Evolution of metals and dust in the universe

Evolution of metals and dust in the universe

Pushpa Khare, 1

V. P.Kulkarni, 2

J. T. Lauroesch, 3

S. M. Fall, 4

D. G. York, 5

D. E. Welty, 5

A. P. S.Crotts, 6

J. W. Truran, 5

and

O.Nakamura 7

1

PhysicsDepartment, UtkalUniversity, Bhubaneswar, 751004,India

2

Departmentof PhysicsandAstronomy,UniversityofSouthCarolina,Columbia,SC29208,USA

3

Departmentof PhysicsandAstronomy,Northwestern University,Evanston,IL60208, USA

4

SpaceTelescopeScience Institute,Baltimore,MD21218, USA

5

Departmentof AstronomyandAstrophysics,Universityof Chicago,Chicago,IL60637,USA

6

Departmentof Astronomy,ColumbiaUniversity,NewYork, NY10027, USA

7

SchoolofPhysicsandAstronomy,UniversityofNottingham,NottinghamNG72RD,UK

Abstract. With theaimofdeterminingtheevolutionofmetals anddustin

theuniverse,wehaveobserved8DampedLyman-alphaAbsorbers(DLAs)with

theMultipleMirrorTelescopeandtheHubbleSpaceTelescopeat0:1<z<1:5,

includingseveralabsorbersdiscoveredin theSloanDigitalSky Survey. These

measurementshavemorethandoubledthesampleofZnandCrmeasurements

atz<1:5andaddedthreemeasurementsatz<0:4,wherenoneexistedbefore.

In contradiction with thepredictions of most chemical evolution models, our

data suggest that the global mean metallicity of DLAs, as measured by the

gasphaseabundanceofZn,atbestevolvesweaklywithredshiftovertherange

0:09 < z < 3:9 and does notseem to rise to the solar level evenat very low

redshifts. Thedust content, as determined by [Cr/Zn], does notshow much

changewithredshift.

Keywords: cosmology: observations{galaxies: abundances{galaxies: evolu-

tion{quasars: absorptionlines

Basedon

Khare,P.,Kulkarni,V.P.,Lauroesch,J.T.,York,D.G.,Crotts,A.P.S.andNakamura,O.,2004,

ApJ,616,86.

Kulkarni,V.P.,Fall,S.M.,Lauroesch,J.T.,York,D.G.,Welty,D.E.,Khare,P.andTruran,J.W.,2005,

1. Introduction

Observations of the UV luminosity density of galaxies in the redshift surveysand the

Hubble Deep Field studies indicate that the global average star formation rate and,

therefore, the chemical enrichment rate of galaxies was high at z 1.5. Most cosmic

chemical evolution models (e.g., Malaney and Chaboyer 1996; Pei, Fall, and Hauser

1999;Somerville,PrimackandFaber2001)thus,predictthattheglobalmeaninterstellar

metallicity in galaxies risesfrom nearly zeroat high redshifts to nearly solarat z =0.

Itisofgreatimportancetodeterminewhethertheobservedglobalmeanmetallicities of

galaxiesagreewithpredictionsofthecosmic chemicalevolutionmodels.

DampedLyman-alphaabsorptionsystems(QSOabsorptionsystemswithHIcolumn

density210 20

cm 2

;hereafterDLAs)havebeenusedtodeterminetheevolutionof

thegasphaseabundancesintheuniversebecause(1)itisbelievedthatDLAsconstitute

mostof theneutralgasin thegalaxies athighredshifts,enoughtoform allstarsvisible

today (Wolfe et al. 1995), (2) in the absence of selection eects, DLAs are expected

toprovideanunbiasedsampleof normalgalaxiesastheyare selectedonlythroughthe

presenceoflargeamountsofneutralhydrogen,(3)DLAshavesuÆcientlylargeamountsof

neutralhydrogensothationizationcorrectionsforH,Zn,Cretcarerelativelysmall,and

(4)DLAsareourprincipalsourceofinformation aboutthemetalcontentofinterstellar

matteringalaxiesover80%oftheageoftheuniverse.

AlargenumberofelementshavebeenobservedinDLAs. Inparticular,Fe,Zn,Si,S

andOhavebeenusedasprobesofthemetallicityinthesesystems. Wepreferto useZn

todeterminethetotal(gas+solidphase)metallicityinDLAsbecause(1)ZntracksFein

mostGalacticstars,(2)itisrelativelyundepletedoninterstellardustgrains,and(3)the

linesofthedominantionizationspecies,Zn II,aremostoftenunsaturated. Abundances

of depleted elements such as Cr, Fe orNi relativeto Zn probe the dust content of the

absorbers.

Theissue ofmetallicityevolutionin DLAshasnotbeenfullyunderstood, themain

reason for the uncertainty being thesmall numberof measurements, especially at z <

1.5(Kulkarniand Fall 2002). This isbecause,thoughfor 0:6<z <1:3, theZn II lines

canbeaccessed withground-basedtelescopes,theyliein blue wavelengthswhere many

spectrographs havelowersensitivity. Also, theLy forz <1:5and theZn II lines for

z<0:5canonlybeaccessedwithspacetelescopes.

