C, N, O Abundances in the Most Metal-Poor Damped Lyman alpha Systems

0.6 MB·English

by  Max Pettini

Mon.Not.R.Astron.Soc.000,1–??(2008) Printed2February2008 (MNLATEXstylefilev2.2)
C, N, O Abundances in the Most Metal-Poor Damped
⋆
Lyman alpha Systems
Max Pettini1, Berkeley J. Zych1, Charles C. Steidel2, and Fred H. Chaffee3
8 1Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
0 2California Institute of Technology, Mail Stop 105-24, Pasadena, CA 91125, USA
0 3W. M. Keck Observatory, 65-1120 Mamalahoa Highway, Kamuela, HI 96743, USA
2

n
a Accepted —;Received—;inoriginalform—
J

4
1 ABSTRACT

]
Thisstudyfocusesonsomeofthemostmetal-poordampedLyαabsorbersknown
h
p inthespectraofhighredshiftQSOs,usingnewandarchivalobservationsobtainedwith
- UV-sensitive echelle spectrographs on the Keck and VLT telescopes. The weakness
o and simple velocity structure of the absorption lines in these systems allow us to
r
measure the abundances of several elements, and in particular those of C, N, and
t
s O, a group that is difficult to study in DLAs of more typical metallicities. We find
a that when the oxygen abundance is less than ∼ 1/100 of solar, the C/O ratio in
[
high redshift DLAs and sub-DLAs matches that of halo stars of similar metallicity

2 andshowshighervaluesthanexpectedfromgalacticchemicalevolutionmodels based
v on conventional stellar yields. Furthermore, there are indications that at these low
9
metallicities the N/O ratio may also be above simple expectations and may exhibit a
2
minimumvalue,asproposedbyCenturi´onandhercollaboratorsin2003.Bothresults
8
canbeinterpretedasevidenceforenhancedproductionofCandNbymassivestarsin
1
thefirstfewepisodesofstarformation,inourGalaxyandinthedistantproto-galaxies
.
2 seenasQSOabsorbers.The higher stellaryields impliedmay haveanorigininstellar
1
rotation which promotes mixing in the stars’ interiors, as considered in some recent
7
model calculations. We briefly discuss the relevance of these results to current ideas
0
on the origin of metals in the intergalactic medium and the universality of the stellar
:
v initial mass function.
i
X Key words:
r
a
1 INTRODUCTION densities and corresponding element abundances straight-
forward,andtherichvarietyofresonanttransitionsatrest-
TheneutralhydrogencloudswhichgiverisetodampedLy-
frame ultraviolet wavelengths allows a detailed picture of
man alpha systems (DLAs; Wolfe, Gawiser, & Prochaska
the chemical composition of many DLAs to be assembled
2005) in the spectra of high redshift QSOs provide impor-
(e.g. Prochaska, Howk,&Wolfe 2003; Rixet al. 2007). The
tantinformationontheearlychemicalevolutionofgalaxies,
elements carbon, nitrogen and oxygen have, however, been
complementingthat obtainedfrom moretraditional studies
comparatively little studied sofar in DLAs,despitethekey
of metal-poor stars and Hii regions in the Milky Way and
roletheyplayin stellar nucleosynthesis.CandOareabun-
nearby galaxies (Pettini 2004; Henry & Prochaska 2007).
dantelementswithintrinsicallystrongatomictransitions;in
The relatively simple physics of interstellar line formation
combination, these two factors result in resonance absorp-
(Str¨omgren 1948) makes the determination of ion column
tion lines which are strongly saturated—and thus unusable
for abundance determinations—even at the low metallici-
⋆ Based in part on data obtained at the W.M. Keck Observa- ties of most DLAs. The problem is particularly acute for
C. For O some weaker transitions are available at far-UV
tory, which is operated as a scientific partnership among the
California Institute of Technology, the University of Califor- wavelengths; these, however, can only be accessed from the
nia, and NASA, and was made possible by the generous finan- ground at redshifts z∼> 2.5. The absorption lines of N, on
cial support of the W.M. Keck Foundation. Based in part on theotherhand,tendtobeweakandareoftenblendedwith
UVESobservations madewiththe European Southern Observa- intergalacticLyαforestlines;nevertheless,theabundanceof
tory VLT/Kueyen telescope at Paranal, Chile, obtained in pro- N in DLAshas been the subject of several studies since at-
gramme078.A-0185(A)andfromthepublicdataarchive.
