The connection between metallicity and metal-line kinematics in (sub-)damped Lyman-alpha systems

0.35 MB·English

by  M. T. Murphy

Mon.Not.R.Astron.Soc.000,1–9(2006) Printed5February2008 (MNLATEXstylefilev2.2)
The connection between metallicity and metal-line kinematics in
α
(sub-)damped Lyman- systems
M. T. Murphy1⋆, S. J. Curran2, J. K. Webb2, H. Me´nager1,2, B. J. Zych1
1InstituteofAstronomy,UniversityofCambridge,MadingleyRoad,CambridgeCB30HA
2SchoolofPhysics,UniversityofNewSouthWales,SydneyNSW2052,Australia
7
0
0
2 Accepted2006December29.Received2006December29;inoriginalform2006October27

n
a
J ABSTRACT
Acorrelationbetweenthemetallicity,[M/H],andrest-frameMgequivalentwidth,Wλ2796,
2 r
is found from 49 DLAs and strong sub-DLAs drawn from the literature over the redshift

1 range 0.2 < zabs < 2.6. The correlation is significant at 4.2σ and improves to 4.7σ when
v the mild evolution of [M/H] with redshift is taken into account. Even when including only
8 the26DLAs(i.e.exlcudingsub-DLAs)whichhaveZn-metallicitiesandWλ2796 > 0.7Å,the
r
2 correlationremainsat> 3σsignificance.SincetheMgλ2796transitionispredominantly
0 saturatedinDLAs(whichalwayshaveWλ2796greaterthan0.3Å),Wλ2796isfarmoresensitive
1 r r
to the kinematicspread of the metalvelocitycomponentsacrossthe absorptionprofilethan
0
itisto[M/H].Thus,theobserved[M/H]–Wλ2796 correlationpointstoastronglinkbetween
7 r
the absorbermetallicity and the mechanism forproducingand dispersing the velocity com-
0
/ ponents.We alsonotethatapproximatelyhalfofthe13knownH2 absorbershaveveryhigh
h Wλ2796 andverybroadvelocitystructureswhichshowcharacteristicsusuallyassociatedwith
p ourtflows.Follow-upultraviolet-andblue-sensitivehigh-resolutionspectraofhigh-Wλ2796sys-
- r
o tems,initiallyidentifiedinlow-resolutionspectra,maythereforeyieldalargenumberofnew
r H discoveries.
2
t
s
Key words: intergalacticmedium– quasars:absorptionlines – cosmology:observations–
a
: galaxies:evolution–galaxies:ISM
v
i
X
r
a
1 INTRODUCTION of which are luminosity bias against faint galaxies and the prox-
imity of some host-galaxies to the QSO sight-line. Indeed, very
The damped Lyman-α absorbers (DLAs) observed along sight-
few DLA host-galaxies have been identified at z & 2 (e.g.
lines to quasars (QSOs) are potentially important pieces in the abs
Møller&Warren 1993; Djorgovskietal. 1996; Christensenetal.
puzzle of galaxy formation. They are the highest column density
2005;Weatherleyetal.2005).
absorption systems, with N(H) > 2 × 1020cm−2, and the first
DLA surveys quickly established that they contain a large frac-
The Mg λ2796/2803 doublet has proved very important
tionofthehigh-zbaryonsavailableforstarformation(Wolfeetal.
for exploring the absorber–galaxy connection at 0.3 . z .
1986;Lanzettaetal.1991).Theearlyestimatesoftheneutralgas abs
mass density at absorption redshifts z ∼ 3 were similar to the 2.6 where it is observable with optical QSO absorption spec-
abs
estimates of the mass density in stars at z = 0. It was there- troscopy.Indeed,manyofthezabs <1.5imagingstudiesaboveini-
tiallyidentifiedabsorptionsystemsviatheeasilyrecognisedMg
fore natural to assume that DLAs act as the neutral gas reser-
doublet (see also, e.g., Lanzetta&Bowen 1990; Bergeronetal.
voirsforstarformation.However,firmlyestablishingthelinkbe-
1992;Steidel&Sargent1992;Drinkwateretal.1993;Steideletal.
tweenDLAsandstarformationreliesonourunderstandingofthe
1994).Also,sincetheionizationpotentialofMgis<13.6eVbut
absorber–galaxy connection. Direct imaging of DLAs (or strong
Mg systems; see below) at z < 1.5 reveals the hosts to be a thatofMgis>13.6eV,thelattertracescoldgas.Attemptshave
abs
therefore been made to identify DLA candidates at low-z via
mixofirregulars,dwarfs,spiralsandlowsurface-brightnessgalax- abs
their Mg absorption (e.g. Raoetal. 1995, 2006; Ellison 2006).
ies (e.g. Bergeron&Boisse´ 1991; LeBrunetal. 1997; Raoetal.
The Mg doublet transitions are also among the strongest metal
2003;Chen&Lanzetta2003).Thisisfurtherborneoutbyablind
21-cm emission survey at z = 0 (Ryan-Weberetal. 2003). How- linesobservedinQSOabsorbers,meaningthat(i)theMgλ2796
rest-frameequivalent widthinDLAsistypicallyWλ2796 & 0.3Å,
ever,quantifyingthemixofDLAhost-galaxymorphologiesfrom r
low- to high-redshift is fraught with selection effects, not least allowingsuchsystemstobeidentifiedinrelativelylow-resolution
spectra,and(ii)mostvelocitycomponentsacrosstheMgprofile
aresaturatedinDLAs.ThisimpliesthatWλ2796 ismainlysensitive
r
⋆ E-mail:[email protected] tothekinematicextentofthevelocitycomponents,∆v,ratherthan
2 Murphyetal.
their column density. A good demonstration of thisisfigure 3of erature(seeSection2.2).ForsystemswithoutapublishedWλ2796
r
Ellison(2006). value we searched for publicly available SDSS (R ∼ 2000) or
In this paper we search for a relationship between Wλ2796 VeryLargeTelescopeUltravioletandVisualEchelleSpectrograph
r
andthemetallicity1,[M/H],ofDLAsandstrongsub-DLAs.Since (VLT/UVES;R & 40000)spectrafromwhichanWλ2796 measure-
r
Wλ2796 is a measure of the kinematic spread of the cold, metal- mentiseasilymade.ForSDSSspectra,ourmethodforobtaining
r
bearing velocity components, such a relationship may constrain Wλ2796 isdescribedindetailinBouche´etal.(2006).UVESspec-
r
themechanismsforproducinganddispersingthemetals.Thismay trawereobtainedfromtheESOdataarchiveandreducedusinga
guideourunderstandingofthephysicalnatureofDLAs. version of the UVESpipeline which we modifiedto improve the
There currently exist only indirect and possibly contradic- flux extraction and wavelength calibration. The extracted echelle
tory constraints on a [M/H]–Wλ2796 relationship for DLAs and ordersfromallexposureswerecombined,using2,with
r
sub-DLAs.Nestoretal.(2003)suggest that[M/H]increaseswith inverse-variance weighting and a cosmic ray rejection algorithm,
increasing Wλ2796. They identified strong Mg systems in Sloan toformasinglespectrum withadispersion of 2.5kms−1pixel−1.