WiththeaimofenhancingtheDLAsampleatz<1:5wehaveobserved8DLAswith

the6.5mMultipleMirrorTelescope(MMT)andtheHubbleSpaceTelescope(HST),with

0.09<z<1:36,theresultsforwhichwepresenthere. Theobservationsarepresentedin

Table1. TargetDLAsandabundances.

QSO z

em z

abs

[Zn/H] [Cr/Zn]

Q0738+313 0.635 0.0912 < 1:1 >-0.1

Q0738+313 0.635 0.2212 <-0.7 >-0.7

Q0827+243 0.939 0.5247 <-0.0 ...

Q0952+179 1.472 0.2378 <-1.0 >-0.6

Q0933+733 2.525 1.4790 -1.6 -0.2

SDSSJ110729.03+004811.1 1.392 0.7405 -0.6 -0.3

SDSSJ172739.03+530229.1 1.444 0.9449 -0.5 -0.4

SDSSJ172739.03+530229.1 1.444 1.0311 -1.4 -0.3

2. Observations

OurMMTsampleoriginallyconsistedof11quasarabsorbersat0:09<z<1:5,forwhich

eitheradampedLylinewasobservedinHST spectra,orastrongDLAwasexpected

onthebasisofMgIIorFeIIlines(RaoandTurnshek2000)availableintheSDSSEarly

DataReleasespectra(Schneider etal. 2002). Inparticular,wechose systemsthathave

W rest

MgII2796

>1:0

A, W rest

FeII2344

>0:8

A and someotherindicator of highN

HI

(Fe I,CI,

MgI,MnII,Ni II,Ca II,CrII,Zn IIorSiII 1808). Later, HSTobservations ofLy

linesinthesesystems(HSTGOprojectNo. 9382;PI:S.Rao)conrmedtheDLAnature

of several of these absorberswhile 3 others were found to be sub-DLAs (systems with

HIcolumn density between10 19

210 20

cm 2

). ForHST observations, the sample

consistedofconrmedDLAswith0.09<z<0.52. Table1liststherelevantpropertiesof

theobserved\classicalDLAs"i.e. systemswithlogN

HI

20:3. Datareductionwasdone

withImageReductionandAnalysis FacilityandSpace TelescopeScience DataAnalysis

System. ForMMT data theZn II 2026 line and Mg I 2026line aswell as theZn

II 2062lineand CrII 2062line areblendedtogether. A prolettinganalysis was

performedforthese linesalongwiththelines2056and2066ofCrIIsimultaneously,

assumingthevelocitydispersionparametertobethesameforallthese ions. Wherever

necessary,thecolumn densityofMgIwasobtainedfromtheequivalentwidthofMgI

2853lineobservedintheSDSSspectra,usingthebvalueobtainedfortheunblendedCr

IIlines.

3. Results and discussion

TheabundancesofZnandthoseofCrrelativetoZnfortheDLAsinoursamplearegiven

inTable1. Formostofthesystems,evenforthoseatthelowestredshifts,theabundances

of Zn are sub-solar. The only super-solarabundance that we have obtainedis for the

systemwithredshiftof0.716towardSDSSJ1323-0021whichhasneutralhydrogencolumn

20 2

To examine the implications of our data for the mean metallicity-redshift relation

of DLAs, we combined our data with other Zn measurements at higher redshifts. We

only included the classical DLAs in our study and, also, excluded systems with radial

velocitieswithin3000kms 1

fromthequasaremissionredshifts,sincethesesystemsmay

be associatedwith theQSOsandmaynotrepresentinterveningpopulationofgalaxies.

Thecombinedsample containsa totalof 87 DLAs with Zn detections or limits, in the

redshiftrange0:09<z<3:90. Wehavenormalizedthemalltothesamesetofoscillator

strengthsandsolarabundancesthat wehaveusedforourdata.

0 1 2 3 4

Redshift

−3

−2

−1 0

log (Z/Zsolar)

PFH99 PF95 MC96 SPF01

−3

−2

−1 0

log (Z/Zsolar)

PFH99 PF95 MC96 SPF01

Maximum Limits

Minimum Limits

Figure 1. Theglobalmetallicity-redshiftrelationdeducedfromdampedLy-absorbers. The

circlesshowthelogarithm ofthe NHI-weightedmean Znmetallicity relative tothe solar value

vs. redshift forthe sampleof87DLAs. Thelled circlesinthetopand bottompanelsshow,

respectively, the metallicity for the maximum-limitsand minimum-limits cases. The unlled

circlesinthe bottom panelshowthe metallicity obtained byusingelements otherthanZnto

constrain the metallicities in casesof Zn limits. Vertical error bars denote 1 uncertainties.