2 M. Pettini et al.
Table 1.JournalofObservations
QSO V orga zem zabs Telescope/ Wavelength Resolution Integration Time S/Nb N(Hi)
(mag) Instrument Range(˚A) (kms−1) (s) (cm−2)
Q0913+072 17.1 2.785 2.61843 VLT/UVES 3310–9280c 6.7 77500 60 2.2×1020
SDSSJ1016+4040 19.5 2.991 2.81633 Kecki/HIRESb 3190–5970c 7.5 15000 12 0.8×1020
SDSSJ1558+4053 18.7 2.635 2.55332 Kecki/HIRESb 3190–5970c 7.5 25200 18 2.0×1020
Q2206−199 17.3 2.559 2.07624 VLT/UVES& 3100–10000c 6.5 68500 100 2.7×1020
Kecki/HIRESr
aMagnitudesareg forSDSSobjects andV forotherQSOs.
bIndicativesignal-to-noiseratioat5000˚A.
cWithsomewavelength gaps
tentionwasfirstdrawntoitbyPettini,Lipman&Hunstead Using this method, we have discovered two of the
(1995). most metal-poor DLAs, with oxygen abundance [O/H]≃
The relative abundances of C, N, and O at low metal- −2.4, in the spectra of the QSOs SDSS J1016+4040 and
licities hold clues to the nature of the stars responsible for SDSSJ1558+4053;highresolutionobservationsofthesesys-
their production in the earliest stages of nucleosynthesis in tems are reported here for the first time. We have also re-
galaxies, and their measurement in DLAshas the potential observedtwoQSOswithknownlow-metallicity,narrow-line
ofclarifyingsomeunresolvedquestionsraisedbyrecentstel- DLAs: Q0913+072 and Q2206−199; the new data add sig-
lar work, as discussed in detail later. The marked increase nificantlytothosealready availablein theliteraturethanks
inthenumberofknownDLAsaffordedbytheSloanDigital to thevery high S/N ratio achieved.
SkySurvey(Prochaska,Herbert-Fort,&Wolfe2005)makes
itpossibletosearchfordampedsystemswiththecharacter-
2.2 Observations
isticswhichfacilitatethemeasurementoftheabundancesof
C, N, and O, primarily low metallicity and simple velocity The observations were performed with the HIRES (Vogt et
structure.Inthispaperwereportnewhighresolutionspec- al. 1994) and UVES (Dekker et al. 2000) echelle spectro-
troscopic observations of four such absorption systems,1 all graphson,respectively,theKeckiandVLT2telescopes.Rel-
with metallicities [O/H]< −2.2 When combined with pub- evant details are collected in Table 1. For the SDSS QSOs
lishedmeasurementsintwosimilar systems,thisdataset— weusedtherefurbishedHIRESwith itsUV-sensitivecross-
althoughmodest—shedsnewlightonthenucleosynthesisof disperser, covering the wavelength range 3190–5970˚A. The
C, N, and O in thelow metallicity regime. UVES observations of Q0913+072 and Q2206−199 used a
variety of dichroics and wavelength settings to cover essen-
tially all of the optical wavelength range from ∼ 3200 −
2 OBSERVATIONS AND DATA REDUCTION 3300˚A to ∼ 9000−10000˚A. The resolution is FWHM≃
6−7kms−1 for spectra recorded with eitherspectrograph.
2.1 Target Selection TheS/N ratio variesconsiderably along thespectrum, par-
ticularly for the UVES data where a given portion of the
The four QSOs observed were selected based on the weak-
spectrummayhavebeencoveredwithanumberofdifferent
ness of metal lines in DLAsalready known to bepresent in
instrument settings; in the penultimate column of Table 1
theirspectra,eitherfromthetrawloftheSDSSQSOsample
we list an indicative valueof S/N at 5000˚A.
by Prochaska et al. (2005) or from earlier published stud-
ies.Ithasnowbecomeapparentthatmetallicityandstrong
metallinewidthsarecorrelatedinDLAs(e.g.Ledouxetal.