r
Digital Sky Survey (SDSS) QSO spectra over the redshift range Wλ2796wasthenmeasuredbydirectsummationofpixelsacrossthe
r
0.9 < z < 2.0 and constructed composite spectra in the ab- Mgλ2796profile.TheuncertaintiesinWλ2796 quotedinTable1
abs r
sorber rest-frame. The Zn lines were stronger in the composite for these UVES spectra are generally dominated by uncertainties
ofWλ2796 >1.3Åabsorberscomparedwiththoseinthecomposite inthecontinuumlevelaroundtheMgλ2796line.Insomecases
r
of1.0 < Wλ2796 < 1.3Åsystems.Assumingthatthemean N(H) whereWλ2796 issohighthattheλ2796profileisblendedwiththat
r r
is not significantly different in these two regimes, they conclude of the λ2803 line(typically Wλ2796 & 3.5Å), larger uncertainties
r
thatsomeevidenceexistsfora[Zn/H]–Wλ2796 correlation.Onthe arereported.
r
otherhand,Yorketal.(2006)findlessdirectevidenceforananti- Formostofthe49absorbersinTable1,Zn-metallicitieswere
correlationbetween[Zn/H]andWλ2796.Fromaseriesofcomposite availablein theliterature. In13 cases we haveused theFeor Cr
r
SDSS spectra with various mean Wλ2796 they find that the dust- abundance intheabsence ofaZnmeasurement. Itiswellknown
r
reddening, E(B−V), caused by the absorbers rises sharply with thattheseelementsaredepletedontodustgrains(e.g.Pettinietal.
increasingWλ2796.WhiletheZnline-strengthsalsoincrease,they 1997)andsowecorrectedtheFeandCrabundanceswiththesim-
r
showaslowerincreasethanE(B−V)overthesamerangeofWλ2796. ple prescription of Prochaskaetal. (2003a): [M/H] = [Fe/H]+
r
Thecompositespectraalsosuggestadustextinctioncurvetypical 0.4=[Cr/H]+0.2.InSection2.2wetestthepossibleimpactthis
of the Small Magellanic Cloud (SMC), leading Yorketal. to as- mighthaveonourresults.MostofthemetallicitiesinTable1have
sume aconstant dust-to-gas ratio,i.e. N(H) ∝ E(B−V),asob- been corrected to the solar scale of Lodders (2003). The values
servedintheSMC.Thus,sincetheZncolumndensityincreases quoted in the literature were used for Q0235+164, Q1622+238,
slowerthantheN(H)inferredfromE(B−V)underthisassump- Q0458−020,Q1122−1649,Q1157+014sincewecouldnotiden-
tion,Yorketal.(2006)arguethat[Zn/H]decreaseswithincreasing tifywhichsolarvaluestheoriginalauthorsused.Sincethecorrec-
Wλ2796.Wediscusstheseresultsfurtherinlightofournewresults tions are typically . 0.1dex, these 5 exceptions have negligible
r
inSection3. impactonourmainresults.
Thispaperisorganisedasfollows.Section2explainsoursam-
ple selection and explores an evident [M/H]–Wλ2796 relationship
r
in that sample. Section 3 compares our results with the [M/H]– 2.2 TheWλ2796–[M/H]correlation
∆vcorrelationrecentlydiscoveredbyLedouxetal.(2006)at(gen- r
erally) higher redshifts. We also discuss whether our observed Figure1(left)showsallthe[M/H]–Wrλ2796 pairs.Table1givesthe
[M/H]–Wλ2796 relationship can be taken as evidence for a corre- formalstatisticalerrorsonbothquantitiesasreportedinthelitera-
r
lation or anti-correlation between absorber metallicity and host- ture.Incaseswherenoerror-barscouldbefoundintheliterature,
galaxymass.Finally,wediscusswhetheralargefractionofstrong uncertaintiesof0.10dexand0.05Åwereassumedasthesetypified
Mg absorbers might arise in outflows from relatively low mass therestofthesample.ForFig.1(left)weadded0.05dexand0.03Å
galaxies. In particular, about half of those absorbers in whichH2 inquadraturetotheinitial[M/H]andWrλ2796 uncertaintiestotake
hasbeendetectedshowsomeevidenceofanoutfloworigin. intoaccountlikelycontinuumfittingerrorswhicharenotincluded
by some authors or, indeed, by our own Wλ2796 measurements in
r
SDSSspectra.WeregardthepointsinTable1withtheseincreased
errorsasourfiducialsample.Kendall’sτforthe[M/H]–Wλ2796cor-
2 CORRELATIONBETWEENMETALLICITYAND relationevidentinFig.1hasaprobabilityP(τ)=2.6×10−5rofbeing
MGIIEQUIVALENTWIDTHINDLASANDSUB-DLAS
duetochancealone;assumingGaussianstatistics,thecorrelation
2.1 Sampledefinition issignificantat>4.2-σ.Thecorrelationisrobusttotheremovalof
distinctsub-samples,someofwhicharediscussedbelow,asshown
WeidentifiedDLAs[N(H)>2×1020cm−2]andstrongsub-DLAs inTable2.
[N(H) > 3×1019cm−2] withexisting metallicitymeasurements Theabsorberscoverquitealargeredshiftrange,0.2 < z <
abs
inthecompilationofKulkarnietal.(2005)andseveralother,typ- 2.6,soevolutioninDLAmetallicitiesmayhavesomewhatwashed
icallymorerecent,sources(ascitedinTable1).Forthesesystems outthecorrelationweobserve.Forexample,ifabsorbersatlower
wesearched the literaturefor Wrλ2796 measurements; Table1 lists zabs are,on average, moremetal-richthanat higher zabs,thenthis
ctheortsaeinstyiesstefmosr sfoomr wehWicrλh279w6evafoluuensdcbooutlhd[nMot/Hb]eafonudnWdrλin279t6h.eUlint-- Twhoiusldwcoauuldseaatdldeiatsiotndaelcdreisapseertshioenstianti[sMtic/Hal]saitganlilfivcaalnuceesooffWtherλ2o79b6-.
