Structure of the Mg II and damped Lyman-α systems along the line of sight to APM 08279+ 5255⋆

14 Pages·2000·0.99 MB·English

Astron.Astrophys.359,457–470(2000) ASTRONOMY
AND
ASTROPHYSICS
α
Structure of the Mg and damped Lyman- systems
II
(cid:63)
along the line of sight to APM 08279+5255
P.Petitjean1,2,B.Aracil1,R.Srianand3,andR.Ibata4
1 Institutd’AstrophysiquedeParis–CNRS,98bisBoulevardArago,75014Paris,France
2 UACNRS173–DAEC,ObservatoiredeParis-Meudon,92195MeudonCedex,France
3 IUCAA,PostBag4,GaneshKhind,Pune411007,India
4Max-Plank-Institutu¨frAstronomie,Ko¨nigstuhl17,69117Heidelberg,Germany
Received27January2000/Accepted9May2000
Abstract. Astudyoftheabsorptionsystemstowardthegravi- absorberislargerthan200h−1 pc.ColumndensitiesofAlii,
75
tationallylensedquasarAPM08279+5255ispresented. Feii,Siii,CiiandOiindicateabundancesrelativetosolarof
MostoftheMgiisystemsintheredshiftrangez (cid:24)1.2–2.07, −2.31,−2.26,−2.10,−2.35and−2.37for,respectively,Fe,Al,
although saturated, show large residuals at the bottom of the Si,CandO(forlogN(Hi)=20.3).Thesesurprizinglysimilar
lines. The most likely interpretation is that individual clouds valuesindicatethattheamountofdustinthecloudisverysmall
withinMgiihalosdocoveronlyoneofthetwobrightestQSO asareanydeviationsfromrelativesolarabundances.Itseems
images.Theseparationbetweenthetwolinesofsightdecreases likelythattheupperlimitsfoundforthezincmetallicityofsev-
from1.7to0.7h−751kpc(qo=0.5,zlens=1)betweenz=1.22and eraldampedLyman-αsystemsazt >3inprevioussurveysis
z=2.07.ThisrevealsthatMgii halosaremadeofacollectionindicativeofatruecosmologicalevolutionofthemetallicityin
ofcloudsofradiussmallerthanabout1h−1kpc. individualsystems.
75
TwostrongMgiiabsorbersatzabs=1.062and1.181arestud-
ied in detail. This is the first time that the Na iλ3303 doubletKey words: galaxies: quasars: absorption lines – galaxies:
isdetectedinsuchhighredshiftsystems.Togetherwiththede-quasars:individual:APM08279+5255
tectionoftheMgiλ2852transition,thisstronglyconstrainsthe
physicalcharacteristicsofthegas.TheN(Nai)/N(Mgi)ratio
isfoundtobelargerthanunity,implyingthatthegasiscooland
neutral. The Doppler parameters measured in individual and 1. Introduction
welldetachedcomponentsareprobablyassmallas1kms−1.
ThegravitationallylensedBroadAbsorptionLine(BAL)QSO
i,Caii,Mgi,Tiii,MniiandFeii APM 08279+5255 (zem = 3.911) has been given much atten-
observedatzabs=1.1801areveryclosetothatobservedalong tion since its discovery by Irwin et al. (1998), as it is one of
thelineofsighttowards23OriinourGalaxy.Theshapeofthe
themostluminousobjectsintheuniverseevenaftercorrection
QSOcontinuumisconsistentwithattenuationbydustatz(cid:24)1
for the gravitational lensing induced amplification. Adaptive-
(AV(cid:24)0.5mag).AltogetheritisfoundthattheHicolumnden- opticsimaginghasrevealedtwomaincomponents(Ledouxet
sityatz=1isoftheorderof1to51021cm−2,thecorresponding al. 1998b), separated by 0.378(cid:6)0.001 arcsec as measured on
metallicityisintherange1–0.3 Z(cid:12),theoveralldust-to-metal HST/NICMOS data (Ibata et al. 1999), and of relative bright-
ratioisabouthalfthatinourGalaxyandtherelativedepletion nessFB/FA =0.773(cid:6)0.007.TheHSTimagesrevealalsothe
ofiron,titanium,manganeseandcalciumissimilartowhatis presence of a third object C with FC/FA = 0.175(cid:6)0.008, lo-
observedincoolgasinthediskofourGalaxy.Theobjectsasso-
cated in between A and B and almost aligned with them. The
ciatedwiththesetwosystemscouldbothcontributetothelens
point-spread-functionmodelfitsonthethreeobjectsareconsis-
together with another possible strong system at zabs = 1.1727 tent with the three components being point-sources, and their
andthestrongLyman-αsystematzabs=2.974. colorsaresimilarwithintheuncertainties.Thereisnotraceof
The probable damped Lyman-α system at zabs = 2.974 has thelensingobjectuptomagnitudeV =23.
19.8 < log N(H i) < 20.3. The transverse dimension of the
A high S/N ratio high-resolution spectrum of APM
08279+5255wasobtainedattheKecktelescope(Ellisonetal.
Sendoffprintrequeststo:P.Petitjean
(cid:63) Based on observations collected at the W.M. Keck Observatory, 1999a,b), and made available to the astronomical community.
Thisspectrum,thoughcomplicatedbythecombinationoflight
whichisoperatedasascientificpartnershipamongtheCaliforniaIn-
travelingalongthreedifferentsightlines,isauniquelaboratory
stitute of Technology, the University of California and the National
Aeronautics and Space Administration. The Observatory was made forstudyingtheinterveningandassociatedabsorptionsystems.
possiblebythegenerousfinancialsupportoftheW.M.KeckFounda- InthispaperwestudythestructureofsixinterveningMgii
tion. systemsat1.2<z<2.07andthephysicalcharacteristicsofthe
458 P.Petitjeanetal.:StructureoftheMgiianddampedLyman-αsystemsalongthelineofsighttoAPM08279+5255
gasintwoverystrongMgiisystemsdetectedatzabs=1.06and 1 1 1
1.18,which,weargue,aredampedLyman-αsystemsandmay
well reveal the lensing galaxies. We also comment on a third
probabledampedLyman-αsystematzabs =2.974.Thispaper 0.5 0.5 0.5
is organized as follows: the data are described in Sect.2; the
structureoftheinterveningMgiisystemsisinvestigatedusing
thecoveringfactoranalysisinSect.3;wedemonstratethatthe 1 1 1
Mgiisystemsatzabs=1.06and1.18aredampedLyman-αsys-
temsinSect.4;wediscussaprobabledampedLyman-αsystem
at zabs = 2.974 in Sect.5. We adopt Ho = 75 km s−1 Mpc−1 0.5 0.5 0.5
andqo=0.5throughoutthepaper.