2.3 Data reduction
2006a;Murphyetal.2007a;Prochaskaetal.2007).Thus,an
efficient way to preselect for further study the most metal- Thereduction of thetwo dimensional images into co-added
poor DLAs is to focus on DLAs in which the associated one-dimensional spectra followed standard procedures. The
strongest metal lines, which are nearly always saturated, HIRESspectra were reduced with themakee pipeline soft-
have very low values of equivalent width—so low as to re- warekindlymaintainedbyT.A.Barlow,whilefortheUVES
main undetected at the moderate resolution and signal-to- spectra we used the pipeline reduction provided bythe Eu-
noise ratio (S/N) of most SDSS spectra. In addition, the ropeanSouthernObservatorybutwith modificationswhich
simple kinematic structure of the absorbing gas implied by improvetheaccuracyofthewavelengthcalibrationandflux
thelow equivalentwidthssimplifies theabundanceanalysis extraction (see Murphy et al. 2007b). These pipelines take
compared with the more usual occurrence of multiple ab- care of the initial stages of bias subtraction, flat-fielding,
sorption components spread over100–200km s−1. wavelength calibration, extraction of individual echelle or-
ders, and flux calibration. The individual spectra of each
QSO were averaged and the echelle orders merged with
1 Three DLAs and one sub-DLA, according to the conventional purpose-builtsoftware3 designed tomaximise thesignal-to-
distinctionbetweenthetwoatneutralhydrogencolumndensities
N(Hi)=2×1020cm−2.
2 Asusual,[O/H]≡log(O/H)DLA−log(O/H)⊙. 3 Availablefromhttp://astronomy.swin.edu.au/∼mmurphy/UVES popler
C, N, O in the most metal-poor DLAs 3
Figure 1. Observed profiles (black histograms) and fitted Voigt profiles (continuous red lines) of selected absorption lines in the
zabs = 2.61843 DLA in Q0913+072. Dashed red lines indicate nearby absorption not attributed to the DLA. The y-axes of the plots
show residualintensity; note the expanded y-scaleof thebottom plot. Thefulllistofabsorption linesanalysed inthis DLAisgivenin
Table3.
4 M. Pettini et al.
noise ratio of the final spectrum which was binned onto a Table 2. Absorption Components of Low Ion Transitions
linear, vacuum heliocentric wavelength scale with approxi- in Q0913+072
mately threebins perresolution element.
Component zabs b Fractiona
Number (kms−1)
2.4 Abundance Measurements
1 2.61828 3.7 0.28
Absorption lines from each DLA were identified and their
2 2.61843 5.4 0.72
profiles fitted with Voigt profiles generated by the VPFIT
(version8.02)softwarepackage,asdescribedbyPettinietal. aFractionofthetotalcolumndensityofSiii.
(2002a) and Rix et al. (2007). Briefly, VPFIT uses χ2 min-
imisation todeducethevaluesofredshiftz,columndensity
N (cm−2),andDopplerparameter b(km s−1) that best re- Table3.IonColumnDensitiesinQ0913+072,zabs=2.61843
producethe observed absorption line profiles. VPFITtakes DLA
into account the instrumental broadening function in its
χ2 minimisation and error evaluation.4 We used the com- Ion Transitionsused logN(X)
pilation of laboratory wavelengths and f-values by Morton
Hi 1215 20.34±0.04
(2003) with recent updates byJenkins & Tripp (2006).
Cii 1036,1334 13.98±0.05
TheDLAsobservedareamong thosewiththesimplest
Ni 1199 12.29±0.12
kinematicstructure,generallyconsistingofonlytwoorthree Oi 1039,1302 14.63±0.01
absorption components separated by ∼ 10 − 20km s−1; Alii 1670 11.78±0.03
this fact, coupled with the high resolution and generally Aliii 1854 611.03
high S/N of the spectra, made the profile fitting procedure Siii 1190,1193,1260,1304, 1526 13.30±0.01
straightforwardanditsresultsunambiguous.Anotherconse- Feii 1608,2344,2374,2382 12.99±0.01
quenceoftheunusuallysimplelineprofilesisthatthesame
set of ‘cloud’ parameters, b and z, was found to reproduce
well all the absorption lines considered in each DLA; nat-
urally, lines arising from the same ground state of a given mostmetal-deficientDLAknown’anddrewattentiontoits
ion were fitted together to give the best-fittingvalue of the role as a probe of early nucleosynthesis. As part of a pro-
column density N(Xi), where N(Xi) is the ith ion stage grammetomeasuretheprimordialabundanceofdeuterium,
of element X. In each DLA, the column density of neutral we re-observed this QSO with UVES in 2007. Including a
hydrogen was determined by fitting the damping wings of few spectra downloaded from the UVES data archive, the
the Lyα line which are its most sensitive measure; we also total exposure time devoted to Q0913+072 is 77500s; the
checked that the resulting value of N(Hi) provides a good corresponding signal-to-noise ratio near 5000˚A is S/N≃60
fit to thehigher order Lyman lines covered by ourspectra. (see Table 1).