servedcorrelationand,dependingontheredshiftdistributionofthe
1 Metallicity is defined as the heavy element abundance with respect
to hydrogen, relative to that of the solar neighbourhood: [M/H] ≡
log[N(M)/N(H)]−log[N(M)/N(H)]⊙ 2 Availableathttp://www.ast.cam.ac.uk/∼mim/UVES popler.html
Metallicityand kinematicsof(sub-)DLAs 3
Table1.DLAsandsub-DLAswherebothMgλ2796Åequivalentwidth,Wλ2796,andmetallicity,[M/H],havebeenmeasured.B1950QSO
r
namesaregiven.N(H)isthetotalneutralhydrogencolumndensity[cm−2],zabsistheDLAredshiftandzemistheredshiftofthebackground
QSO.ErrorsonWλ2796 and[M/H]arequotedfromtheliterature butinmanycases donotinclude continuum-fitting uncertainties. [M/H]
r
ismeasuredrelative tothesolarabundances ofLodders(2003)inmostcases(seetext).Theelementusedtodetermine[M/H]isZnwhere
possible;whenthisisnotavailablewecorrectedfordustdepletionvia[M/H]=[Fe/H]+0.4=[Cr/H]+0.2(seeProchaskaetal.2003a).
QSO logN(H) zabs zem Wrλ2796[Å] Ref. [M/H] M Ref. Notes
0013−0029 20.8 1.973 2.08694 4.7±0.3 R03 −0.72±0.09 Zn P02 a
0058+0155 20.1 0.61251 1.954 1.68±0.03 HIRES +0.05±0.21 Zn P00 —
0100+130 21.4 2.3091 2.681 0.85±0.01 UVES7,19 −1.54±0.09 Zn PW99 PHL957
0235+164 21.7 0.52385 0.94 2.34±0.05 LB92 −0.22±0.15 Zn J04,P03a b
0302−223 20.4 1.00945 1.409 1.16±0.04 R06 −0.56±0.12 Zn P00 e
0405−4418 21.0 2.5505 3.00 2.38±0.25 UVES10,11,14 −1.32±0.11 Zn LE03 CTQ0247
0405−4418 21.1 2.595 3.00 1.29±0.09 UVES10,11,14 −1.04±0.10 Zn LE03 a,CTQ0247
0454+039 20.7 0.8596 1.345 1.45±0.01 C00 −0.99±0.12 Zn P00 c
0458−020 21.7 2.03945 2.286 1.69±0.07 W93 −1.19±0.09 Zn PW99 b
0512−3329 20.5 0.931 1.569 1.94 E06b −1.09±0.11 Fe L05 —
0515−4414 20.5 1.1508 1.713 2.34±0.02 UVES1,5,16 −0.95±0.19 Zn D00 a
0528−250 20.6 2.141 2.813 1.89±0.08 UVES6,9,11 −1.45±0.07 Zn C03a d
0551−366 20.4 1.962 2.318 5.80±0.08 R03 −0.11±0.09 Zn L02b a
0738+313 20.8 0.2212 0.635 0.61±0.04 B87 −1.24±0.2 Cr K04 b
0827+243 20.3 0.5247 0.939 2.90 RT00 −0.62±0.08 Fe K04 b
0841+1256 21.3 2.3745 2.50 0.76±0.07 UVES2,15 −1.47±0.15 Zn PW99 —
0933+732 21.6 1.4789 2.528 0.95±0.08 R06 −1.58±0.25 Zn K04 e
0935+417 20.3 1.3726 1.98 1.04 P04 −0.78±0.2 Zn M95 —
0945+436 21.5 1.223 1.89149 1.12±0.05 SDSS −0.95±0.10 Zn P03a —
0949+527 20.1 1.7678 1.87476 3.88±0.05 SDSS +0.02±0.10 Zn P06b —
0952+179 21.3 0.2378 1.472 0.63±0.11 RT00 −1.45±0.2 Cr K05 b
0957+561A 20.3 1.391 1.413 2.25 E06b −0.91 Fe C03b c
0957+561B 19.9 1.391 1.413 2.13 E06b −0.63 Fe C03b c
1101−264 19.5 1.838 2.145 0.94 E06b −1.06±0.07 Fe D03 —
1104−1805 20.9 1.6614 2.319 0.96 L99 −0.99±0.02 Zn L99 —
1104+0104 21.0 0.7404 1.3924 2.95±0.03 R06 −0.60±0.2 Zn K04 —
1122−1649 20.5 0.6819 2.40 1.72 P04 −1.00±0.15 Fe L02a,P03a d
1151+068 21.3 1.7736 2.762 0.52±0.05 R03 −1.53±0.14 Zn P97 —
1157+014 21.8 1.94362 1.986 1.38±0.06 R03 −1.36±0.12 Zn L03 b
1209+093 21.4 2.5840 3.297 3.20±0.50 UVES8,18 −1.04±0.11 Zn P03b e
1210+1731 20.6 1.892 2.543 1.09 E06b −0.86±0.10 Zn D06 —
1215+333 21.0 1.999 2.606 1.03±0.15 SS92 −1.25±0.09 Zn P99 d
1223+1753 21.5 2.4658 2.936 1.54±0.06 UVES3,4,13 −1.58±0.10 Fe P01 e
1229−0207 20.8 0.39498 1.045 2.17±0.07 LB92 −0.45±0.19 Zn B98 b
1247+267 19.9 1.22319 2.038 0.42 E06b −1.02±0.25 Zn P99 d
1320−0006 20.2 0.716 1.38839 2.13±0.05 SDSS +0.61±0.20 Zn P06a —
1328+307 21.3 0.692154 0.849 0.33 SM79 −1.20 Zn MY92 b,3C286
1331+170 21.2 1.77642 2.084 1.33±0.09 R03 −1.26±0.10 Fe P01 a,b
1351+318 20.2 1.14913 1.326 1.25 E06b −0.27±0.17 Zn P99 d
1354+258 21.5 1.4205 2.006 0.61 RT00 −1.58±0.16 Zn K05 c
1451+123 19.9 2.255 3.246 1.20±0.08 UVES2 −1.08±0.23 Zn D05 e
1622+238 20.4 0.6561 0.927 1.29 SS92 −0.87±0.25 Fe P03a d,3C336
1629+120 19.7 0.9008 1.795 1.20±0.09 R06 −0.18 Zn E06a d
1727+5302 21.2 0.9448 1.444 2.832±0.070 R06 −0.52±0.2 Zn K04 —
1727+5302 21.4 1.0312 1.444 0.922±0.057 R06 −1.39±0.2 Zn K04 —
2206−1958 20.5 1.9205 2.56 1.99±0.05 UVES3,17,19 −0.37±0.07 Fe P01 —
2231−0015 20.5 2.066 3.015 1.80 E06b −0.86±0.10 Zn D04 e
2343+125 20.3 2.4310 2.549 3.22±0.15 UVES6,12,17 −0.74±0.08 Zn D04 a,e
2348−1444 20.6 2.279 2.94 0.32 E06b −1.84 Fe D06 d
Notes:aH2 absorptiondetected;b21-cmabsorptiondetected;c21-cmabsorptionnotdetected;dradio-loudbutnotsearchedfor21-cmabsorption;eradio-quiet.160.A-9022UVES
commissioning;265.O-0063Ledoux;365.O-0158Pettini;465.P-0038Srianand;566.A-0212Reimers;666.A-0594Molaro;767.A-0022D’Odorico;867.A-0146Vladilo;968.A-0106
Petitjean;1068.A-0361Lopez;1168.A-0600Ledoux;1269.A-0204D’Odorico;1369.B-0108Srianand;1470.A-0017Petitjean;1570.B-0258Dessauges-Zavadsky;16072.A-0100Murphy;
17072.A-0346Ledoux;18073.B-0787Dessauges-Zavadsky;19074.A-0201Srianand.