1 1 1
2. Data
AhighS/Nratio,high-resolutionspectrumofthezem =3.911 0.5 0.5 0.5
quasarAPM08279+5255wasobtainedwiththeHIRESechelle
spectrograph at the 10m Keck-I telescope (Ellison et al.
1999a,b). This data was made public together with a low-
-50 0 50 -50 0 50 -50 0 50
resolution spectrum of the quasar and a high-resolution spec-
trumofastandardstar.Wehavecorrectedthehigh-resolution
spectrumofAPM08279+5255forsmalldiscontinuitiesinthe 1 1 1
continuum,whichareprobablyduetotheinappropriatemerging
ofdifferentorders.Thesediscontinuitieshavebeenrecognized
bycomparingthehighandlow-resolutionspectra.Thelatteris 0.5 0.5 0.5
alsousedfornormalizationofthehigh-resolutiondata.Atmo-
spheric absorption features were identified from the standard
star spectrum. Voigt profile fitting of the absorption features 1 1 1
have been performed using the context FITLYMAN (Fontana
&Ballester1995)oftheEuropeanSouthernObservatorydata
0.5 0.5 0.5
reductionpackageMIDASandthecodeVPFIT(Carswelletal.
1987). We have measured the final spectral resolution by fit-
tingthenarrowatmosphericabsorptionlineswhicharefreeof
blending. We find FWHM (cid:24) 8 km s−1 (b (cid:24) 4.8 km s−1) at 1 1 1
6900A˚,R=37500,andusethisvaluethroughoutthepaper.
0.5 0.5 0.5
3. Structureoftheinterveningz>1Mgiiabsorbers
Ellison et al. (1999b) have already noted that the two lines of
-50 0 50 -50 0 50 -50 0 50
someoftheMgiiλλ2796,2803doubletscannotbefittedwith
thesamecolumndensityandDopplerparameter.Ascanbeseen Fig.1.Coveringfactor(bottompanels)ofsixMgiisystemsobserved
onFig.1,mostofthesystemshaveadoubletratioclosetounity
alongthelineofsighttoAPM08279+5255calculatedfromthepro-
inspiteofhavingresidualintensitiesinthenormalizedspectrum filesoftheMgiiλ2796(toppanels)andMgiiλ2803(middlepanels)
closeto0.5. absorptions.
If the background source is a point source and if the
Mg iiλ2796 absorption line is resolved, then the residual in-
tensityofthenormalizedspectrummeasuredatanyvelocityv from different images, it is possible that the column densities
withrespecttothecentroidoftheline,isequaltoe−τ(v),where are different along different lines of sight and, as a limiting
τ(v) is the optical depth at v and the residual intensity of the case,thattheabsorbingclouddoesnotcoveralltheimages.We
Mgiiλ2803lineise−τ(v)/2.ItisapparentfromFig.1thatthe discussthesedifferentpossibilitiesbelow.
aboveconditionisnotfulfilledformostoftheMgiidoublets.
Therearetwopossibilitiestoexplainthis.Ifthelinesarenot
3.1. Unresolvednarrow-components
resolved,themeasuredresidualintensityisaffectedbyconvolu-
tionofthetrueabsorptionprofilewiththeinstrumentalprofile. If the absorption profiles are made up of unresolved compo-
Thiscanintroduceartificialresidualsatthebottomofthesat- nents, the observed residual intensity does not correspond to
uratedabsorptionfeatures(seee.g.Lespine&Petitjean1997). the real optical depth. Given the resolution of the spectrum
Alternatively, as the observed light is a combination of light (R (cid:24) 37500), a saturated Mg iiλ2796 line can have a resid-
P.Petitjeanetal.:StructureoftheMgiianddampedLyman-αsystemsalongthelineofsighttoAPM08279+5255 459
Fig.2.BestfitoftheMgiidoubletatzabs=1.5523withfivenarrow
components(b<1.5kms−1)andacoveringfactorf =1(reduced
χ2of1.5).
ual intensity in the normalized spectrum of 0.5 if its Doppler
parameterissmallerthan1.5kms−1.Inthatcase,theresidual
intensityoftheMgiiλ2803lineisintherange0.5–0.6,depend-
ing on the actual column density. The two equivalent widths
differbynomorethan20%(seee.g.Lespine&Petitjean1997).
The system at zabs = 1.5497 can be indeed fitted this way
using10componentswithbvaluesintherange1.1–1.7kms−1.
Inthatcasethewelldetachedcloudintheredwing(seeFig.1)
is fitted with two adhoc nearly identical components though
theMgiiλ2796profileisperfectlyfittedwithasingleresolved
componentmodel.Howevertheone-componentmodelcannot
explain the strengths of the two Mg ii lines without invoking
partialcoverage(seebelow).
Weusethezabs =1.5523systemtoillustratethecase.The Fobigta.i3n.eBdewstitfihtsfivoeftnhaerrzoawbs(b=<1.515.523kmMgs−ii1)ancodmFpeoinieanbtssoarnpdtiofn=lin0e.6s
finalspectrumhasadefectinthecenteroftheMgiiλ2796line. (top panel, reduced χ2 of 0.9) and three broader (b > 2.5 km s−1)
WehavethereforeusedthreeindividualexposuresofhighS/N componentsandf =0.45(bottompanel,reducedχ2 of0.6).Thefit
ratio (Ellison private communication) to correct for this. The
parametersaregiveninTable1.
finalopticaldepthvariationsfromonespectrumtotheotheris
about 2%. Fig.2 shows the best fit to the doublet considering
fullcoverage(f=1).Fivenarrowcomponents(b<1.5kms−1) f =0.45andreducedχ2 =0.6(seeFig.3andTable1).Theb
are needed. Although good (reduced χ2 of 1.5), the fit is not valuesarelargerthan2.5kms−1relaxingtherestrictiononthe
completely satisfactory. We have fitted consistently the Mg ii temperature.
together with the Fe ii lines considering that only one of the A statistically acceptable fit is difficult to find for the
brightestsourcesiscovered.Withfivecomponents,agoodfit systems at zabs = 1.221 and 1.5523. The red wing of the
(reduced χ2 of 0.9), shown on Fig.3, is obtained if B is not Mg iiλ2803 line at zabs = 1.221 is blended with another ab-
covered(f =0.6).Detailsofthesubcomponentparametersare sorptionfeaturewhichwefounddifficulttoidentify.Itcouldbe
given in Table 1. As the Mg ii doublet ratio is close to one, Mgiλ2852atzabs =1.1727.Thissystemispossiblydetected
small b values are needed even with the assumption of partial byFeiiλλ2344,2382,Mniiλ2576andCaiiλ3934(seeFig.4).
coverage. Theotherlinesareeitherbelowthedetectionlimitorblended.
