
anorderofmagnitude(Prochaska&Wolfe2009).Second, – Blackhole–bulgemassrelation:Establishingagalaxy’s
once moleculargasforms,it formsstars, to first order,ac- mass through molecular gas dynamics can be used to
cordingtoastarformationlaw(e.g.,Kennicutt1998,Bigiel test the evolution of the black hole – bulge mass rela-
etal.2009,Leroyetal.2009,Krumholzetal.2009,Daddi tion back to the first galaxies. Indeed, in many cases,
etal.2010b,Genzeletal.2010).Inlow–densityenvironem- themolecularlineobservationsaretheonlymeanstoget
nts, the general idea is that, once a giant molecular cloud galaxydynamicsforgalaxieswithopticallybrightAGN
becomesself-gravitating,star formationproceedsvialocal (e.g., Walter et al. 2003, Shields et al. 2006, Riechers
processesinherenttotheGMC.Iftheuniversalstarforma- et al. 2008, 2009). The local relation suggests that the
tionlawweretoholdtohighredshift,thentheclassicSFHU formation of black holes and galaxies are closely tied,
(cid:13)c 2006WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim www.an-journal.org
Astron.Nachr./AN(2006) 791
buttheoriginofthisrelationshipremainsamystery(see ulargasintheseobjects(>1010M ,i.e.comparabletothe
⊙
Sec.4.2). submillimetergalaxies,e.g.,Solomon&VandenBout2005,
– ISMphysicsandchemistry:Detailedstudiesofthephys- Coppinetal.2008).Quiteremarkably,suchreservoirshave
ical conditionsin the ISM of primevalgalaxies can be nowbeendetectedinasizablesampleofquasarsatredshift
performed using numerous molecular line transitions, 6 and greater, i.e. at the end of cosmic reionization when
includingexcitationstudiesofCO,observationsofdense theUniversewaslessthanoneGyrold(Walteretal.2003,
gas tracers like CN, HCN and HCO+, and other key 2004,Bertoldietal.2003,Maiolinoetal.2007,Carillietal.
astrochemicaltracers.Such observationsdeterminethe 2007,Wangetal.2010).Resolvedimagingofafewquasar
temperature and density of the ISM, molecular abun- hosts (Walter et al. 2004, Riechers et al. 2008a, b, Riech-
dances and the fraction of dense gas. To date, only a ersetal.2009)implythatthemoleculargasisextendedon
few extreme high–redshiftsystems have been detected kpcscalesaroundthecentralblackhole.Itremainspuzzling
inthesedensegastracers(e.g.VandenBoutetal.2003, howsuchamountsofcoldmoleculargas,richin CandO,
Riechersetal.2006,2007a,2007b,Garcia–Burilloetal. can be distributedovermanykpcscales on short(few 100
2006) Myr)timescales.
The high FIR luminosity of the quasars, in conjunc-
tionwiththeirmassivemoleculargasreservoirsimplyhigh
3 Recent progress onmolecularlines
star formation rates in the host galaxies of SFR > 1000
observations inearly galaxies
M year−1.Thisfindingiscorroboratedbythedetectionof
⊙
[CII] emission in a quasar host at z=6.42 (Maiolino et al.
3.1 SubmillimeterGalaxies(SMGs)
2005, Walter et al. 2009). In fact, spatially resolved mea-
Surveys at submm wavelengths have revealed highly ob- surementsof the [CII] emission were able to constrainthe
scured,extremestarburstgalaxies(SFR>1000M year−1) sizeofthestarbursttobeemergingfromthecentral∼1.5kpc
⊙
athighredshift,theso–calledsubmillimetergalaxies(SMGs, ofthequasarhost.Thisimpliesextremelyhighstarforma-
e.g.,Smailetal.2007,Blainetal.2002).Subsequentstud- tion rate surface densities (∼1000M⊙yr−1kpc−2), at the
ieshavedetectedlargereservoirsofmoleculargasinthese maximumwhatisallowedtheoreticallybasedonEdington–
systems (> 1010M ; e.g., Frayer et al. 1998, 1999, Gen- limitedstarformationofaradiation–pressure–supportedstar-
⊙
zeletal.2003,Greveetal.2005,Tacconietal.2006,2008, burst(Walteretal.2009a).
