On the link between associated MgII absorbers and star formation in quasar hosts

0.34 MB·English

by  Yue Shen

DRAFTVERSIONJANUARY19,2013
PreprinttypesetusingLATEXstyleemulateapjv.05/04/06
ONTHELINKBETWEENASSOCIATEDMgIIABSORBERSANDSTARFORMATIONINQUASARHOSTS
YUESHEN1ANDBRICEMÉNARD2,3,4
DraftversionJanuary19,2013
ABSTRACT
Afew percentofquasarsshowstrongassociatedMgII absorption,with velocities(voff) lyingwithinafew
thousandkms- 1 fromthequasarsystemicredshift. Theseassociatedabsorptionlinesystems(AALs)areusu-
2
allyinterpretedasabsorbersthatareeitherintrinsictothequasaranditshost,orarisingfromexternalgalaxies
1
0 clustering around the quasar. Using composite spectra of ∼1,800 MgII AAL quasars selected from SDSS
2 DR7 at 0.4.z.2, we show that quasars with AALs with voff <1500kms- 1 have a prominent excess in
[OII]λ3727emission (detectedat>7σ) atrestrelativeto thequasarhost, comparedto unabsorbed quasars.
n
We interpretthis [OII] excessas due to enhancedstar formationin the quasarhost. Our resultssuggestthat
a a significantfractionof AALs with v <1500kms- 1 are physicallyassociated with the quasarand its host.
J off
AALquasarsalsohavedustreddeninglyingbetweennormalquasarsandtheso-calleddust-reddenedquasars.
7
WesuggestthattheuniquepropertiesofAALquasarscanbeexplainediftheyarethetransitionalpopulation
2
fromheavilydust-reddenedquasartonormalquasarsintheformationprocessofquasarsandtheirhosts. This

scenario predicts a larger fraction of young bulges, disturbed morphologiesand interactions of AAL quasar
]
O hostscomparedtonormalquasars. Theintrinsiclinkbetweenassociatedabsorbersandquasarhostsopensa
newwindowtoprobemassivegalaxyformationandgalactic-scalefeedbackprocesses,andprovidesacrucial
C
testoftheevolutionarypictureofquasars.
.
h Subjectheadings:blackholephysics—galaxies:active—quasars:general—quasars:absorptionlines
p
-
o
1. INTRODUCTION icalprocesseswithin quasarhosts is intrinsicquasarabsorp-
r
t tion lines. Historically most of the focus has been on broad
s In the popular Sanders & Mirabel picture for quasar evo-
absorptionlines(BALs,usuallydefinedasabsorptiontroughs
a lution (e.g., Sandersetal. 1988; Sanders&Mirabel 1996;
[ Canalizo&Stockton 2001; Hopkinsetal. 2008), gas-rich broaderthan2000kms- 1),whichareundoubtedlyintrinsicto
major mergersbetween galaxies triggerintense starbursts in thequasar. Herewefocusonanotherclassofnarrowabsorp-
2
tion lines called associated absorption lines (AALs), whose
v ultraluminous infrared galaxies (ULIRGs) and channel am-
absorption velocity is close to the systemic velocity of the
8 ple fuel into the nuclear region to feed the black hole (e.g.,
background quasar (z ≈z ). AALs are traditionally de-
7 Hernquist 1989). The black hole (BH) growth is initially finedasnarrowabsorpatbiontreomughs(.500kms- 1)withave-
7 buried in dust until some feedback process (either stellar-
locityoffsetv within±3000kms- 1ofthesystemicredshift
0 driven or quasar-driven) blows the dust away and a clas- off
. sical quasar emerges. The major merger event also trans- of the quasar 5. Strong (EW>0.6Å) low-ionization MgII
4
forms the host morphology from disks to a spheroid (e.g., associated absorbers are present in a few percent of the en-
0
1 Toomre&Toomre 1972). The feedback processes invoked tirequasarpopulation(e.g.,VandenBerketal.2008). These
1 in this model are also thoughtto be responsible for shutting absorption systems are generally believed to be close to the
: downstarformationinthebulge,andforestablishingthetight quasarand are explainedby either(ora combination)of the
v correlations between BH mass and bulge properties as ob- followingscenarios: absorptionby externalgalaxiescluster-
Xi servedinlocaldormantmassivegalaxies(e.g.,Tremaineetal. ingaroundthequasar(e.g.,Weymannetal.1979;Wildetal.