ItshouldbenotedhoweverthataDopplerparametersmaller Confirmation of the Ca ii lines, which are free of blending,
than 1.5 km s−1 corresponds to a temperature smaller than would be particularly important as, probably, the presence of
3500K,asurprizinglysmalltemperatureforthisgaswhichis this additional system, together with the presence of the three
mostprobablyionized.Allowingforasmallernumberofcom- dampedsystemsatzabs =1.062,1.181and2.974(seebelow)
ponents, we can find a good fit with only three components, shouldbetakenintoaccountinanymodelofthelens.
460 P.Petitjeanetal.:StructureoftheMgiianddampedLyman-αsystemsalongthelineofsighttoAPM08279+5255
&Petitjean2000).Giventheuncertainties,ifonlyoneline-of-
1 sight is completely absorbed, the covering factor should be in
1
therange0.4–0.6.Ofcourse,itislargerifthesecondline-of-
0.9
0.5 sight is not completely clear. We have computed the covering
0.8
Ca II 3934 Fe II 2344 factor for the Mg ii systems using the method described by
0.7 10
1 Srianand & Shankaranarayanan (1999). This assumes that the
0.9 lines are resolved. It can be seen in Fig.1 that for the three
0.5
0.8 systemswithzabs<1.7,thecoveringfactorrangesbetween0.5
Ca II 3969 Fe II 2374 and0.6whereasforthesystemswithz>1.7,thecoveringfactor
0.71 10
islarger(butalwayslessthan0.8)withthepossibleexception
0.5 0.5 of the zabs = 2.0668 system. The latter system is quite weak
howeveranduncertaintiesarelarge.Thevaluesofthecovering
Mg I 2852 Fe II 2382
10 0 factorforthethreelowerredshiftsystemssuggestthattheclouds
1
cover one of the two brightest background sources only. This
0.9
0.5 hastobeinvestigatedinmoredetail,though.
0.8
Mg II 2803 Ti II 3242
110 0.711
3.2.2. Opticaldepthsalongdifferentsightlines
00..55 00..55 In this section we investigate the effect of the optical depth
Mg II 2796 Mn II 2576 beingdifferentalongdifferentsightlines.Inordertomakeour
00 00
-200 0 200 -200 0 200 analysissimpler,weconsideronlytwoimages,AandB,with
fractionalfluxcontributionsF1=0.6andF2=0.4.Supposethe
Fig.4.ProbableMgiisystematzabs =1.1727.Mostofthelinesare optical depth along the two sight lines are τ1 and τ2 then the
blendedbutthesimilarityoftheFeii,MgiiandCaiiprofilessupports measuredresidualintensitiesare,
theidentification.Verticaldottedlinesindicatethepositionsofthefour
probablecomponents. R(2796) = F1e−τ1 +F2e−τ2 (1)
R(2803) = F1e−τ1/2+F2e−τ2/2
Table1.Fitparametersforthezabs=1.5523system Theresidualintensitiescanbewrittenas(Srianand&Shankara-
narayanan1999),
z bb (cid:6) N(MgII)a (cid:6) N(FeII)a (cid:6)
R(2796) = 1−f +fe−τ (2)
Bestfitwithfivecomponents;f =0.6
R(2803) = 1−f +fe−τ/2
1.552165 1.7 0.5 11.51 0.15
1.552219 1.1 0.5 11.88 0.05 11.67 0.12 where, f and τ are the resulting covering factor and optical
1.552270 1.3 0.2 14.47 0.27 12.79 0.05
depthandtherefore,
1.552338 3.4 0.2 12.47 0.10 12.23 0.06
1.552399 0.8 0.1 14.67 0.09 12.34 0.07 [1−R(2803)]2
f = (3)
Bestfitwiththreecomponents;f =0.45 1+R(2796)−2R(2803)
1.552170 2.9 0.5 11.67 0.10 11.29 0.11 FromEqs.(1)and(3)onecanderivef asafunctionofF1,F2,
1.552274 4.2 0.6 13.02 0.18 12.82 0.09
1.552378 4.6 0.7 12.85 0.20 12.44 0.07 τ1andτ2.Thisanalysisassumesthattheabsorptionprofilesare
resolvedintheHIRESspectrum.AsF1andF2areknownfrom
baliongkamritsh−m1ofthecolumndensityincm−2 observation,thecoveringfactordependsonτ1andτ2only.
In order to investigate the parameter space we have com-
putedandplottedonFig5,thecoveringfactor(paneld),resid-
3.2. Partialcoveringfactor ualintensitiesofthelines(panelsaandb)andtheirratio(panel
c)asafunctionofτ2fordifferentvaluesofτ1.Inpanels(a)and
3.2.1. Computingacoveringfactor
(b),thetwohorizontaldottedlinesshowtherangeofobserved
Wecaninterprettheobservationsintermsofacoveringfactor values for the Mg iisystems with zabs<1.7. In panel (c) the
whichisthefractionofthebackgroundsourcecoveredbythe dottedlinesgivethemeasuredvaluesforthethreelow-redshift
absorbing cloud. The relative brightness of the three sources Mgiisystems.
are FA,B,C/Ftot = 0.513, 0.397 and 0.090 (Ibata et al. 1999). As expected, when τ1 = τ2, f = 1 (albeit with various
Ifoneline-of-sightiscompletelyabsorbed(conditionimposed ratiosofresidualintensities)andforτ1 =/ τ2 thecoveringfac-
by the fact that the doublets are saturated) and the other free tors are less than 1.0. When τ1 (cid:21) τ2 (respectively τ1 (cid:20) τ2),
of absorption, then the covering factor is 0.40, 0.49, 0.51 and the covering factor is in the range 0.6–1.0 (respectively 0.4–
0.60if,respectively,Bonly,B+C,AonlyandA+Carecovered 1.0).Conversely,theobservedresidualintensities,R(2796)and
(thecaseConlyisveryunlikelyasCisclosetoAandlocated R(2803),togetherwiththemeasuredcoveringfactor,f,canbe
in between A and B, Ibata et al. 1999; see however Srianand usedtoconstrainτ1andτ2.