References:SM79(Spinrad&McKee1979),B87(Bouladeetal.1987),W93(Wolfeetal.1985),LB92(Lanzetta&Bowen1992),MY92(Meyer&York1992),SS92(Steidel&Sargent
1992),A94(Aldcroftetal.1994),M95(Meyeretal.1995),P97(Pettinietal.1997),B98(Boisseetal.1998),L99(Lopezetal.1999),P99(Pettinietal.1999),PW99(Prochaska&Wolfe
1999),C00(Churchilletal.2000),D00(delaVargaetal.2000),P00(Pettinietal.2000),RT00(Rao&Turnshek2000),P01(Prochaskaetal.2001),P02(Petitjeanetal.2002),L02a
(Ledouxetal.2002),L02b(Ledouxetal.2002),C03a(Centurio´netal.2003),C03b(Churchilletal.2003),D03(Dessauges-Zavadskyetal.2003),L03(Ledouxetal.2003),LE03
(Lopez&Ellison2003),P03a(Prochaskaetal.2003a),P03b(Prochaskaetal.2003b),R03(Ryabinkovetal.2003)[andreferencestherein],D04(Dessauges-Zavadskyetal.2004),J04
(Junkkarinenetal.2004),K04(Khareetal.2004),P04(Pe´rouxetal.2004),A05(Akermanetal.2005),K05(Kulkarnietal.2005),L05(Lopezetal.2005),D06(Dessauges-Zavadskyetal.
2006),E06a(Ellison2006),E06b(S.E.Ellison,privatecommunication),P06a(Pe´rouxetal.2006),P06b(Prochaskaetal.2006),R06(Raoetal.2006).‘SDSS’or‘UVES’indicateswe
haveobtainedWrλ2796fromSloanDigitalSkySurveyspectraorfromVLT/UVESspectra.
4 Murphyetal.
Figure1.Correlationbetweenabsorbermetallicity,[M/H],andMgrest-frameequivalentwidth,Wλ2796.Theuppervelocityscaleistheapproximatevelocity
r
spreadcalculatedsimplyfrom∆v[kms−1]≈70[kms−1Å−1]×Wλ2796[Å].Left:OriginaldatafromTable1withadditional[M/H]-andWλ2796-uncertainties
r r
of0.05dex and 0.03Å added inquadrature (see text). Thecorrelation is significant at the 4.2σlevel. Right: The[M/H] values have been corrected for
cosmologicalmetallicityevolutionaccordingtoequation(1).Thisreducesthescatter,improvingthecorrelationtothe4.7σlevel.
Table2.Statisticsfordifferentsub-samples.Naisthesamplesize.DWrλ2796Eandh[M/H]iarethemedianrest-frameMgequivalentwidthandthemedian
metallicity,respectively.Columns5–8showthecorrelationstatisticsfortheoriginaldatafromTable1(withincreasederrors)whilecolumns9–12showthe
samestatisticsafterthemetallicitiesarecorrectedforredshiftevolution(seetext).P(τ)isKendall’sτtwo-sidedprobabilitythatnocorrelationexistsbetween
[M/H]andWλ2796.Weconvertthistoasignificance,S(τ),assumingGaussianstatistics.InallcasesaSpearmantestgivesamoresignificantcorrelation.A
r
andBaretheinterceptandslope,respectively,ofthefittothe[M/H]versus∆vdata(seeSection3.1).
Originaldata Redshift-correctedmetallicities
Sample Na DWrλ2796E h[M/H]i P(τ) S(τ) A B P(τ) S(τ) A B
[Å] [dex] [10−3] [σ] [dex] [dex] [10−3] [σ] [dex] [dex]
0:Fiducial 49 1.38 −0.99 0.026 4.2 −4.29±0.38 1.69±0.20 0.0032 4.7 −3.24±0.35 1.62±0.18
1:[Zn/H]only 36 1.33 −0.97 0.48 3.5 −4.28±0.46 1.70±0.24 0.038 4.1 −3.14±0.40 1.60±0.21
2:DLAsonly 40 1.42 −1.02 0.028 4.2 −3.85±0.28 1.41±0.14 0.0011 4.9 −2.86±0.28 1.38±0.14
3:Wλ2796>0.7Å 42 1.69 −0.93 2.3 3.1 −4.96±0.50 1.99±0.25 1.7 3.1 −3.70±0.44 1.83±0.22
r
4:1+2+3 26 1.57 −0.97 2.2 3.1 −4.25±0.35 1.59±0.17 0.35 3.6 −2.97±0.34 1.44±0.16
5:Wλ2796<1.4Å 25 0.96 −1.20 9.7 2.6 −4.54±0.70 1.91±0.39 1.8 3.1 −3.67±0.63 1.94±0.35
r
6:Wλ2796>1.4Å 24 2.21 −0.73 100 1.6 −5.88±1.32 2.33±0.58 17 2.4 −5.33±0.88 2.50±0.39
r
7:zabs<1.4 24 1.56 −0.82 20 2.3 −3.98±0.57 1.64±0.30 46 2.0 −3.28±0.60 1.67±0.31
8:zabs>1.4 25 1.33 −1.19 0.31 3.6 −4.16±0.36 1.52±0.19 0.048 4.1 −3.09±0.34 1.53±0.17
9:NoH2 43 1.29 −1.00 0.11 3.9 −4.52±0.46 1.83±0.24 0.025 4.2 −3.38±0.42 1.71±0.22
10:NoH2or21-cm 35 1.25 −0.99 0.92 3.3 −4.80±0.54 1.97±0.29 0.40 3.5 −3.49±0.48 1.78±0.25
11:H2systems 6 2.78 −0.84 15 2.4 −4.06±0.63 1.44±0.29 91 1.7 −3.26±0.75 1.55±0.32
12:21-cmsystems 9 1.38 −1.20 95 1.7 −3.65±0.68 1.37±0.33 12 2.5 −2.74±0.63 1.30±0.31
sample,maysomewhatalteritsslope.Toexplorewhetherthisisin- statistics:thegeneralscatteraroundthe[M/H]–Wλ2796 correlation
r
deedthecase,wemakeasimplecorrectiontothemetallicitiesin clearlydecreasesuponcorrection;thetableshowsthatthesignifi-
Table1accordingto canceofthe[M/H]–Wλ2796 correlationincreasesinalmostallsub-
r
samplesoncethemetallicitiesarecorrectedforredshiftevolution.