2002). Althoughsuchaviolentrouteisnotrequiredfortrig- 2008); absorption by halo clouds of the quasar host galaxy
r
a gering low-to-intermediate luminosity Active Galactic Nu- (e.g., Heckmanetal. 1991); absorption by material from a
cleus (AGN) activity, this merger-based framework seems starburstwindofthequasarhost(e.g.,Heckmanetal.1990);
successful in reproducing an array of observations of lumi- ororiginatingfrom the vicinity of the black hole (.200pc)
nousquasarsand massive red ellipticals in the cosmological basedonpartialcoverageanalysisorvariabilitystudies(e.g.,
context(e.g.,Wyithe&Loeb2003;Hopkinsetal.2008;Shen Hamannetal. 1995; Barlow&Sargent 1997). On the other
2009). Nevertheless, manyfundamentalissues remain unre- hand,classicalinterveningabsorbersystems(withzab≪zem)
solvedin thismodel, in particular, the nature andphysicsof arenotphysicallyassociatedwiththequasarandareabsorp-
these fueling and feedback processes along the evolutionary tions due to cosmologically intervening foregroundgalaxies
sequenceremainlargelyelusive. alongthequasarline-of-sight(LOS)(Bahcall&Spitzer1969;
Ausefultooltoprobethephysicalconditionsanddynam- Bergeron1986). Thestrengthoftheseinterveningabsorbers
isshownto correlatewith the associated starformationrate,
1Harvard-Smithsonian Center forAstrophysics, 60Garden St., MS-51, measuredwithin10kpc(Ménardetal.2011).
Cambridge,MA02138,USA;[email protected] Using composite spectra, VandenBerketal. (2008)
2CITA,University ofToronto, 60St. George Street, Toronto, Ontario,
M5S3H8,Canada 5 Some earlier studies used a larger velocity offset cut ∼ 5000-
3Institute for Cosmic Ray Research, University of Tokyo, Kashiwa 6000kms- 1todefineAALs. Thisispartlyduetothefactthatthesystemic
2778582,Japan
redshiftformanyhigh-redshiftquasarsisbasedonthecentroidofbroademis-
4DepartmentofPhysicsandAstronomy,JohnsHopkinsUniversity,Balti- sionlines suchas CIV, which has alarger uncertainty compared with the
more,MD21218,USA;[email protected]
systemicredshiftdeterminedfromMgIIornarrowemissionlines.
2 SHEN&MÉNARD
FIG. 1.—Anexample ofourfitting procedure toidentify MgIIAALs.
Theblackandgraylines showtheSDSSspectrumandfluxdensityerrors
around the MgII region. The red line shows our pseudo-continuum plus
emissionlinemodelfit. Thedashedlinemarksthequasarsystemicvelocity
basedonthecentroidofthebroadMgIIline. Thecyanlineshowsthecon-
tinuum+emission linesubtracted spectrumandthemagenta lineshowsthe
double-Gaussianfittotheabsorptiondoublet.ForthisexampleonlyNpair=1
absorptiondoubletwasidentified.
showed that AAL quasars on average have dust extinction
almost twice that observed in quasars with intervening ab-
sorbers(e.g., Yorketal. 2006), and the associated absorbers FIG.2.— DistributionofvelocityoffsetvoffofassociatedMgIIabsorbers.