P.Petitjeanetal.:StructureoftheMgiianddampedLyman-αsystemsalongthelineofsighttoAPM08279+5255 461
111111 111111
000000......888888
000000......888888
000000......666666
000000......666666
000000......444444
000000......444444
000000......222222
000000 111111 222222 333333 444444 555555 000000 111111 222222 333333 444444 555555
111111 111111
000000......888888 000000......888888
000000......666666 000000......666666
000000......444444 000000......444444
000000 111111 222222 333333 444444 555555 000000 111111 222222 333333 444444 555555
Fig.5. Panels a and b give, respectively, the observed residuals R(2796) and R(2803) in the normalized spectrum and in the center of the
Mgiiλλ2796,2803linesasafunctionofτ2fordifferentvaluesofτ1.τ1andτ2aretheopticaldepthsinthecenteroftheMgiiλ2796linealong,
respectively,lineofsightnumberonetowardsimageAandlineofsightnumbertwotowardsimageB(seeEqs.(1)and(2)).Itisassumedthat
thefractionalfluxcontributionsaref1 =0.6andf2 =0.4.Thetwohorizontaldottedlinesshowtherangeofobservedvalues.Panelcgives
theratiooftheresidualintensitiesofthetwoabsorptionlinesasafunctionofτ2fordifferentvaluesofτ1.Thedottedlinesshowtheobserved
valuesinthethreesystemsatzabs<1.7.Inpaneldthederivedcoveringfactorf (Eq.(3))isshownasafunctionofτ1fordifferentvaluesof
τ2.Measuredvaluesareindicatedbydottedhorizontallines.
The covering factor estimates for zabs< 1.7 systems are addition,whenconsideringtotalequivalentwidths,theconse-
in the range 0.5 and 0.6 (see Fig.1). This, together with the quence of contamination by weak lines is small unlike in the
observed residual intensities, indicates that the absorbing gas case of the analysis of the residual intensities. The constrains
issaturatedalongonesightlineonlywithopticaldepthratios aremuchweakerhowever.
as large as ten. For example, the well detached component in Let us assume that the absorption is saturated along
theredwingofthezabs =1.5497systemhasf = 0.6(with line of sight number one and optically thin along line of
a typical error of 0.02), a residual intensity ratio (cid:24)0.84 (with sight number two. Therefore Wreal(2796) = Wreal(2803) and
1 1
a typical error of 0.02) and R(2796)’0.40 at the core of the Wreal(2796)=2(cid:2)Wreal(2803).Thecombinedobservedequiv-
2 2
line.Thisimpliesthatthecontributiontothisabsorptioncomes alentwidthratiois:
mainlyfromthelineofsighttowardA+Cwiththeopticaldepth
alongBbeingmorethananorderofmagnitudesmaller. Wobs(2803) 1 + W1real(2796)F1
= 2 W2real(2796)F2 (4)
Wobs(2796) 1+ W1real(2796)F1
3.3. Equivalentwidthratio W2real(2796)F2
Theprecisedeterminationofthestrengthoftheabsorptionlines where F1 and F2 are the fractional flux contributions of the
along different lines of sight should await HST/STIS spectro-
two distinct background sources. In Fig.6, we have plotted
scopic observations. Though we derived some information on Wreal(2796) /Wreal(2796) versus Wobs(2803) /Wobs(2796)
1 2
this in the previous section using the absorption line residual for F1/F2 = 0.65 (1 is B; 2 is A+C), 1 (1 is A or B+C; 2 is
intensities,theresultsdependcruciallyontheassumptionthat
B+C or A) and 1.5 (1 is A+C; 2 is B). The vertical dashed-
theabsorptionlinesareresolved.Wecancomplementthepre-
dotted lines correspond to the observed doublet ratios of the
viousanalysisusingthetotalequivalentwidthsandtheirratios systemsatzabs =1.221,1.5523andofthereddestcomponent
withoutmakinganyassumptionaboutthespectralresolutionIn ofthezabs=1.5497system.
462 P.Petitjeanetal.:StructureoftheMgiianddampedLyman-αsystemsalongthelineofsighttoAPM08279+5255
(qo =0.5)betweenz =1.22andz =2.04andismorethanan
order of magnitude smaller than the radius of Mg ii halos at
intermediateredshift.Althoughevolutionisprobable,itwould
be really surprizing that the Mg ii systems studied here with
Wr <0.5A˚ atzabs >1.2havecharacteristicdimensionsmore
thananorderofmagnitudesmallerthanwhatisderivedatlower
redshift.Iftrue,thiswouldsuggestthatthestructureoftheMgii
halosattheseredshiftsdifferssubstantiallyfromthatatlower
redshifts.
Amorelikelyexplanationoftheseobservationsisthatthe
halos are composed of a collection of clouds (see Petitjean &
Bergeron 1990; Srianand & Khare 1994) and that individual
cloudscoveronlyonesightline.Thenumberdensityofclouds
is large enough so that the total covering factor of the halo is
closetoone,consistentwithobservationsofassociatedgalaxies.