[M/H] =0.52+[M/H]+0.27z . (1)
corr abs
The numerical coefficients are the y-intercept and slope from an Visually,thecorrelationinFig.1(left)appearstighterforthe
unweighted least squares fit to [M/H] versus z using the fidu- lower-Wλ2796 half of the sample compared to that for the upper
abs r
cial sample. This has the effect of correcting each metallicity by half.Thatis,thevariancein[M/H]aboutthegeneral[M/H]–Wλ2796
r
theexpectedmeanvalueattheabsorber’sredshift.Theslopeand trend appears smaller at low-Wλ2796. However, an F-test reveals
r
interceptareconsistentwiththosederivedfromlargersamplesof onlymarginalevidenceforthis:afitof[M/H]versusWλ2796isper-
r
DLAscoveringsimilarredshiftranges(e.g.Prochaskaetal.2003a; formed[seeequation(2);Section3.1],thesampleissplitintotwo
Kulkarnietal. 2005). Fig. 1(right) shows the redshift-corrected sub-samples at Wλ2796 = 1.4Å and the variance in [M/H] about
r
metallicitiesversusWλ2796andTable2givestheredshift-corrected the fit is calculated for each sub-sample. The ratio of the vari-
r
Metallicityand kinematicsof(sub-)DLAs 5
ancesaboutthefitis1.83,butthisshouldoccur15percentofthe 3 DISCUSSION
time by chance alone given the sizes of the sub-samples. Using
The[M/H]–Wλ2796correlationfoundinFig.1isadirectdemonstra-
the redshift-corrected metallicities [Fig. 1(right)], the ratio of the r
tionthatthemetallicityofstrongMgabsorbersiscloselyrelated
variances reduces to1.64, whichhas an associated probability of
totheirkinematics(butnotnecessarilythekinematicsofthehost
24per cent. Themarginally increased scatter at high Wλ2796 does
r galaxy; see below). SincetheMgλ2796 transition isso strong,
reduce the significance of the correlation in the high-Wλ2796 sub-
r most velocity components in absorbers with Wλ2796 > 0.3Å are
samplecomparedtothelow-Wλ2796 sub-sample(seeTable2);one r
r saturatedandsoWλ2796 measuresthetotalvelocityspreadamong
possibilityisthatFig.1,particularlytheredshift-correctedversion r
thecomponents,∆v.Equivalently,itisameasureofthenumberof
inFig.1(right),definesalowerbound‘envelope’tothemetallicity
velocity components across the profile(e.g. Petitjean&Bergeron
at a given Wλ2796 instead of a normal correlation. This should of
r 1990). Thus, the [M/H]–Wλ2796 correlation demonstrates that the
coursebecombinedwithanupperboundenvelopeathighmetal- r
mechanism responsible for producing individual Mg velocity
licitiessincearbitrarilyhighvaluesareunphysical.Alargersample
componentsandfordispersingthemovervelocityranges∆v∼20–
withrobustcontrolovermetallicityuncertaintieswouldberequired
1000kms−1isrelatedto–andpossiblyalsodetermines–themetal-
toidentifythesefeaturesandtodistinguishbetweenatruecorrela-
licityoftheabsorber.
tionandalowerboundenvelope.
TheresultsinFig.1bearontheapparentdiscrepancybetween
Wehavealsosplitthesampleintotworedshiftbinsaboutthe theconclusions drawnbyNestoretal.(2003),whoseresultssug-
fiducialsample’smedian,zabs =1.4.Table2showsthatthe[M/H]– gesta[M/H]–Wrλ2796correlation,andYorketal.(2006)whoseanal-
Wrλ2796 correlationisstillwell-definedinthehigh-zabs sub-sample ysissuggestsananti-correlation.Ourobserved[M/H]–Wrλ2796 cor-
butitissomewhatlessstatisticallysignificantatlow-z .However, relationsupportstheformerandseemsinconsistentwiththelatter.
abs
thefitstobothsub-samples,asdescribedinSection3.1,yieldcon- OnepossibleexplanationmaybethatYorketal.assumeaconstant
sistentresults. dust-to-gas ratioas afunction of N(H), i.e. E(B−V) ∝ N(H).
Theybase thisassumption ontheir observation that theMg ab-
Threemainobjectionsmightberaisedtosomeofthedatawe sorbers they study seem to redden the background QSOs with,
useinTable1:(i)SomeofthemetallicitiesarenotderivedfromZn on average, an SMC-like dust extinction law. Even if this proves
andsomightbesusceptibletodust-depletioneffects,eventhough to be true for strong Mg absorbers, this relation has certainly
we have made a crude correction to any metallicitiesderived us- not been demonstrated in DLAs or sub-DLAs. Indeed, from the
ing the heavily depleted Fe and Cr ions; (ii) Absorbers with low detection of SMC-like dust-reddening in > 300 SDSS DLAs at
Wλ2796 mightcontainasignificantnumberofvelocitycomponents z > 2.2, Murphy et al. (2007, in preparation) find that the
r abs
whicharenot completelysaturated. WhileWλ2796 stillprovidesa mean E(B−V) ≈ 7×10−3 appears nottochange overtherange
r
measureofvelocityspread,∆v,inthesesystems,Wλ2796 willalso 20.3 6 log[N(H)/cm−2] 6 21.7. One caveat may be important
r
dependsomewhatontheopticaldepthsoftheunsaturatedcompo- here: possible magnitude bias. Nestoretal. assumed that N(H)
nents.Therefore,astheMgIIvelocitycomponentsweakenatlower is not a strong function of Wλ2796 based on the observations of
r
[M/H],onemightexpectacorrelationbetween[M/H]andWλ2796 Raoetal.(2006).ThedatainTable1alsorevealnoapparenttrend
r
whichhaslittletodowiththecorrelationfoundathigherWλ2796. inN(H)withWλ2796.However,Raoetal.selectedrelativelybright
r r
OnemightconsiderthatonlysystemswithWλ2796 > 0.7Åshould backgroundQSOswhichcouldprovidereasonablesignal-to-noise
r
beusedsincetheprofileisevenmorelikelytobecompletelysat- ratios in ultraviolet (UV) Hubble Space Telescope (HST) spec-
urated;(iii)Finally,thereliabilityofsub-DLAmetallicitiesisnot tra.Yorketal. suggest that dustyabsorbers arethereforeunlikely
aswellestablishedintheliteratureasthatforDLAs.Itispossible to exist in Raoetal.’s sample and that this dust-bias (relative to
thatwedonotfullyunderstandtheionizationcorrectionsrequired deeper surveys likeSDSS)mayinvalidate Nestoretal.’sassump-
insomecasesor,indeed,themagnitudeofthesystematicsinvolved tion.Fromthispointofview,itmaynotbesurprisingthatourre-
inderivingN(H)insub-DLAs.Forexample,VoigtprofileN(H) sultsagreewiththeconclusionofNestoretal.sincemost,butnot
estimatesclosertotheflatpartofthecurveofgrowthnaturallyhave all,oftheabsorbersinoursampleoccultrelativelybrightQSOs.
