The H$_{2}$ density within spiral and irregular galaxies at high redshift: estimating CO detection limits

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by  Mercedes Mollá

AstrophysicsandSpaceSciencemanuscriptNo.
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The H density within spiral and irregular galaxies at high redshift:
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estimating CO detection limits
MercedesMolla´ · EduardoHardy · A´ngelesI.D´ıaz
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Received:date/Accepted:date

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Abstract We have computed a grid of chemical evolution Inourchemicalevolutionmodelsthegasphasehastwo

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modelsforalargesetofspiralandirregulartheoreticalgalax- components, the diffuse and the molecular phases of gas,
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iesofdifferenttotalmass.Inourmodels,thegasphasehas whichwetreatseparately.Thisway,itispossibletofollow
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9 twocomponents,thediffuseandthemolecularone(H2).It theevolutionoftheexpecteddensityofH2withredshift.We
6 ispossible,therefore,tofollowthetime(orredshift)evolu- willshowhereourpredictionsabouttheH densityathigh
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tionoftheexpecteddensityoftheH phase.Wewillshow redshift,andwewillanalyzeiftheymightbedetectablewith
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the predictions of this gas density at hight redshift, which ALMA.
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0 mightbedetectedwithALMA,inthistypeofgalaxies.
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h Keywords Galaxies:Spirals · Galaxies: Irregulars ·
p Interstellarmedium·Moleculargas 2 Agridofmultiphasechemicalevolutionmodels
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o
r Wehavecomputedawidegridofchemicalevolutionmod-
st 1 Introduction elsforalargesetoftheoreticalspiralandirregulargalaxies
a ofdifferenttotalmass[5]byusingthemultiphasechemical
:
v The molecular interstellar medium plays a critical role in evolutionmodelfrom [3,4,6]. We have simulatedtheoreti-
Xi theevolutionofgalaxiessinceitprovidesthematerialfrom calmodelswith44differentradialdistributionsoftotalmass
which stars form. There are observations [9] of rotational and10star-formingefficiencies,withvaluesintherange[1–
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a transitionsofCOwithin1.0<z<6.5,mostlyinquasarsand 0], which we denoted by N from 1 to 10, for each one of
Early Universe Molecular Emission Line Galaxies (here- them.The440resultingmodelsprovidethe time evolution
inafterQSO’s and EMGs, respectively)which demonstrate forspecificradialregionswithineachgalaxymodeled,with
thatmolecularcloudsexistalreadyattheepochofthegalaxy thecorrespondingmassineachgaseousorstellarphase,as
formation.However,thecurrentsampleisscarceforz>3, wellastheirelementalabundances.Thistypeofmodelwas
where the study of the star and galaxy formation is spe- already applied to several individual spiral galaxies in [7],
cially interesting. ALMA is the instrument that may solve whereitwasshownthatthemodelisadequatelycalibrated
thisproblem. andthatitmaybeusedforgalaxiesofdifferentmorpholog-
icaltypesanddifferenttotalmasses.
M.Molla´
Astrof´ısicadePart´ıculas,CIEMAT,Avda.Complutense22, Thecodeassumesthatatotalmass,Mgal,initiallyasdif-
28040-Madrid(Spain),E-mail:[email protected] fuse gas, is located within a protogalaxy or halo. The gas
E.Hardy falls onto the equatorial plane forming out the spiral disk
NRAO,CasillaElGolf16-10,Santiago(Chile), ataratedefinedbyacollapsetime-scalewhichdependson
E-mail:[email protected]
M . In each zone, halo or disk, several radial regions are
TheNationalRadioAstronomyObservatoryisafacilityoftheNa- gal
defined.Wefollowseparatelytheevolutionineachregion.
tionalScienceFoundationoperatedundercooperativeagreementby
AssociatedUniversities,Inc. Theassumedstarformationinthehaloisapowerlawof
thediffusegas.Inthedisk,however,weassumethatthestar
A.I.D´ıaz
Dpto.deF´ısicaTeo´rica,UniversidadAuto´nomadeMadrid, formationoccursin two steps: first, molecular clouds(H2)
Cantoblanco,28049-Madrid(Spain),E-mail:[email protected] formfromthediffusegas(HI)alsofollowingapowerlaw.
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Fig.1 Radialdistributionsofmoleculargassurface densityatz=0.Eachpanelrepresents theresultsforadifferent rotationvelocitygalaxy,
whichmeansadifferentgalaxytotalmass.Curvesarefordifferentstarformationefficienciesintherange1(thehighestefficiency,N=1)to0
(thelowestefficiency,N=10)aslabelledinpanela).
Then, stars form by two means: 1) cloud-cloud collisions, TheevolutionofthetotalmassofH ingalaxiesisrep-
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and 2) by the interactionof the massive stars with the sur- resentedinFig2.Themagentaopenstarsareestimatesfrom
roundingmolecularclouds:Thisway,inourmodelsthegas [1]fornearbyQSOs.Thepredictedpresenttimemolecular