Element abundances were deduced from the observed AselectionoftheabsorptionlinesinthisDLAisshown
ratios N(Xn)/N(Hi), where N(Xn) denotes the column in Figure 1. The metal lines consist of two closely spaced
density of the ion stage which contains the dominant frac- components; profile decomposition with VPFIT returns b-
tionofelementXinHiregionsoftheMilkyWayinterstellar values of 3.7 and 5.4km s−1 and a velocity separation of
medium (e.g. Morton et al. 1973). For the elements consid- 12.4km s−1 (see Table 2). Table 3 lists theabsorption lines
eredhere,theseareeithertheneutrals(Oi,Ni)orthefirst analysed and the corresponding values of column density;
ions(C ii,Alii,SiiiandFeii).However,wehavealsomea- the errors on N(X) are the formal values returned by VP-
suredthecolumndensitiesofhigherionsstages,whentheir FIT.MostofthemetallinesinthisDLAareweak;inpartic-
absorption lines are present in our spectra, in order to as- ular, from Figure 1 it can be seen that both Ciiλ1334 and
sess the corrections, if any, that need to be applied to our Ciiλ1036areunsaturated(theselinesarenearlyalwayssat-
abundance determinations to account for the possible con- urated in DLAs), as is Oiλ1039. Although both Cii lines
tributions by ionised gas to the absorption lines observed. arepartiallyblendedwithbroadabsorptionatlongerwave-
Whenelementabundancesarereferredtothesolarscale,we length(modelled byVPFITwithtwocomponents),theim-
have used the compilation by Asplund, Grevesse, & Sauval pact of these contaminating features on the determination
(2005). ofN(Cii)isrelativelyminor:aconservativeestimateofthe
error on logN(Cii) deduced from several VPFIT trials is
±0.05.
Wehaveadefinite(4σ)detectionofNiλ1199.55inboth
3 INDIVIDUAL ABSORPTION SYSTEMS
velocitycomponents;thetwoother,weaker,membersofthe
3.1 Q0913+072; DLA at zabs =2.61843 Niλλ1199.55,1200.22,1200.71tripletareblendedwithLyα
forest lines.
The metal-poor DLA in front of this bright QSO has been
Turningtoabsorptionlinesfromionisedgas,Ciiiλ977,
known since thesurveyby Pettiniet al. (1997). It was sub-
Siiiiλ1206, and Feiiiλ1122 are blended with Lyα forest
sequentlystudiedathigherspectralresolutionbyLedouxet
lines, while Niiλ1084 is undetected (which is unsurprising
al.(1998)andErnietal.(2006)whoconsideredittobe‘the
given the weakness of the Ni lines). We do, however, have
a tentative detection of Aliiiλ1854; given the weakness of
4 VPFIT is available from http://www.ast.cam.ac.uk/˜ this feature, we consider the column density of Aliii to be
rfc/vpfit.html an upperlimit.
C, N, O in the most metal-poor DLAs 5
Figure 2. Observed profiles (black histograms) and fitted Voigt profiles (continuous red lines) of selected absorption lines in the
zabs = 2.81633 DLA in SDSSJ1016+4040. Dashed red lines indicate nearby absorption not attributed to the DLA. The y-axes of the
plotsshowresidualintensity.ThefulllistofabsorptionlinesanalysedinthisDLAisgiveninTable5.
6 M. Pettini et al.
Table 4. Absorption Components of Low Ion Transitions Table 6. Absorption Components of Low Ion Transitions
in SDSSJ1016+4040 in SDSSJ1558+4053
Component zabs b Fractiona Component zabs b Fractiona
Number (kms−1) Number (kms−1)
1 2.81604 5.0 0.11 1 2.55264 9.1 0.01
2 2.81633 6.9 0.89 2 2.55298 8.8 0.06
3 2.55332 6.1 0.80
aFractionofthetotalcolumndensityofCii. 4 2.55334 25.1 0.13
aFractionofthetotalcolumndensityofSiii.