largerrandomuncertaintiesandmaybecomeincreasingly,system-
aticallyinfluencedbyweakerLy-αforestlines.However,Table2
shows that the [M/H]–Wλ2796 correlation is fairly robust against
r
removal of non-Zn metallicities, systems with Wrλ2796 < 0.7Å or 3.1 ComparisonwithhigherredshiftDLAs
sub-DLAs.Removingallthreecasestogetheralsoleavesa> 3σ
correlation (> 3.5σ after correcting the metallicities for redshift Athigherredshifts(1.6 < zabs < 3.0),Wolfe&Prochaska(1998)
tentativelynotedacorrelationbetween ∆vandmetallicityfroma
evolution).
sample of 17 DLAs. Pe´rouxetal. (2003) found a similarly ten-
Notethatoursamplecontainsonlyonesystemwith[M/H]. tative [Zn/H]–∆v correlation from a larger sample of DLAs and
−1.7.ThisispartiallyaselectioneffectsincetheZnlinesbecome sub-DLAs over the redshift range 1.4 < z < 4.5. However,
abs
undetectably weakat lowmetallicities.However, veryfew DLAs Ledouxetal.(2006,hereafterL06)veryrecentlyfoundarelatively
andsub-DLAsareknownwithlowermetallicitiesatredshifts0.2< tightandclear[M/H]–∆vcorrelationfromasampleofUVESspec-
z <2.6andcomparisonwithothermorecompletesamples(e.g. tracontaining70DLAsandsub-DLAsovertherange1.7<z <
abs abs
Kulkarnietal.2005;Akermanetal.2005)shows thatour sample 4.3.Theydefined∆vfromdifferenttransitionsofseverallowion-
isfairlyrepresentativeoftheoverallDLAmetallicitydistribution. ization species (O, Si, Fe, Cr, S) with moderate optical
Finally, we have included 3 systems in Table 1 which are within depths such that the strongest velocity component absorbed be-
5000kms−1oftheemissionredshiftofthebackgroundQSO.These tween 10 and 60per cent of the continuum. Since many of the
systemsdonotshowanysignsofbeingassociatedwiththeQSO DLAs in the L06 sample are at z > 2.6 where the Mg lines
abs
centralengineandsoareincludedhere.Removingthemfromthe arenotobservablewithopticalspectroscopy,acompletecompari-
samplemakesanegligibledifferencetoourresults. sonbetweentheL06sampleandoursisdifficult.Nevertheless, it
6 Murphyetal.
is clear that, since Wλ2796 is a measure of ∆v, the [M/H]–Wλ2796 interpretingtheobservedredshiftevolutionofthenumberdensity
r r
correlationinFig.1persistsathigherredshifts. ofstrongMgsystemswithsimplecross-sectionarguments.
L06 fit their [M/H]–∆v data using the least squares bisector Ofcourse,themaindifferencebetweentheB06absorbersam-
methodofIsobeetal.(1990),findingthebest-fitrelationshiptobe pleandtheonestudiedhereisthatB06usedstrongMgabsorbers
[M/H] = (1.55±0.12)log∆v[kms−1]−(4.33±0.23). For com- while we study DLAs and strong sub-DLAs. It ispossible that a
parison,weperformedfitsusingthesamebisectortechniqueafter mass–Wλ2796 correlationdoesexistfor DLAsifthesharpdropin
r
convertingourWλ2796valuesto∆vaccordingtothesimplerelation halo-massobservedbyB06forMgsystemswithWλ2796 >1.0Å
r r
∆v[kms−1] ≈ 70[kms−1Å−1]×Wλ2796[Å]. The proportionality weredominatedbysystemswithneutral hydrogen columndensi-
r
constantisderivedfromafitofWλ2796 versus∆v,withtheinverse tiesbelowthelowerlimitimposedhere,i.e.N(H)=3×1019cm−2.
r
of the variances on Wλ2796 used as weights, for the 18 absorbers However,thisseemsveryunlikelygiventhatthefractionofstrong
r
whichappearinbothoursampleandthatofL06.Thefiducialsam- MgsystemswhichareDLAs[i.e.haveN(H) = 2×1020cm−2]
pleisbest-fittedbytherelation increasesfrom∼15to∼65percentovertherangeWλ2796 = 0.6–
r
3.3Å(Raoetal.2006).Itisthereforeimportanttoconfirmorrefute
[M/H]=(1.69±0.20)log∆v[kms−1]−(4.29±0.38), (2) theB06results,whichderivefromSDSSDataRelease3.Bouche´
etal.(2007,inpreparation)willpresentananalysisbasedonasig-
whichisconsistentwiththerelationfoundbyL06.
nificantlyincreasedsampleofabsorbersfromDataRelease5.