Knudsenetal.2009,Ivisonetal.2010).Generalproperties The central black hole masses of the quasars that are
oftheSMGsaresummarizedinthereviewbySolomon& currently detectable reach masses of many 109 M⊙. If the
VandenBout(2005)andthedetectionstodatearesumma- local relation seen between the centralblack hole and sur-
rizedinFigure1(asdiscussedinSec.4.3). roundingbulgemass(e.g.,Magorrianetal.1998,Ferrarese
High–resolutionimagingofSMGsbyTacconietal.(2006, etal.2000,Gebhardtetal.2000)weretoholdatthesehigh
2008) have shown that their molecular gas reservoirs are redshifts,onewouldexpectbulgemassesoforder1012M⊙.
compact,withamediandiameterof∼4kpc.Thismorphol- Suchhighmassesexceeddynamicalmassestimatesofafew
ogy can be explained if these galaxies are the results of sources by more than an order of magnitude (Walter et al.
mergers, in which the molecular gas settles in the centre 2004,Riechersetal.2008a,b,2009),whichinturnindicates
of two interacting galaxies (leading to a starburst). Recent thatblackholesbuiltuptheirmassesmorequicklythanthe
COimagingofoneofthemostdistantSMGssuggeststhat stellarbulges(atleastinsomequasars).
cold mode accretion may also play a role in powering the
ongoingstarformation(Carillietal.2010). 3.3 ‘Normal’starforminggalaxies
Chapmanetal.(2005)haveshownthatthemedianred-
shift of SMGs is z∼2.3 — the existence of a substantial Morerecently,majorprogresshasbeenmadeondetecting
high–redshift(z>4)tail of the submillimetergalaxypopu- CO emission from more normal star forming galaxies at
lationthathostmoleculargashasrecentlybeenestablished highz.Thesecomeintheformofnear-IRandcolorselected
galaxy samples at z ∼ 1.5 to 3. The BzK color selection
byanumberofstudies(Schinnereretal.2009,Daddietal.
2009a,2009b,Carillietal.2010,Coppinetal.2010). techniqueinparticularhasrevealedasampleofstarforming
galaxiesatz ∼ 1.5to2.5,selectedatnear-IRwavelengths,
WhilethestudyoftheseSMGshasopenedanewwin-
with stellar massesinthe range1010 to 1011 M , andstar
dowontheopticallyobscuredUniverseathighredshift,the ⊙
formationrates∼ 100M yr−1 (Daddietal.2004).These
fact remains that these sources are rare, likely represent- ⊙
galaxieshaveaspacedensity> 10timesthosethesubmm
ing the formation of the most massive galaxies in merger-
galaxies.
drivenhyper-starburstsat high redshift,with gas depletion
timescales<0.1Gyr, Daddietal.(2008)showedthatstarformingBzKgalax-
ies (sBzK) uniformly contain molecular gas reservoirs of
comparable mass to the submm galaxies, and yet they are
3.2 Quasars
forming stars at ∼10 times lower rates. This leads to high
Likewise,surveysofmoleculargasinquasarhostgalaxies gas depletion times of up to ∼1 Gyr (see Fig. 1). Follow–
haverevealedthepresenceofmassivereservoirsofmolec- up observations showed that massive gas reservoirs have
www.an-journal.org (cid:13)c 2006WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
792 Walter,Carilli&Daddi:MolecularGasatHighRedshift
Fig.1 Comparison of molecular gas masses and total IR bolometric luminosities (taken from Daddi et al. 2010b)for
all high–redshifted systems detected in CO to date: BzK galaxies (red filled circles; Daddi et al. 2010a), z ∼ 0.5 disk
galaxies(redfilledtriangles;Salmietal.inprep.),z =1–2.3normalgalaxies(Tacconietal.2010;browncrosses),SMGs
(blueemptysquares;Greveetal.2005,Frayeretal.2005,Daddietal.2009a,2009b),QSOs(greentriangles;Riecherset
al. 2006,Solomon& VandenBout2005),localULIRGs (blackcrosses; Solomonet al. 1997),local spirals(black filled
square: Leroy et al. 2009;black filled triangles: Wilson et al. 2009).The two nearby starbursts M82 and the nucleusof
NGC253arealsoshown(seeDaddietal.2010bforreferences).Thesolidline(slopeof1.31intheleftpanel)isafitto
localspiralsandBzKgalaxiesandthedottedlineisthesamerelationshiftedinnormalizationby1.1dex.Forguidance,
twoverticallinesindicateSFR=2and200M yr−1intherightpanel.FormoredetailsseeDaddietal.(2010b).