Fig.6.EquivalentwidthratiooftheMgiiλ2796absorptionlinealong However,individualclouds,regularlyspreadoverthevelocity
profile by kinematics, cover only one image of the lens. The
twodifferentline-of-sights1and2versustheobserveddoubletratio
forthreedifferentfluxratiosF1/F2=0.65(solidline),1(dashed-dotted numberdensityofcloudsisnotlargeenoughfortheabsorption
line)and1.5(dashedline).TheMgiidoubletisassumedtobesaturated material to cover the two lines of sight at all velocities. The
alongline-of-sight1andopticallythinalongline-of-sight2.Thedou- distance over which the optical depth, and hence the column
bletratiosobservedforsixMgiisystemstowardAPM08279+5255 density of Mg ii, changes by at least one order of magnitude
areshownasverticaldashed-dottedanddottedlinesforzabs<1.5and at zabs<1.7 is smaller than (cid:24) 1h−751 kpc. In contrast, the two
zabs>1.5respectively.Redshiftsareindicatednexttothelines. strongMgiisystemsatzabs =1.06and1.18(seebelow)have
covering factor equal to one (the lines are saturated and go to
thezerolevel)overmorethan200kms−1.Theselattersystems
From Fig.6, it can be seen that the zabs = 1.221 system arelikelytoariseduetoabsorptionthroughcentralregionsof
(doubletratioof0.96)requirestheratiooftheequivalentwidths galaxieswherethenumberofcloudsissolargethatsaturated
ofMgiiλ2796alongthetwolinesofsighttobelargerthan8. absorptionoccursalongbothlinesofsightwhatevertheradius
andthecolumndensitiesalongthetwosightlinesdifferbymore oftheindividualcloudsmightbe.
thananorderofmagnitude.
4. ThetwoMgiisystemsatzabs=1.062and1.181
3.4. Dimensionofindividualclouds
ThepresenceofastrongMgiisystem(Wrλ2803(cid:24)2.4A˚)at
Rauchetal.(1999)haveobservedstrongvariationsofCiiand zabs(cid:24)1.18wasalreadymentionedbyIrwinetal.(1998).There
Siiiabsorptionsatz=3.538alongtwosightlinesseparatedby is an additional even stronger Mg ii system at zabs = 1.062
only13h−1pc.However,asthevelocitydifferencebetweenthe (Wrλ2803 (cid:24) 3.3 A˚). Although the Mg ii lines are redshifted
quasarandthesystemisonly6000kms−1,itcannotbeexcluded in the Lyman-α forest and may be blended with Lyman-α in-
that the latter system is somehow associated with the quasar. terveningabsorptions,theexistenceofthesystemisconfirmed
Variationsofthestrengthofmetallinesystemshavealsobeen by numerous lines redshifted redward of the quasar Lyman-α
reported along adjacent lines of sight with larger separations emission.Asthetwobrightestimagesofthelensedquasarhave
(5–10 kpc) by Monier et al. (1998) and Lopez et al. (1999). similarmagnitudes,itisexpectedthatthelinesofsighttoboth
Each time however a damped Lyman-α system is seen along imagespassthroughthecoreofthelensingobjectwherestrong
one of the sightlines. Contrary to these previous studies, the Mg ii absorption is likely to occur. The gravitational lensing
Mgiisystemsweexamineherearemostlikelytobeassociated maythusresultfromthecumulativeeffectofthetwogalaxies
withhalosofinterveninggalaxies. associatedwiththesetwoabsorbingsystemstogetherwiththe
From the detection of associated galaxies, radii of the or- objectsresponsibleforthepossiblesystematzabs=1.1727(see
derof35h−1 kpchavebeenderivedforMgiihalosproducing Sect.3.1)andtheotherdampedLyαsystematzabs=2.974.Ab-
absorptionswithequivalentwidthsWr>0.3A˚ atz<1(Berg- sorptionsfromMgii,Feii,Caii,Mnii,TiiiandNaiareseen
eron & Boisse´ 1991, Steidel 1993). Dimensions of the same inbothsystems(seeFigs.7,8).
orderhavebeenderivedfromthestudyofMgiisystemsseen
along two lines of sight separated by 3 arcsec (Smette et al.
1995). The latter authors find a lower limit of 22 h−1 kpc for 4.1. Naiabsorptions
50
theradiusofMgiiabsorberswithWr>0.3A˚ at0.5<z<1.3. In both Mg ii systems, the weak Na iλλ3303,3303 doublet is
IfweassumethatthelensinggalaxyofAPM08279+5255 detected (see Figs.7 and 9). The fact that the two lines of the
is at zlens (cid:24) 1 (see next section), the separation between the doubletareseenwithconsistentstrengthsgivesconfidencethat
twolinesofsighttoAandBdecreasesfrom1.7to0.7h−1kpc theidentificationiscorrect.Wehaveidentifiedlinesfromother
75
P.Petitjeanetal.:StructureoftheMgiianddampedLyman-αsystemsalongthelineofsighttoAPM08279+5255 463
Fig.7.Absorptioninafewtransitionsonavelocityscalewithorigin Fig.8.Absorptionsinafewtransitionsonavelocityscalewithorigin
at zabs = 1.06230. Vertical dashed lines mark velocity components at zabs = 1.18070. Vertical dashed lines mark velocity components
discussedinthetext. discussedinthetext.
sitiesofotherspeciesthatarefoundtobesurprisinglycloseto
metallinesystemsinthevicinityofthedoublet.Thespectraof
what is observed in typical interstellar clouds (see below and
APM08279+5255andthestandardstararecomparedinFig.9
Table 2). This leaves little doubt that the systems are indeed
to rule out the possibility that the absorption features are of
dampedLyman-αsystems.
atmosphericorigin.
In our Galaxy, such high Na i column densities are seen
The column densities obtained by Voigt profile fitting are
only in dense and cool gas (see below). The typical b values
large,logN(Nai)=12.9and13.5at,respectively,zabs=1.0626 oftheNaidiffusecomponentsinboththelocalandlow-halo
and1.1801.Theseparationofthetwoprincipallinesofsightis
(cid:24)1.9h−1 kpc at z (cid:24) 1. It is therefore possible that the clouds gasisabout0.7kms−1correspondingtoT <500K(Weltyet
75 al.1994).Althoughtheconclusionisveryuncertaingiventhe
seen by their Na i absorptions do not cover the two brightest
resolutionofthespectrum(R(cid:24)37500)andthedoublenature
linesofsight.Inthatcase,however,thecolumndensitycould
of the background source, the lines we observe are consistent
beevenlargerbyafactoroftwo(seeSect.4.4).
withbvaluesassmallas1kms−1 (seebelow).Fromthis,we
Using the measurements by Sembach et al. (1993) and
deriveanupperlimitonthetemperatureofT <2000K.