Table 2 shows the intercept A and slope Bof the[M/H]–∆v
relationshipforthedifferentsub-samples.Inparticular,wefindno
evidenceforsignificantdifferencesinAorBinthelow-andhigh- 3.3 ArestrongMgabsorberscausedbyoutflows?
z sub-samples.ThisissimilartotheresultsofL06whofindthe
abs
slopeandintercepttobestatisticallyconsistentintworedshiftbins B06 interpret their observed mass–Wrλ2796 anti-correlation as evi-
splitatz =2.43.WealsonotethatthemedianWλ2796and[M/H] dencethatoutflowsbecomethedominantmechanismforproduc-
ofourloawbs-zabs sub-samplearemarginallyhigherthranathigh-zabs: ingMgabsorbersasWrλ2796 increasesbeyond∼1.0Å.Theyrea-
DWrλ2796E=1.56Åandh[M/H]i=−0.82fortheformercompared son that outflows are more easily ejected from lower mass star-
forminggalaxiesandso,whenviewedalongarandomQSOsight-
withDWrλ2796E=1.33Åandh[M/H]i=−1.19forthelatter.Again, line, are more likely to produce more Mg velocity components
L06findsimilartrends.
over a larger velocity range. In this outflow picture, Wλ2796 and
r
thenumberofMgvelocitycomponentsmaybeinverselyrelated
to the mass and could depend more strongly on the recent star-
formationhistoryofthehost-galaxy.L06mentionthatsomefrac-
3.2 Mass–metallicitycorrelationoranti-correlation?
tionofDLAscouldbecausedbyoutflows,buttheyagainattribute
The [M/H]–Wλ2796 (or [M/H]–∆v) correlation in DLAs and sub- theoutflowvelocityspreadtobedirectlyrelatedtothedepthofthe
r
DLAs provides an important link between the kinematics of the hostgalaxy’spotentialwell.
absorberandthemetal-enrichmenthistoryoftheabsorber.How- AninitialobjectiontotheoutflowmodelforDLAsandstrong
ever, one would like to go a step further and link the metallicity Mgsystemsmightbethatsuchhot,highlyionizedmaterialmay
withthemassofthehaloinwhichtheabsorberresides.Thisstepis notproducevelocitycomponentsofcoldgaswhichcanbetraced
potentiallyconfusing.Forexample,manyauthorshave,inthepast, with low-ionization species such as Mg. However, the outflows
tacitly assumed that the absorption-line kinematics are a reliable from local starburst galaxies are well-known to contain clouds
tracerofthehost-galaxykinematics.Thatis,∆visassumedtopos- of cold gas and dust (Lehnert&Heckman 1996; Martin 1999;
itively correlate with the circular velocity of the host-galaxy. By Heckmanetal. 2000; Rupkeetal. 2005; Martin 2006). Dense,
presuming this detail of the absorber–galaxy connection – which molecular gasisalsoknown tobeentrainedinsmallerscaleout-
shouldinsteadbeobservationallydetermined–onewouldconclude flows (e.g. Nakaietal. 1987) and even the ionization cones of
thatWλ2796 (and∆v)iscorrelatedwiththegalaxymass.L06make nearby active galactic nuclei which show starburst activity (e.g.
r
this latter assumption and use it to constrain the implied mass– Irwin&Sofue1992;Curranetal.1999).Moreover,Normanetal.
metallicityrelationship.Indeed,theyfindthistobeconsistentwith (1996)identifiedMgabsorptioninspectraoftwoQSOsight-lines
theluminosity–metallicityrelationshipderivedfromlocalgalaxies. withimpactparameters25h−1kpcand55h−1kpcnearthegalaxy
However, the assumption that galaxy or halo mass is corre- NGC520,alocalstarburstwithsuper-winds,adisturbedmorphol-
latedwithWλ2796 ischallengedbynewobservations.Bouche´etal. ogy and filamentary Hα emission along its minor axis. Thus, it
r
(2006,hereafterB06)identified1806MgsystemswithWλ2796 > seems clear that outflows from star-forming galaxies do contain
r
0.3Åand∼250000luminousredgalaxies(LRGs)withinprojected coldgas.
co-movingdistancesof13h−1Mpcoftheabsorbers.Theratioofthe Ifindeedmanyhigh-Wλ2796Mgabsorbersarecausedbyout-
r
absorber–LRGcross-correlationtotheLRG–LRGauto-correlation flows and these outflows do indeed entrain cold clouds (or cold
providesameasureofthebias-ratiobetweenabsorbersandLRGs. gas clumps cool out of the hotter outflow medium), do we ob-
In hierarchical structure formation scenarios the bias scales with servetracersof thecold, dustygas,such asmolecular hydrogen?
halo mass, so the clustering of LRGs around the Mg absorbers H isidentifiedviaUVabsorptionintheLymanandWernerbands
2
providesameasureoftheabsorberhalomass.Withthisapproach, andisknowntoariseinverycold,dustyvelocitycomponents(e.g.
B06foundthathalomassandWλ2796areanti-correlated.Theyalso Ledouxetal.2003).Itisthereforenotablethat3ofthe5highest-
r
foundthatbycombiningthepreviouslyobservedluminosity-and Wλ2796 systemsinFig.1areknowntocontainH .Indeed,thetwo
r 2
Wλ2796-dependence of the absorber cross-section, or by combin- highest-Wλ2796 systems, those towards Q0013-0029 and Q0551-
r r
ing the cross-section’s luminosity-dependence with the observed 366, both contain H (Ledouxetal. 2002; Petitjeanetal. 2002)
2
incidenceprobabilityofstrongMgabsorbers,d2N/dz dWλ2796, and have Wλ2796 values at the extreme of the Wλ2796 distribution
abs r r r
one finds that luminosity (or mass) and Wλ2796 should be anti- obtained from large SDSS Mg surveys (e.g. Nestoretal. 2003;
r
correlated. Prochteretal. (2006) reached a similar conclusion by Prochteretal. 2006). The median Wλ2796 for the H absorbers in
r 2
Metallicityand kinematicsof(sub-)DLAs 7
Figure2.Montageofthevelocitystructuresofstrongtransitionsinthe13knownH2-bearingDLAs/sub-DLAsintheliterature(6areinoursample)andthe
21-cmabsorbersinoursample.Thetransitionusedtoillustratetheextentanddistributionofabsorptioncomponentsismarkedoneachpanel.Inmostcases
theFeλ2382lineisused.Feλ2600isusedfortheabsorbertowardsQ0458−020.WhenFeλ2382wasnotavailable(particularlyathigh-zabs),Oλ1302,
Siλ1526,Alλ1670orMgλ2796isused.AllsystemsareshownonacommonvelocityscaleregisteredtotheredshiftofthestrongestH2 or21-cm
componentineachsystem.Theblackhistogramisthedatanormalizedbyalocalcontinuumfitwhilethegrey/redlinenearzerofluxshowsthenormalized
1σerrorarray.Theapproximateextentofthetransitionsismarkedbythebracketbelowthezerolevel.Notethesimilarityintheprofilesformanyofthe
H2-bearingsystems.Thesebroadandcomplexstructuresmaybecharacteristicofoutflows.The21-cmprofilesgenerallydonotshowsuchoutflowsignatures,
butthesampleistoosmallforstrongconclusionstobemade.
oursampleis2.8Åcomparedwith1.4Åforthesampleasawhole Hence,eveniftheseoutflow-likeabsorptionprofilesarethemselves
(Table2).EvenwithonlysixH systems,aKolmogorov-Smirnov rare,H maybefoundwithintheminalargefractionofcases.