hastwocomponents:HIandH . gas masses are similar to those ones of local objects with
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a maximum for z=0 in agreement with the observational
Sincewehavecomputedseparatelythediffusegasevo- limit log(MH2) = 10.4, shown by a green solid line, ob-
tainedforthoseauthors.Theredfulldotsaretheestimates
lutionandthemolecularcloudsevolution,weobtainthera-
from[9]forEMGs.Theobservedmoleculargasmassesare
dialdistributionsofthemoleculargasmass,whichisrepre-
decreasing for decreasing z in agreement with the most of
sentedinFig.1.Thereweshowtheradialdistributionsofthe
our model results. Only the models with low star-forming
moleculargasdensityatthepresent-timefor4valuesofthe
efficiencies(coloredlines)arestillproducingH .
rotationvelocity,ortotalmassoftheGalaxy.Thus,panela) 2
showstheradialdistributionsproducedinalowmassgalaxy On the other hand, following that work [9], and taking
whiled)representsamassivegalaxy.Panelc)isthemodeled into account the estimates for the star formation rate and
galaxymostsimilartoourMilkyWayGalaxyandpaneld) molecular gas total mass, these objects might be bulges or
isanintermediatemassgalaxy.Ineachoneofthesepanels starbursts in centrally concentrated disks, but not elliptical
modelsfordifferentstarformationefficiencies,namedbyN, galaxiesasitwasinitiallyassumed.Ourmodelsarealsoin
arerepresented.Thelinesincolorcorrespondtothelowest agreementwiththis statement:the totalmass ofH forthe
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star-formingefficiencies(highN values),whichmaintaina mostmassivespiralgalaxies(withVrotintherange100-300
higher density than in cases where stars are formed more km.s−1)andhighstarformationefficiencies(N<5)areal-
efficiently (black lines). The distributions are close to ex- waysbelowoftheEMGsdataregionsinceinthosecasesthe
ponentialin the externalregionsbut are flatter at the inner molecular gas is very quickly consumed. Only the models
regions,mostlywhentheNislow,asisinfactobservedin formassive galaxies-panelsc) and d) with the lowesteffi-
Nature[8]. ciencies show values closer to the observationsof EMG’s.
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Fig.2 Theevolutionofthetotalmassofmoleculargasforsomeofourmodels.Eachpanelrepresentstheresultsforagivengalaxytotalmass(or
Vrot),using,foreachone,10differentSFRefficiencies.LinesmeanthesameasinFig.1Theblacklinescorrespondtothehighestefficiencies.
Atthishighredshift,theseparticularmodelsshowstarfor-
mationonlyatthecentralregionsandthereforetheywould
appearasconcentrateddisks,withalargequantityofH .
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3 TheCOdetectionlimits
Observation of emission from CO rotational transitions is
the usual means of detecting interstellar molecular clouds.
CO is a very stable molecule and the most abundant after
H .Following[2]levelsJ=4−5arethebestonestodetect
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moleculargasathighredshifts.
Inorderto verifyif ourdensitiesofmoleculargasmay
bedetectedwithALMA,weneedtotransformthelimiting
flux for this instrument to a limit in gas density. The flux
limitdependsontheobservedfrequency.Wetakethehigh-
estvaluefromthereport”SciencewithALMA”,andweuse
Fig.3 Theexpectedradialdistributionsofmoleculargasdensitynor-
Eq.1from[9],tocalculatetheminimumLCOnecessaryfor malizedtoRcar(halfopticalradius)forthreeredshiftsaslabelled.
detection.This luminosity is transformedinto gasmass by
using,asthoseauthors,aconstantconversionfactora =0.8
(which assumes independence of redshift z). The result is foreachtotalmassradialdistribution.Weshowintoppanel
shownasathicklineinFig3,whereweshowtheexpected a)theresultsforz=0(presenttime)whilepanelsb)andc)
radial distributions of molecular gas density normalized to representsthesamemodelsforthehigherredshiftsz=2.95
acharacteristicradiusRcar,takenasthehalfopticalradius, (t ∼1.2 Gyr)and z=4.25 (t ∼0.6 Gyr),respectively.For
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Fig.4 ExpectedevolutionofthefluxdensityofCOlinesasafunctionofzforspiralgalaxieswithfourrotationvelocities.Linemeaningasin
Fig.1
z=0mostmodeledgalaxiesshowdistributionswithvalues plausibleobjectstobeobservedwithALMAwillbethecen-
higher than the limiting value which may be detected with tersoflow-intermediateefficienciesmassivegalaxies.
ALMA downto the opticalradius,whileforhighredshifts
only our inner disks, mainly of low efficiencies (colored)
models,haveafluxdensitysufficientlyhightobedetected. 4 Conclusions
Therefore, if the red points of Fig. 2 would correspond to
– At redshift z > 3 only the central regions of galaxies
spiralgalaxies,theymustrepresent,effectively,theircentral
mightbeobservedinCOwithALMA
parts.
– EMGsarenotnecessarilythelargestorthemostmassive
InFig.4werepresenttheexpectedevolutionoftheflux
stellardisks.
density of CO lines as a function of z, for an integrated
galaxy,computedfromthemoleculargasdensitys (H )ob-
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tainedfromourmodels,transformedtofluxesbyusingthe
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