Diplas&Savage(1994),Bowenetal.(1995)findthat,inour
Galaxy, log N(H i) = 0.688 log N(Na i) + 12.16. Note that
thiscorrelationholdsuptocolumndensitieslogN(Hi)>21 4.2. Commentsoneachsystem
(see e.g. Ferlet et al. 1985). Applying this correlation for the
zabs=1.0626and1.1801absorbersgivesneutralhydrogencol- 4.2.1. zabs=1.062
umndensitiesoftheorderoflogN(Hi)(cid:24)21.0and21.4respec- Asubsetoftheabsorptionsdetectedinthissystemisshownin
tively,forgas-phasemetallicitiescomparabletowhatisseenin Fig.7.Itcanbeseenthatstrong(butunsaturated)absorptions
the interstellar medium in our Galaxy (the neutral hydrogen from Ti ii and Ca ii are detected. The profile of the Ti ii ab-
columndensitiescouldbeevenlargerifthemetallicityinthese sorptionisspreadoverabout250kms−1butdoesnotshowany
intermediate-redshift systems is smaller than in our Galaxy). edge leading pattern (Prochaska & Wolfe 1998). We have se-
SuchlargevaluesforN(Hi)aresupportedbythecolumnden- lectedtoexaminetwocomponentsatzabs=1.0613and1.0631
464 P.Petitjeanetal.:StructureoftheMgiianddampedLyman-αsystemsalongthelineofsighttoAPM08279+5255
4.2.2. zabs=1.181
Asubsetoftheabsorptionsdetectedinthissystemisshownin
Fig.8. The profile of the Mg i absorption is spread over more
than 200 km s−1 but, as for the previous system, it does not
show any edge leading pattern. A number of absorption lines
areopticallythinormoderatelysaturatedandreliablecolumn
densitiescanbederivedeventhoughdifficultiesarisefrommost
of the components being badly defined (see Fig.8). We have
selected for study two subcomponents which are clearly seen
in all absorption profiles at zabs = 1.1799 and 1.1801. They
areindicatedonFig.8byverticaldashedlines,andthecolumn
densitiesobtainedfromVoigt-profilefittingaregiveninTable2.
Dopplerparametershavebeenconsideredtobeidenticalforall
species.Forzabs=1.1799wefindb=2.5kms−1.
Forthecomponentatzabs =1.1801inwhichNaiabsorp-
tion is detected, the Doppler parameter is estimated by fitting
thewell-definedlinesofthesodiumdoubletafterhavingtaken
intoaccounttheeffectofNaiλ3303.3beingpartiallyblended
withanatmosphericfeature(seeFig.9).Weobtainabestvalue
b = 1.1+1.0 km s−1. The column densities derived using the
−0.5
twovaluesb(cid:6)1σdifferbylargefactors.Wehavethereforere-
finedthedeterminationofbandN usingthefollowingindirect
argument.
TheratiooftheMgitotheNaicolumndensitiesinneutral
gascanbewritten
N(Mgi) Mgi Na δ Z
= (cid:2) (cid:2) Mg (cid:2) Mg, (5)
N(Nai) Mg Nai δ Z
Na Na
where δ is the depletion of the element due to the presence
Fig.9. Portions of the APM 08279+5255 spectrum centered on the
expectedpositionsofNaiλ3303.3(toppanel),Naiλ3303.9(middle of dust and Z the abundance. Assuming that (i) the relative
panel)andMgiλ2852(bottompanel)atzabs=1.181.TheNaicom- abundance of Na to Mg is solar, ZMg/ZNa = 19, (ii) the rel-
ponent at zabs = 1.1801, detected by the two lines of the doublet is ative depletion into dust-grains is δMg/δNa > 0.3 (Savage &
indicatedbyaverticaldashedline.Thespectrumofthestandardstar Sembach 1996) and (iii) (Mg i/Mg ii)(cid:2)(Na ii/Na i) > 0.15
isoverplottedinthemiddlepanel.Metallinesfromothersystemsare in cold and neutral gas (Pe´quignot & Aldrovandi 1986), we
indicatedbycrosses. derive N(Mg i)/N(Na i) (cid:21) 0.8. This estimation is certainly
very approximate. However, this simple argument shows that
the latter ratio cannot be much smaller than 0.5. If we as-
because they show well detached absorptions in Ti ii, Mn ii, sumeb=1.5kms−1,thenwefindN(Mgi)/N(Nai)=0.045
Caiiand/orFeiiplusthecomponentatzabs=1.0626inwhich which is definitively too small. We therefore have fitted the
we see Na i. Column densities are listed in Table 2. We have absorption lines decreasing b from 1.5 to 0.5 km s−1 to
adjustedthebestvaluesforbfromthefittothelinesthatarefree find the largest N(Mg i)/N(Na i) ratio. We find a maximum
ofanyblending,consideringforsimplicitythatallspecieshave N(Mgi)/N(Nai)=0.3forb=0.8kms−1,logN(Mgi)=13.0
thesameDopplerparameterandassumingcompletecoverage. andlogN(Nai)=13.5.
We find b = 1.5, 1.5 and 1.9 for, respectively, zabs = 1.0613,
1.0626and1.0631.
4.3. Physicalstateofthegas
Caiisnotdetectedandthe3σupperlimitonthecolumnden-
sityinthethreecomponentswehaveselectedis<10.43,10.43 Table 2 contains the column densities measured in the five
and10.20.TheMgiiλ2796,2803andFeiiλλλ2382,2600,2586 subcomponents defined above. The penultimate column gives
lines are badly saturated. Moreover, they are redshifted in the forcomparisonthecolumndensitiesmeasuredbyWeltyetal.