Table 5.Ion Column Densities in SDDSJ1016+4040,zabs =
2.81633 DLA
Table 7.Ion Column Densities in SDDSJ1558+4053,zabs=
Ion Transitionsused logN(X) 2.55332 DLA
Hi 1215 19.90±0.11
Ion Transitionsused logN(X)
Cii 1334,1036 13.66±0.04
Ni 1199.5 612.76 Hi 1215 20.30±0.04
Oi 1302,988.77 14.13±0.03 Cii 1334,1036 14.22±0.06
Siii 1260,1526 12.90±0.05 Ni 1199.5,1200.2,1200.7 12.66±0.07
Oi 1302,988.77,988.65 14.54±0.04
Alii 1670 11.92±0.06
Siii 1190,1193,1260,1304, 1526 13.32±0.02
3.2 SDSSJ1016+4040; DLA at zabs =2.81633 Feii 1096,1144,1260,1608, 13.07±0.06
This QSO is one order of magnitude fainter than
Q0913+072. We recorded its spectrum from 3190˚A to
5970˚A on the night of 2007 May 6 with the refurbished (b = 6.1km s−1) component at zabs = 2.55332; other,
HIRES spectrograph on the Kecki telescope, using the UV weaker, components are seen at positive and negative ve-
cross-disperser and UV sensitive CCD array. With a total locities in Cii and Siii and probably originate in ionised
exposure time of 15000s we achieved a modest S/N≃ 12 gas (Figure 3 and Table 6). The entries in Table 7 are for
near5000˚A.AscanbeseenfromFigure2,whatappearsas the neutral component only. The absorption lines in this
a likely damped Lyα line in the low resolution SDSS spec- DLAarestrongerthanintheprevioustwocases. Neverthe-
trumbreaksupintothreedistinctcomponentsatthehigher less, we cover at least one unsaturated transition for every
resolution of the HIRES data. However, the central one of species considered except possibly for Cii: both λ1334 and
thesethreeabsorptionlinesdoesexhibitdampingwings;we λ1036arejustsaturated.EvenforCii,however,wecande-
deduceN(Hi)=(8±2)×1019cm−2.Suchsystemsaresome- terminethecolumndensitywithconfidencebecause:(i)the
times referred to as sub-DLAs since their column densities absorption profile (of the central, dominant component) is
arelowerthanthehistoricaldefinitionofaDLAasasystem well fitted by the same b-value which fits best all the other
with N(Hi)>2×1020cm−2 (Wolfe et al. 1986). lines; (ii) larger values of b are incompatible with the ob-
AllthemetallinesinthisDLAareweak(seeFigure2). served width of theline which is resolved in our data. Note
The first ions exhibit two narrow velocity components with that while we cannot exclude, in principle, that narrower
b = 5.0 and 6.9km s−1 separated by 23km s−1, with the componentsmaybepresentwithinthemarginallysaturated
latter accounting for ∼ 90% of the total column density Cii line profiles, their unrecognised presence would lead to
(see Table 4). The former, at zabs = 2.81604, probably an underestimate of [C/H] and C/O, further strengthening
arises in ionised gas sinceit isnot detectedin theOiλ1302 the main conclusion of our analysis of the behaviour of the
andλ988.77 lines(thelatterbeingthestrongest memberof C/O ratio at low metallicities, as discussed in Section 5.
the Oiλλ988.77,988.65,988.58 triplet shown in Figure 2). AsforthecaseoftheDLAinSDSSJ1016+4040,weare
Therefore, we have not included this component in the val- unable to deduce reliable column densities of higher ionisa-
uesofN(Cii)andN(Siii)attributedtotheDLAinTable5. tionstagesthanthoselistedinTable7becausetherelevant
Ni is below our detection limit; similarly, absorption lines absorption lines are either blended, undetected, or outside
from higher ion stages than those dominant in Hi regions thewavelength range of ourHIRES spectrum.
are either blended (e.g. Siiiiλ1206), or undetected.
3.4 Q2206−199; DLA at zabs=2.07624
3.3 SDSSJ1558+4053; DLA at zabs =2.55332
Themetal-poorDLAinthisQSOhasbeenknownsincethe
We observed this QSO with HIRES on the night of 2006 early echelle observations by Pettini et al. (1990) and by
June17,achievingS/N≃18near5000˚Awitha25200sex- Prochaska & Wolfe (1997). Here we combine spectra from
posure.Theechelledataconfirmthatthestrongabsorption the UVES and HIRES data archives to achieve S/N≃ 100
feature near 4320˚A in the discovery SDSS spectrum is in- at5000˚Aandextensivespectralcoveragefrom ∼3100˚Ato
deedadampedLyαlinewithN(Hi)=2.0×1020cm−2 (see 1µm.