Positive(negative)valuesindicateblueshifted(redshifted)fromthesystemic
haveahigherionizationstate thaninterveningabsorbers. To redshift. The black histogram shows the whole sample while the red one
someextent, thissupportstheidea thatAALsarephysically representsasubsetofquasarswithz<1.4forwhichwehaveemissionline
associatedwiththequasar(oratleastaffectedbytheradiation redshiftsbasedon[OII]or[OIII].Systemswithvoff>1500kms- 1arefound
tobesimilartoclassicalinterveningabsorbersystems.
field of the quasar), but a directlink between AALs and the
quasarisstillelusive(cf.,Wildetal.2008).
Using∼1800quasarswithassociatedMgIIabsorbersfrom
the SDSS, we will show that there is a link between AALs
and [OII] emission from the quasar host, suggesting that a
substantial fraction of AALs are intrinsic to the quasar and
itshost. WefurtherarguethatthepropertiesofAALquasars
are in favor of them being the transitional population in the
Sanders& Mirabelpicture, withsignaturesoffeedback. We
describeoursamplein§2andpresentourmainresultsin§3.
In §4 we present an evolutionarypicture to interpret our re-
sultsanddiscussitsimplications.
2. DATA
WesearchalltheDR7quasars(Schneideretal.2010)with
MgII coverage (∼ 85k quasars) for associated MgII ab-
sorption. This restricts our sample to 0.4 . z . 2 (¯z ∼
1.2), which extends to lower redshift than the sample in
VandenBerketal. (2008). The unabsorbed continuum plus
emissionlinefluxismodeledwithaχ2 fitwithrejectionsof
absorptiontroughs(Shenetal. 2011). We referthereaderto
thatpaperonthedetailsofourfittingprocedure.Inshort,we
fit the restframe 2200-3090Å regionwith a power-law con-
tinuumplus an iron template, and a set of Gaussians forthe
MgII line. Duringthe fitting, we mask out3σ pixeloutliers
below the 20 pixel boxcar-smoothed spectrum to minimize
the effects of narrow absorption troughs. We then subtract
the pseudo-continuumplus emission line emission from the
spectrumtogettheresidualspectrum.MgIIλλ2796,2803ab- FIG. 3.—Top: Mediancompositespectrafordifferentsamples,stacked
inthequasarrestframeandnormalizedat3000Å.ThethreeAALsamples
sorptiondoubletsarethenidentifiedonbothsidesoftheMgII
(intervening-like:yellow,blueshifted:cyan,andredshifted:magenta)showin-
emission line by fitting double-Gaussians to the absorption creasing reddening relative to the control sample (black), but are substan-
doublet.Weonlykeepabsorbersthataredetectedat>3σfor tiallylessreddenedthantheSDSSdust-reddenedquasars(withcolorexcess
bothGaussiancomponents.Wemeasurerestframeequivalent ∆(g- i)>0.3)showninred.Bottom:Fluxratiosbetweencompositespectra
ofthethreeAALsamplesandthecontrolnormalquasars. WhiletheAAL
widths(EW)oftheseAALsanduncertaintiesofEWsarees-
samplesshow(byconstruction)MgIIabsorptionwithdifferentvelocities,an
timated from the Gaussian fits. Fig. 1 shows an example of excessof[OII]emissionisseenfortheblueshiftedandredshiftedsamples
ourfittingresults. Althoughthisfittingrecipewasdeveloped atthequasarsystemicredshift.
to reduce the effects of narrow absorptions on the emission
QUASARSWITHMGIIAALS 3
TABLE1
THESAMPLEOFAALS
SDSSDesignation Plate Fiber MJD redshift EW2796 EWErr2796 EW2803 EWErr2803 voff voff,SDSS voff,HW
(hhmmss.ss+ddmmss.s) (Å) (Å) (Å) (Å) (kms- 1) (kms- 1) (kms- 1)
000045.77+255106.1 2822 339 54389 1.4446 0.61 0.07 0.50 0.07 1300 2726 2069
000140.70+260425.5 2822 322 54389 0.7653 0.48 0.04 0.47 0.04 47 54 113
000219.64+260029.2 2822 367 54389 1.2507 3.07 0.18 3.00 0.14 -421 -588 -289
NOTE.—ThelistofAALsusedinthispaper. Inthelastthreecolumnswelisttheabsorbervelocityoffsetcalculatedfromthefiducialsystemic
velocity(basedonthebroadMgIIcentroid),theSDSSredshiftintheDR7quasarcatalog,andtheimprovedredshiftfromHewett&Wild(2011).