Lymanα forest, which prevents any fit of the lines. However, (1999)intheneutralgastoward23Ori.Thisgasisfoundtohave
the unsaturated Fe iiλ2260 features detected at zabs = 1.0613 temperatureT (cid:24)100K,hydrogendensitynH (cid:24)10–15cm−3
and1.0626(seeFig.7)giveareliableestimateoftheFeiicol- (andthereforetotalthicknessof12–16pc)andelectronicden-
umn density, log N(Fe ii) = 13.90(cid:6)0.80 and 14.10(cid:6)0.80 for sityne(cid:24)0.15(cid:6)0.05cm−3.ThelastcolumnofTable2givesthe
b=1.5kms−1,thecontinuumbeingadjustedlocally.Thenon- measurementsforthestrongestcomponentofthewarmneutral
detection of Fe iiλ2367 at zabs = 1.0631 gives an upper limit gastowardµCol(Howk&Savage1999).Thisgasisfoundto
logN(Feii)<14.60. have T (cid:24) 6000–7000 K and ne (cid:24) 0.3 cm−3. The two sets of
P.Petitjeanetal.:StructureoftheMgiianddampedLyman-αsystemsalongthelineofsighttoAPM08279+5255 465
Table2.Columndensitiesa
Redshift 1.0613 1.0626 1.0631 1.1799 1.1801 23Orib µCold
HI 20.74 19.86
H2 18.30 15.50
NaI <11.90 12.91(cid:6)0.04 <12.80 <12.40 13.50(cid:6)0.11 13.36 11.6e
MgI 11.6:c bl bl 11.61(cid:6)0.06 13.0(cid:6)0.74 13.81 12.55
MgII 15.68 15.08
CaI <10.43 <10.43 <10.20 <10.80 <10.33 10.20
CaII 11.31(cid:6)0.11 bl 11.71(cid:6)0.03 11.33(cid:6)0.39 11.79(cid:6)0.35 12.10 12.19
TiII 11.80(cid:6)0.05 <12.00 11.90(cid:6)0.02 <11.66 <11.20 11.23 11.78
MnII bl bl 12.70(cid:6)0.40c bl bl 13.15 12.48
FeI <11.50 <11.63 <11.40 <11.40 <11.50 11.34
FeII 13.90(cid:6)0.80 <14.10 <14.60 13.36(cid:6)0.07 <15.00 14.38 14.13
CH+ <12.80 <12.70 <13.00 <13.20 <13.30 13.06
CH <13.40 <13.60 <13.50 <13.40 <13.60 12.69
alogarithmof,incm−2;Dopplerparametersaretakentobeb=1.5,1.5,1.9,2.5,0.8kms−1forthefivecomponentsrespectively;thecomponents
areassumedtocoverallimages;
bWeltyetal.(1999);
cThecontinuumisfittedlocally;
dHowketal.(1999);
eHobbs(1978).Asign“bl”meansthatnomeasurementispossibleduetoblendingeffects
columndensitiesaresimilarexceptforN(Nai)whichismuch FromN(Hi)=N(Nai)(cid:2)(Na/Nai)/Z(Na)andusingthetwo
smallertowardµCol.Thisillustratesdirectlythatatleastinthe upperlimitsontheNai/Naiiratiosderivedabove,wecanwrite
componentatz(cid:24)1.1802towardAPM08579+5255whereNai N(Hi)>19.9and19.5/(Z(Na)/Z(cid:12)(Na))/δ(Na)atzabs=1.0626
isdetected,thegasismostlikelytobeneutralandcold.More- and1.1801respectively,whereδ(Na)isthefractionofsodium
over,notealsothatinourGalaxy,aratioN(Nai)/N(Caii)>1, remaininginthegasphaseafterdepletionintodust-grains.This
asobservedatzabs=1.1802towardAPM08579+5255,ischar- factor is equal to about 0.1 in the ISM (Savage & Sembach
acteristicofcoldgasinthedisk.Indeed,alongthelineofsight 1996).Thisaddssupporttoargumentspresentedpreviouslythat
to the LMC, such large ratios are observed only at the sys- thesesystemsaredamped.Toillustratethediscussion,simple
temicvelocitiesoftheLMCandtheGalaxy;gasinbetweenhas photo-ionizationmodelsusingthecodeCloudy(Ferland1996)
N(Nai)/N(Caii)<1(Vidal-Madjaretal.1987,Vladiloetal. have been constructed. The absorbing cloud is modelled as a
1993). planeparallelslabwithuniformdensity,solarchemicalcompo-
FromtheupperlimitonCaiandFeiwecanderive,inthe sitionandneutralhydrogencolumndensity5(cid:2)1020cm−2.The
componentswhereN(Caii)andN(Feii)aremeasured,alower elementsareconsideredtobedepletedintodust-grainsasinthe
limitfortheelectronicdensityforagivenionizingfield.Indeed, coolcloudobservedtowardζOph(Savage&Sembach1996).
ne=(Xo/X+)(cid:2)(Γ/α),whereΓisthephotoionizationrateand TheshapeoftheUVfluxistakentobeapower-lawFν/ν−1.0.
αtherecombinationcoefficient.Thecorrespondingelectronic Theresultingcolumndensitiesofvariousspeciesalongaline-
densitiesforaGalacticionizingfield(Pe´quignot&Aldrovandi of-sightperpendiculartotheslabaregivenversustheionizing
1986),arene<0.13and3cm−3forFeandCaatzabs=1.0626 parameterinFig.10.Asdiscussedabove,theN(Mgi)/N(Nai)
and1.1801respectively.Notethatthedeterminationofneinthe ratio can be smaller than one only if magnesium is more de-
interstellarmediumfromtheratioofsinglyionizedtoneutral pletedintodustgrainsthansodium.Notethateverymodelthat
speciesishighlyuncertainprobablybecauseofcontamination producesenoughNaihastemperaturelessthan100K.
ofthesinglyionizedcolumndensitydeterminationbyadjacent Finally, we do not detect any CHλ4300 and CH+λ4232
components(Weltyetal.1999).Writingthesamerelationfor absorption.ThelimitsonthecolumndensitiesarelogN(CH)
sodiumandequatingtheexpressionofne obtainedforsodium <13.5andlogN(CH+)<13.0atbothzabs=1.06and1.18(see
and iron or calcium leads to upper limits on the Na i/Na ii Table2).ThisisjustwhatwouldbeexpectedinourGalaxyalong
ratiowhichdependsonlyontheshapeoftheionizingspectrum anotherwisesimilarlineofsight.Indeed,alongthelineofsight
and not on its absolute value. With the only assumption that to23Ori,logN(CH+)=13.06andlogN(CH)=12.69(Welty
theionizingspectrumhasthesameshapeasinourGalaxyand et al. 1999; see Table 2). More generally, the column density
using the coefficients derived from Pe´quignot & Aldrovandi ofCHisobservedtoincreasefrom1.5to7.5(cid:2)1013 cm−2 for
(1986;seeWeltyetal.1999),wefindlogNai/Naii<−1.3and linesofsightwithEB−V increasingfrom0.5to1.5(Gredelet
0.1 from the constraints obtained on Fe and Ca in the 1.0626 al.1993).Itwouldbeofprimeinteresttoobtainbetterdatain
and1.1801systemsrespectively. thiswavelengthrangetobetterconstrainthemoleculecolumn
densities.