Figure 3). Even at this high resolution and signal-to-noise ratio
The neutral gas is concentrated in a single, narrow thisDLAappearstoconsist ofasingle, narrowcomponent:
C, N, O in the most metal-poor DLAs 7
Figure 3. Observed profiles (black histograms) and fitted Voigt profiles (continuous red lines) of selected absorption lines in the
zabs = 2.55332 DLA in SDSSJ1558+4053. Dashed red lines indicate nearby absorption not attributed to the DLA. The y-axes of the
plotsshowresidualintensity.ThefulllistofabsorptionlinesanalysedinthisDLAisgiveninTable7.
8 M. Pettini et al.
Figure 4. Observed profiles (black histograms) and fitted Voigt profiles (continuous red lines) of selected absorption lines in the
zabs = 2.07624 DLA in Q2206−199. Dashed red lines indicate nearby absorption not attributed to the DLA. The y-axes of the plots
showresidualintensity.ThefulllistofabsorptionlinesanalysedinthisDLAisgiveninTable9.
C, N, O in the most metal-poor DLAs 9
Table 8. Absorption Components of Low Ion Transitions theseDLAshaveattained a levelof metal enrichment com-
in Q2206−199 parable to that typical of intergalactic Lyα forest clouds at
the same redshifts (e.g. Simcoe et al. 2004). Although the
Component zabs b Fractiona sourceoftheseintergalacticmetalsisstillsomewhatcontro-
Number (kms−1) versial(seeRyan-Weberet al. 2006 andreferences therein),
itseemslikelythatsuchmetal-poorDLAshaveexperienced
1 2.07624 6.5 1.00 verylittle, if any,star formation in situ.
It is also interesting to note that no DLA has yet been
aFractionofthetotalcolumndensityofSiii.
found with metallicity less than 1/1000 solar, despite the
fact that such metal-poor stars do exist in the halo of our
Table9.IonColumnDensitiesinQ2206−199,zabs=2.07624 Galaxy. In drawing such comparisons, however, it must be
DLA bornein mind that such extremely metal-poor stars consti-
tute a tiny fraction of the stellar populations of the Milky
Ion Transitionsused logN(X) Way.Itmaysimplybethecasethatwithonlyafewhundred
metallicity measurements in DLAsso far, we simply donot
Hi 1215 20.43±0.04
Cii 1334, 1036 14.41±0.03 havethestatistics necessary topickoutsuchoutliersin the
Ni 1199.5,1200.2, 1200.7 12.79±0.05 metallicity distribution.
Nii 1083 12.46±0.10 Finally, we draw attention to the fact that in the four
Oi 1302, 1039 15.05±0.03 systems in which this ratio could be measured, [O/Fe]≃
Alii 1670 12.18±0.01 +0.25 to +0.5 (see Table 10) consistent with the enhance-
Aliii 1854, 1862 11.59±0.04 ment of the alpha-capture elements relative to iron seen in
Siii 1193, 1260,1304,1808 13.65±0.01 halo stars of similar metallicity. At these low DLA metal-
Siiii 1206 612.86 licities, the depletion of Fe onto dust grains is expected to
Feii 1121, 1260,1608,2344,2374, 2382,2600 13.33±0.01
beminimal (Vladilo2004; Akermanet al.2005) sothat the
observed N(Oi)/N(Feii) ratio gives a good indication of
inherent departures from the solar relative abundances of
the profiles of all the absorption lines from species which these two elements. As we show in section 4.1, differential
are dominant in Hi regions are well reproduced bya model ionisationcorrectionsbetweenOiandFeiiarealsounlikely
‘cloud’ with b = 6.5km s−1 (Figure 4 and Tables 8 and to account for this effect.