⊙
beendetectedineachBzKgalaxythatwastargeted(Daddi mationefficiencies,andlowermoleculargasexcitation(e.g.
et al. 2010a). The implied gas fractions are very high and Dannerbaueretal.2009,Aravenaetal.2010).
the gas mass in these galaxies is comparable to or larger
than the stellar mass, and the gas accounts for 50–65%of
3.4 DifferentStarFormationLawsatHighRedshift?
the baryons with the galaxies’ half–light radius. New ob-
servationsofanevenlargersampleof‘normal’starforming Thedetectionofmoleculargasinsystemsspanningawide
galaxieshaveconfirmedand extendedthese findings(Tac- range of parametersnow allows one to investigatethe star
conietal.2010). formationlaw(i.e.thedependenceofthestarformationrate
Inafewcases,themoleculargasemissionintheseob- ontheavailablegas)athighredshift.InFigure1(left),we
jects could even be spatially resolved (see Tacconi et al. showthesummaryplotofDaddietal.2010b.Herethein-
2010, Fig. 2, for a particularly striking example) allowing fraredluminosities(aproxyfortheSFR)isplottedasfunc-
for a determination of the dynamical mass. A good con- tion of molecular gas mass (M ) of all high–redshiftob-
H2
strainton thedynamicalmass also allowsoneto derivean jectsdetectedinCOtodate,includingsubmillimetergalax-
independentmeasurementoftheCO–to–H2conversionfac- ies, quasars and BzK,BM/BX galaxies (see figure caption
tor(asdoneintheBzKsamplebyDaddietal.2010a).The for references). As discussed in Daddi et al. (2010b), two
interestingresultofsuchananalysisisthattheBzK’shave differentconversionfactors(toderiveM )havebeenused
H2
aconversionfactorsimilartotheGalaxy,i.e.afactorof∼5 forthehighredhiftgalaxypopulationsinthisplot:theGalac-
largerthanwhatisfoundinlocalULIRGs. tic one for the local spiral galaxies and the BzK/normal
In conclusion these systems likely represent the major galaxiesatz∼1.5–2.5andalower‘ULIRG’conversionfac-
starformationepochinearlymassivegalaxies(presumably torfortheULIRS,submillimetergalaxiesandquasars.Plot-
drivenbycoldmodeaccretion).Inthecurrentpicturethese ted this way, there are two sequences emerging (each of
galaxiesmayturnintopassivelyevolvinggalaxiesatz ∼1, which can be fitted with a power law of slope ∼1.3), one
which may eventually turn into ‘red and dead’ cluster el- for the ‘starbursts’ (ULRIGS, SMGs and QSOs) and one
liptical galaxies seen today. They are substantially differ- for the ‘disk’ galaxies. It should be stressed that the off-
entfromthequasarandsubmillimeterpopulationdiscussed set in sequencesseen hereis notonlydue to the choiceof
above in terms of their larger spatial sizes, lower star for- conversionfactors(e.g.,Fig.13inDaddietal.2010a).The
(cid:13)c 2006WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim www.an-journal.org
Astron.Nachr./AN(2006) 793
righthandpanelshowsthesamedata,butthey–axisshows areobservedtocomparethevariousgalaxypopulations.
the ratio of infraredluminosityto moleculargasmass(i.e. The EVLA will enable detailed studies of the ground
thethegasdepletiontime)plottedasafunctionoftotalin- transitionsofCOuptohighredshift.Ontheotherhand
fraredluminosity.Itisobviousfromthisfigurethatthegas ALMAwillenablestudiesofmoleculargastracersthat
depletion times in the BzKs/normal galaxies are about an arecurrentlyverydifficulttoobserve(e.g.[CII],[NII],
orderofmagnitudelongerthaninthesystemsthatarepre- [OIII] and other fine structure lines, e.g., Walter et al.
sumablyundergoingmergers(suchas the ULIRGs, SMGs 2009b).
andQSOs;seealsodiscussioninGenzeletal.2010).These
Acknowledgements. Itisourpleasuretoacknowledgeourcollab-
different star formation efficiencies appear to be regulated
orators,inparticularFrankBertoldi,PierreCox,HelmutDanner-
by the dynamical timescales of the systems (Daddi et al.
bauer, Roberto Maiolino, Roberto Neri, Dominik Riechers, Ran
2010b,Genzeletal.2010). Wang,andAxelWeiss.
These initial molecular gas observations of the differ-
entpopulationofstarforminggalaxiesduringtheepochof
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