466 P.Petitjeanetal.:StructureoftheMgiianddampedLyman-αsystemsalongthelineofsighttoAPM08279+5255
16 blendat(cid:1)v =−60kms−1 (seeFig.8).Forthecomponentat
zabs=1.1801,weobtainlogN(Mgi)=13.8.
Notethatinthiscase,theNaiandMgicolumndensitiesare
nearlyidenticaltowhatisobservedtoward23Ori(seeTable2).
We therefore conclude that N(Mg i)/N(Na i) (cid:24) 1 is a robust
measurementinthissystem.
14 Mg i is the line with the largest saturation among those used
to derive column densities quoted in Table 2. Therefore, the
column densities derived from weaker lines should not differ
fromwhatisquotedinTable2bymorethanafactoroftwo.
4.5. Metallicityanddustcontent
12
In the following we use the conventional definition [X/H] =
log(Z/Z(cid:12)), with Z(X) the metallicity of species X relative to
hydrogen. Ca ii and Ti ii are both detected at zabs = 1.0613
and 1.0631. The column densities are consistent with what is
seenintheinterstellarmediumofourGalaxy(Stokes1978;see
Table2).However,logN(Caii)/N(Tiii)(cid:24)−0.5and−0.2at,
10-4 -3.5 -3 -2.5 -2 respectively,zabs =1.0613and1.0631whentherelativesolar
log Ionization parameter metallicityislogZ(cid:12)(Ca)−logZ(cid:12)(Ti)=1.38.Variousexpla-
nationsforthisdiscrepancycanbeinvoked,amongstthemthe
Fig.10.ResultsofphotoionizationmodelswithZ =Z(cid:12),logN(Hi) mostlikelyare:(i)CalciumismostlyintheformofCaiii,(ii)
=20.7,constantdensityandplaneparallelgeometry.Depletioninto
dust-grainsisassumedtobethesameasinthecoolcloudtowardζOph Calciumismoredepletedintodust-grainsthanTitanium.Note
(Savage&Sembach1996).Columndensitiesaregivenversusioniza- that the relative metallicities [Ca/Fe] and [Ti/Fe] are both ob-
tionparameter;theshapeoftheUVfluxisapower-lawFν /ν−1.0. servedtobe(cid:24)+0.3for[Fe/H]<−1inlate-typestars(The´venin
1998).NotealsothattheCaiii/CaiiratioderivedinourGalaxy
isintherange5–10whichismuchsmallerthanthediscrepancy
mentioned above. This favors the explanation that Calcium is
4.4. Consequenceofpartialcoveringfactor
heavilydepletedintodust-grains(seebelow).
FromthedetectionoftheNaiλλ3303.3,3303.9doublet,wecan In the zabs = 1.0631 component, Mn ii is also seen
derivethatthetwocomponentsatzabs=1.0626and1.1801arise and log N(Mn ii)/N(Ti ii) (cid:24) +0.8. As the solar metal-
incold,denseandneutralgas(seeSect.4.3).Itisthereforepos- licity of Mn is −6.47, the relative solar metallicity is
siblethatthedimensionofthecloudislessthan(cid:24)1.9kpcwhich log Z(cid:12)(Mn) − log Z(cid:12)(Ti) = = +0.6. This is consistent with
istheseparationofthelinesofsighttothetwobrightestimages similardepletionofMnandTiasobservedinwarmgas(Sav-
attheredshiftoftheabsorberassumingthatthelensingobject age&Sembach1996).Therefore,inthiscomponent,wecannot
isatthesameredshift.Indeed,largevariationsofNaicolumn ruleoutthatthelowCaiicolumndensityisduetoionization.
densityhavebeenreportedinthenearbyinterstellarmediumon In the zabs = 1.0613 component, Fe ii is also seen with
verysmallscales(Meyer&Lauroesch1999).Wehavetherefore log N(Fe ii)/N(Ti ii) (cid:24) +2.1. The solar metallicity of iron
investigatedtheimpactonthecolumndensitymeasurementsof and titanium are, respectively, −4.49 and −7.07 and the rel-
the assumption that the cloud covers the three images. If it is ative solar metallicity is log Z(cid:12)(Fe) − log Z(cid:12)(Ti) = +2.58.
the case that the cloud covers only one of the images the col- Thereisnodifferentialionizationcorrectionforthesetwoele-
umn density is larger. The discrepancy cannot be very large, ments.Thediscrepancy,(cid:24)+0.5dex,betweenthetworatioscan
however,asmostofthelinesusedforcolumndensitydetermi- only be explained by a larger depletion of titanium compared
nationareweak.Thestronglinesarecompletelysaturatedand to iron into dust-grains by (cid:24)0.5 dex as in the cool gas of the
blended, which prevents in any case any determination of the ISM(Savage&Sembach1996).Indeed,fromnucleosynthesis
lineparameters. alone, we would expect titanium to be enhanced compared to
WehaveconsideredtheMgiandNailinesinthezabs=1.1801 iron (The´venin 1998) contrary to what is observed. We there-
component. It can be seen on Fig.8 that if only one image is fore conclude that depletion into dust-grains is present in this
covered, it cannot be the brightest as the residual normalized system.ThelowCaiicolumndensitycanindeedbeexplained
fluxintheMgiabsorptionissmallerthan0.5.Asthefluxratio byalargedepletionofcalciumintodust-grainsasisobserved
ofthetwobrightestcomponentsis1.2,weartificiallyplacedthe intheISM.
zerolevelat0.4onthescaleofFig.8.Voigtprofilefittingofthe The fact that Ti ii is not detected in the zabs = 1.18 system
NaidoubletgiveslogN(Nai)=13.7andb=1kms−1which is surprising, although the limit on the column density is not
is within a factor of two of what has been derived previously stringentandonlyafactoroffoursmallerthanwhatisseenin
(seeTable2).FourcomponentshavebeenusedtofittheMgi thezabs=1.06components.