9). The availability of a number of transitions with differ-
ingf-valuesformostionsensuresthatthecolumndensities
are well determined. The Cii lines are just saturated but, 4.1 Ionisation Corrections
for the reasons outlined above, this is unlikely to affect our
Asexplainedinsection2.4,therelativeelementabundances
conclusionsconcerningtherelativeabundancesofCandO.
listed inTable10 werederivedbydividingthecolumnden-
ThankstoitshighS/Nratioandwidewavelengthcoverage,
sitiesN(Xn)byN(Hi),whereN(Xn)isthedominantion-
ourcombinedUVES+HIRESspectrumdoescoveranumber
isation stage of element X in Hi regions. Strictly speak-
oftransitionsfrom ionsthatonlyoccurinHiiregions(Nii,
ing, this commonly adopted assumption only holds true
Aliii,andSiiii;seeTable9);theircolumndensitiesarelower
for O out of the elements considered here. It has been
than those of the corresponding Hi region species. Finally,
known for a long time that charge-exchange reactions keep
itcanbeseenfromFigure4thatwehaveacleardetectionof
N(Oi)/N(Hi) very close to O/H for a wide range of phys-
all threelines of theNiλλ1199.55,1200.22,1200.71 triplet.
ical conditions (e.g. Field & Steigman 1971). For elements
whichinHiregionsaremostlysinglyionised—thatis,C,Al,
Si and Fe in our case—the possibility exists that some Hii
4 ELEMENT ABUNDANCES gas—at thesame velocityas theHiandthusindistinguish-
able kinematically—may contribute to column densities of
OurabundancemeasurementsinthefourDLAsarecollected
their first ions; if true, this would lead to an overestimate
in Table 10. We have searched the literature for other ex- of the corresponding element abundances.5 Conversely, N
amples of DLAs (or sub-DLAs) where the Cii and Oi ab- maybeoverionisedrelativetoH,sothateveninHiregions
sorption lines are unsaturated, so that the corresponding N(Ni)/N(Hi)<(N/O) leading to an underestimate of the
column densities can be determined, and found only two
N abundance.
other cases, also included in Table 10, from the work by
Ofcourse,oneofthemajormotivationsforusingDLAs
Dessauges-Zavadsky et al. (2003) and P´eroux et al. (2007).
inabundancestudies,isthefactthatwhenthecolumnden-
Bothofthesecasesareofsomewhatlowerhydrogencolumn sityofneutralgasexceedsN(Hi)=1×1020cm−2 suchion-
density and at higher redshifts than the four reported here
isationcorrectionsareexpectedtobesmallandcomparable
but,giventhepaucityofC/O measurementsuptonow,we
to other sources of error, as concluded by a number of de-
shall consider them in thepresent analysis.
tailedanalyses(e.g.Viegas1995;Vladiloetal.2001).Given
As can be seen from Table 10, all four DLAs in the
present work have metallicities of less than 1/100 of solar.
The DLA in front of Q0913+072 remains the most metal- 5 It is worthwhile pointing out here that the neutral atoms of
poor in [Fe/H], but its oxygen abundance of ∼ 1/250 solar thesespecies,whichwehaveneglected,makenegligiblecontribu-
is similar to those of the two newly discovered DLAs from tionstotheabundancemeasurements,becauseeveninHiregions
the SDSS. With [O/H]≃ −2.4 and [C/H]∼ −2.5 to −2.7, N(Ci)≪N(Cii),andsimilarinequalitiesapplytoAl,SiandFe.
10 M. Pettini et al.
Table 10.ElementAbundances
QSO zabs logN(Hi) [O/H]a log(C/O)b log(N/O)c [Al/H]a [Si/H]a [Fe/H]a
Q0913+072 2.61843 20.34 −2.37 −0.65 −2.34 −2.99 −2.55 −2.80
SDSSJ1016+4040 2.81633 19.90 −2.43 −0.47 6−1.37 ... −2.51 ...
SDSSJ1558+4053 2.55332 20.30 −2.42 −0.32 −1.88 −2.82 −2.49 −2.68
Q2206−199 2.07624 20.43 −2.04 −0.64 −2.26 −2.69 −2.29 −2.55
OtherObjects:
J0137−4224d 3.665 19.11 −2.39 −0.24 ... ... −2.22 ...
J2155+1358e 4.21244 19.61 −1.77 −0.55 ... −2.12 −1.87 −2.13
a[X/H]=log(X/H) −log(X/H) .Solarabundances fromAsplundetal.(2005).
DLA ⊙
bThesevaluesarenot referredtothesolarabundance.Forreference,log(C/O) =−0.27(Asplundetal.2005).
⊙
cThesevaluesarenot referredtothesolarabundance.Forreference,log(N/O) =−0.88(Asplundetal.2005).