Horizontal bars show the bin extent. Eachpoint is plotted at the median redshiftinits bin.

Theshort-dashed,long-dashed,solid,andshort-dash-long-dashedcurvesshow,respectively,the

meanmetallicity(not correctedfor dustobscuration) inthecosmicchemicalevolution models

ofPeiand Fall (1995),Malaney and Chaboyer(1996),Pei etal. (1999), andSomervilleetal.

(2001).

Of the87systemsin thecombinedsample,51are detectionsand36 arelimits. We

asdetections, anda\minimum-limits" sampletreatingthe Zn limitsas zeros. For any

individual system, these cases cover the full range of possible values the Zn column

densitiescantake. Forthesakeofpictorialillustration,wedividedeachofthesesamples

intosixredshiftbinswith14or15absorberseach. ResultsareshowninFigure1. Clearly,

theN

HI

-weightedmean metallicityof DLAsdoesnotrisefast enoughto reach solaror

near-solarmetallicitiesatz=0.

Forthebinneddata(withsixbinsintherange0:09<z<3:90),thelinearregression

slope of the logarithm of the metallicity vs. redshift relation is 0:180:06 for the

maximum-limits sample and 0:220:08 for the minimum-limits sample. The linear

regressionintercept of the metallicity-redshift relation (which corresponds to predicted

present-day metallicity) is 0:740:15 for the maximum-limits sample, 0:750:18

fortheminimum-limitssample. TheresultschangeverylittleiftheZnupperlimitsare

replacedwiththeconstraintsfromotherelements.

0 1 2 3 4

Redshift

−2

−1.5

−1

−0.5 0 0.5 1

[<Cr/Zn>]

DLAs Sun

Galactic Halo Clouds Galactic Warm Clouds Galactic Cold Clouds

Figure2. BinnedunweightedmeanoflogarithmicabundanceofCrrelativetoZnh[Cr/Zn]ivs.

redshiftfor DLAsfromoursampleand otherdata fromtheliterature. Thezero leveldenotes

thesolarlevel.

The refractoryelementssuch as Si, Ti, Cr, Mn, Fe, Co, and Ni are expected to be

depletediftheDLAscontaindust,whilevolatileelementslikeZnarealmostundepleted

ininterstellarclouds. [Cr/Zn]is,therefore,takentobeameasureoftheamountofdust.

ofthe dustcontentwecombinedourdata withthose from theliterature. Asbefore we

only used the classical DLAs which were separated by > 3000 km s 1

from the QSO

redshift. We,also, excluded caseswhere onlyupperorlowerlimitsare available forCr

and/orZn, and scaledthe measurementsto thesameset of oscillator strengthsfor the

ZnIIandCrIIlinesusedbyus. InFig. 2,weplot[Cr/Zn]vs. redshiftforthiscombined

sample of 41 DLAs. Our MMT data have provided 4 new [Cr/Zn] measurements for

DLAs at z < 1:5 and nearly doubled the existing sample of [Cr/Zn] measurementsat

theseredshifts. InadditionourMMTdatahaveprovided[Cr/Zn]ratioforonecandidate

DLAand threesub-DLAs . A linearregressiont totheunbinned[Cr/Zn]vs. redshift

datagivesaslopeof0:070:01,andaninterceptof 0:480:02implyingaslowdecrease

in[Cr/Zn]withdecreasingredshift.

Tosummarize,inallthe3DLAsatz<0:4forwhichourobservationsoermeaningful

constraintsonthe Zn abundance, the metallicities appear to besubstantiallysub-solar

(10 20%solar). TheglobalmeanmetallicityofDLAsshowsatmostaslowincrease

withdecreasingredshiftanddoesnotappeartoriseuptosolarornear-solarvaluesatz=

0in contradictionwith thechemical evolutionmodels. Thissuggeststhat theobserved

DLAs, especially those at low redshifts, probablydier from the generalpopulation of

galaxiesrepresentedbytheglobalstarformationhistoryoftheuniverse.

ThisweakevolutionintheDLAglobalmeanmetallicitycouldbepartiallyexplained

bythefactthatoursampleatredshiftssmallerthan0.6seemstobedominatedbydwarf

orlowsurface brightnessgalaxies. It is possible that current DLA samples, especially

thoseat lowredshifts,arebiasedagainstmoreenrichedgalaxies becausethelattermay

cause more dust obscuration of the background quasars. We note, however, that the

low-redshift samples are still small, and the fact that mostof the DLAs in our sample

are metal-poorcouldbean eect ofsmall numberstatistics. Metallicity measurements

of more low-redshift DLAs are necessary to further improve the statistics of the mean

metallicity-redshift relationand to studythescatter aroundthemean. Observations of

low-redshift DLAs towardoptically faint quasars are especially necessaryin the future

to improve the constraints on the mean metallicity-redshift relation for DLAs, and to

quantify potentialdustselectioneects.

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