2 2
testgivesaprobabilityofjust9percentthattheequivalentwidths
Fig. 2 also shows the velocity structures of the 21-cm ab-
oftheH absorbersaredrawnfromthesameparentpopulationas
2 sorbers in our sample for which UVES spectra were available.
therestofthesample.
Thesevelocitystructuresaretypicalofthoseinthegeneralpopula-
tion,seeminglyconsistentwiththefactthatthe21-cmabsorbersare
In Fig. 2 we compare the velocity structures of all the 13
generallyindistinctfrommostothersystemsinthe[M/H]–Wλ2796
knownH -bearingDLAsandsub-DLAs.Itisstrikinghowsimilar r
2 plane(Fig.1).LikeH absorption,21-cmabsorptionshouldarise
thevelocity structuresarefor the systemstowards Q0515−4414, 2
incoldclouds,butthereisnoevidenceinFig.2tosuggestthat21-
Q0551−366 and Q0013−0029: each is spread over ∆v &
cmabsorbersalsoariseinthecomplicatedandbroadoutflow-like
700kms−1 and comprisesat least threegroups of stronger veloc-
profiles like the H systems. However, the 21-cm sample is very
itycomponentsseparatedbyfew(ifany)weakercomponents.The 2
smallandisdominatedbylow-z systems;outflowsarelikelyto
systems towards Q1444+014 and Q2343+125 have very simi- abs
belesscommon below z ∼ 1due to thegeneral decline inthe
lar profiles and these are not dissimilar to the above three. Also, abs
globalstar-formationratedensity(see,e.g.,Bondetal.2001).
thesystemstowardsQ2348−011 andQ0528−250 bothhavetwo
broad,saturatedregionsseparatedbyanarrowregionoflowopti- L06notethat,undertheassumptionofamass-metallicityre-
caldepth.Suchstructuresandsymmetrieshavebeendiscussedby lationship, the absorbers with the highest equivalent widths and
Bondetal.(2001)andEllisonetal.(2003)asbeingthecharacter- metallicitieswillbemosteasilydetectedindirectimagingobser-
isticsignsofabsorptioningalacticoutflows.Itisalsoconceivable vations.Thispredictionisalsonaturalintheoutflowpicturewhere
thattheseprofilesareproducedingalaxymergers(butseediscus- Wλ2796 and the number of Mg velocity components are related
r
sioninBondetal.2001). to both the mass and the recent star-formation history of the ab-
sorberhost-galaxy.WhileoneexpectsthehighestWλ2796andhigh-
r
Although no strong conclusions can be reached due to the est[M/H]systemstobehostedbylowermassgalaxies,theyshould
samplesize,Fig.2neverthelesssuggeststhatapproximatelyhalfof be actively star-forming and show strong Hα emission. If indeed
theknownH -bearingsystemsmayariseinoutflows.Ifindeedcold H -bearing systems more often probe star-forming galaxies with
2 2
gasanddust isentrained inoutflowsandthegasiswell-shielded outflows, they may be more reliable tracers of the cosmological
thenH shouldformonthesurfaceofdust-grains.Evenifthishap- evolution of metallicity than other DLAs, many of which might
2
pens invery few clouds, the largenumber of clouds across these not be closely linked to star-formation sites/processes in high-z
profiles significantly increases the probability of observing one galaxies.WehavealreadyadvancedthispossibilityinCurranetal.
or more H -bearing clouds towards any given background QSO. (2004)andMurphyetal.(2004)basedonacomparisonofH ab-
2 2
8 Murphyetal.
sorbermetallicitieswiththoseofthegeneralDLApopulation.The identifymoreH -bearingsystems.Giventhe[M/H]–Wλ2796 corre-
2 r
H absorbersshowedafaster,morewell-definedincreasein[M/H] lationweobserve,thismaybesimilartothemetallicity-selection
2
withdecreasing redshift, although thesample was quitesmall. A approach of Petitjeanetal.(2006).However, ithastheadvantage
muchlargersampleofH -bearingsystemsisrequiredtocompute thatonecandetermineWλ2796withonlymoderate-resolutionspec-
2 r
theN(H)-weightedmetallicityevolutioninawaycomparablewith tra–orinferitfromtheequivalentwidthofbluertransitionswhen
generalDLAsamples. z & 2.5–which need not be blue- or UV-sensitive(e.g. SDSS
abs
spectra).Onecanthenfollow-upthehighestWλ2796 systemswith
r
high-resolution,UV-orblue-sensitivespectratosearchforH ab-
2
sorption.
4 CONCLUSIONS
For49DLAsandsub-DLAswithpublishedmetallicitieswehave
measured or gathered from the literature their Mg rest-frame ACKNOWLEDGMENTS
equivalent widthstosearchfor anempiricalrelationshipbetween
WethankN.Bouche´ formanydiscussionsandcriticalcomments
thekinematicspread,∆v,intheabsorbingmetal-linevelocitycom-
whichsignificantlyimprovedthepaperandS.Ellisonforproviding
ponents and the total absorber metallicity. The vast majority of
herWλ2796measurements.WealsothankH.-W.Chenforcomments
DLAs and strong sub-DLAs have Wλ2796 > 0.3Å which means r
thatmost Mgvelocitycomponents arcrosstheabsorptionprofile onanearlydraft.MTMthanks PPARCfor anAdvanced Fellow-
ship.Someofthisresearchwasbasedonobservationsmadewith
aresaturated.Thus,Wλ2796providesasimplemeasureof∆vwhich
r ESOTelescopesattheParanalObservatoriesunderprogrammeIDs
canbederivedevenfromrelativelylow-resolutionQSOspectra.
listedinTable1. Thisresearchhas made useof the NASA/IPAC
Wefindacorrelationbetween[M/H]andWλ2796atthesignifi-
r ExtragalacticDatabase(NED)whichisoperatedbytheJetPropul-
cancelevelof4.2σ[Fig.1(left)]whichincreasesto4.7σwhenthe
sionLaboratory,CaliforniaInstituteofTechnology,undercontract
[M/H] values are corrected for mild cosmological evolution over
withtheNationalAeronauticsandSpaceAdministration.Thisre-
thesample’sredshiftrangeof0.2<z <2.6[Fig.1(right)].Even
abs search has also made use of NASA’s Astrophysics Data System
for our most conservative sub-sample, where weinclude only 26
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ChurchillC.W.,MellonR.R.,CharltonJ.C.,VogtS.S.,2003,ApJ,593,
anti-correlatedwithWλ2796.Thisresult,together withthe[M/H]–
r 203
Wλ2796relationshipobservedhere,impliesthattheabsorbermetal-
r CurranS.J.,RydbeckG.,JohanssonL.E.B.,BoothR.S.,1999,A&A,
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