⊙
dP´erouxetal.(2007).
eDessauges-Zavadskyetal.(2003).
theimportanceofthispointforthemeasurementoftheC/O The DLA which provides the most extensive information
ratio,however,weverifiedthatthisisindeedthecaseforthe in this respect is the one in line to Q2206−199. Compar-
twoDLAsinoursmallsamplewherecolumndensitiesofions ing the measured N(Nii)/N(Ni), N(Aliii)/N(Alii), and
from Hiiregions could bemeasured in our spectra. N(Siiii)/N(Siii)ratios in Table9 (all theseratios are <1)
Specifically, we run a series of photoionization models with the values computed by cloudy for different densi-
withthesoftwarepackagecloudy(Ferlandetal.1998;Fer- ties, we find that in this DLA logn(H)/cm−3 > −2 so
land 2000)6, approximating the DLAs as slabs of constant that the ionisation corrections for carbon and nitrogen are
density gas irradiated by the metagalactic ionising back- |IC(C),IC(N)| < 0.1. This is also the case for the DLA
ground (Haardt & Madau 2001) and the cosmic microwave in Q0913+072, although in this case we rely solely on the
background. We assumed a metallicity of 1/100 solar and N(Aliii)/N(Alii)ratio.P´erouxetal.(2007)andDessauges-
no differential depletions due to dust grains (Akerman et Zavadsky et al. (2003) reached similar conclusions for the
al. 2005), buttheresults ofthecomputationsarenot sensi- two sub-DLAs included in Table 10, based on similar pho-
tivetotheseassumptions.Wevariedthegasvolumedensity toionisation analyses. ForthetwoDLAsin theSDSSQSOs
from log[n(H)/cm−3] = −3 to +3 and stopped the calcu- we have no comparable information and therefore cannot
lation when the measured column density of neutral gas in come to definite conclusions concerning ionisation correc-
each DLA was reached. From the cloudy output we then tions. However, even in these two cases, it is unlikely that
calculated for each element observed the ionization correc- to|IC(C),IC(N)|>0.2. Largerionisation corrections would
tion implyvolumedensitiesn(H)<0.001cm−3andthereforelin-
earsizes for theabsorbing structuresinexcess of ∼100kpc
IC(X)=log N(X) −log N(XN) (1) (for N(Hi)= 2×1020cm−2); it seems unlikely that gas in
(cid:20)N(H)(cid:21) (cid:20)N(HI)(cid:21)
intrinsic computed suchlarge volumeswould maintain internalvelocity disper-
which is negative when the abundance of an element has sions of only a few km s−1 as required by the narrow line
been overestimated by neglecting theionized gas. widths.
Figure 5 illustrates the results of this exercise for the Insummary,ourbest estimates oftheionisation struc-
DLA in Q2206−199 (the conclusions are similar for the ture of the sample of six DLAs and sub-DLAs consid-
other DLAs considered here). As expected, Oi tracks Hi ered here leads us to conclude that the measured ratios
very closely at all densities. At very low densities N is ove- N(Cii)/N(Oi)andN(Ni)/N(Oi)reflectthetruevaluesof
rionisedrelativetoH(thatissomeNispresentasNiieven C/O and N/O in theDLAstowithin 0.2 dex and probably
in Hi regions), while the column densities of the other ele- to within 0.1 dex in most cases.
mentsconsideredhavesomecontributionfromionisedgasso
that, for example, N(Cii)/N(Hi)>C/H. However, for the
elementswhicharethefocusofthepresentwork,C,N(and 5 THE C/O RATIO AT LOW METALLICITIES
O), the ionisation corrections amount to less than 0.1dex
provided the gas density is greater than n(H)≃ 0.01cm−3. Oneoftheaimsofthepresentworkistoinvestigatethebe-
Whenn(H)>0.1cm−3,theionisationcorrectionsforallthe haviour of the C/O ratio at low metallicities, motivated by
elements in Table 10 are ∼< 0.05dex. the results of Akerman et al. (2004) and Spite et al. (2005)
which we now summarise. Akerman et al. (2004) measured
We can obtain an indication of the density which ap-
the abundances of C and O in 34 halo stars of metallicities
pliestothegasintheDLAsfromconsiderationoftheratios
of different ions stages, since they also depend on the den- from [Fe/H]= −0.7 to −3.2 and combined such data with
analogous measurements in disk stars from the literature.
sity, once the radiation field has been fixed (see Figure 6).
The pattern that emerged from their study is illustrated in
Figure 7. Briefly, in halo stars [C/O]≈ −0.5 (correspond-
6 http://www.nublado.org/ ing to an observed log(C/O) ≈ −0.77 since in the Sun