Thefullcatalogisavailableintheelectronicversion.
line measurementsratherthan a dedicated recipe forfinding plewith[OII]or[OIII]-basedredshifts.Thedispersioninthe
narrowabsorptionlines,itdoesareasonablygoodjobiniden- v distributionis∼600kms- 1,largerthanthetypicaluncer-
off
tifyingAALs. Withthismethodwerecovered∼80%ofthe tainty ∆v ∼350kms- 1 arising from systemic redshift un-
off
AALsintheDR3sampleofVandenBerketal.(2008),where certaintiesbasedonMgII. Itisthusdominatedbyanintrinsic
mostofthe“missing”AALsareweakabsorptionswithother- dispersion. Ourcompositespectra(see§3)also confirmthat
wisenormalquasarproperties. However,wedonotintendto thisdispersionisnotcausedbyincorrectsystemicredshifts.
quantifythecompletenessinourAALselectionasfunctions Asearlierstudiesalreadyshow(e.g.,Weymannetal.1979;
of S/N and absorber strength, as such a task would require VandenBerketal. 2008), there is a substantial fraction of
a more dedicated narrowabsorptionfinderand Monte-Carlo AALs that are redshifted from the systemic velocity. These
simulations, which are unnecessary for the purposes of this systemscouldeitherbetrulyinfallingabsorbers,ortheemis-
paper. sion linesused to estimate the systemic redshifthavesignif-
To define an AAL one needs to know the systemic red- icantoutflowingvelocity. Atv >1500kms- 1,thedistribu-
off
shift of the quasar. Stellar absorption features are generally tion flattens out and is consistent with the expectation from
unavailable due to the overwhelmingquasar light. The red- cosmologicallyinterveningabsorbers,asseeninearlierstud-
shifts based on narrow lines such as [OII] and [OIII] are ies (e.g., Wildetal. 2008). We thus argue that these high-
generally consistent with stellar absorption redshifts within velocity AALs are mostly classical intervening absorbers,
.100kms- 1(e.g.,Richardsetal.2002). Theredshiftsbased whichwillbeconfirmedbyourcompositespectrain§3.
on the broad MgII emission line are consistent with those Wedivideoursampleintothreecategories:1)AALquasars
based on [OII] or [OIII] with a dispersion of ∼350kms- 1 with voff > 1500kms- 1 (the “intervening-like” sample); 2)
(e.g.,Richardsetal.2002;Shenetal.2011,theirfig.17).The AAL quasars with 0<v <1500kms- 1 (the “blueshifted”
off
high-ionizationbroadCIVlineisknowntobesystematically sample); 3) AAL quasars with - 3000<voff <0kms- 1 (the
blueshifted(∼700kms- 1) from low-ionizationlines such as “redshifted”sample). Wehave∼750quasarseachinthelat-
MgII, with a large dispersion of ∼700kms- 1 (e.g., Gaskell tertwosamples,and∼370quasarsintheformer.
1982; Tytler&Fan 1992; Richardsetal. 2002; Shenetal.
2011), and hence will bias the systemic redshift determina- 3. COMPOSITESPECTRA
tionathighredshift. TostudytheaveragepropertiesofthesethreeAALquasar
HereweadoptthecentroidofthebroadMgIIemissionline samples we create high S/N composite spectra in the rest-
as the quasar systemic redshift to calculate the absorber ve- frame of the quasar. SDSS spectra integrate all the light
locity offset, and use 3000kms- 1 as the velocity cut to de- received within a 1.5′′ fiber radius, corresponding to ∼ 10
fineanAAL.Wecomparedtheabsorbervelocityoffsetswith kpc at z∼1, hence they cover a substantial fraction of the
thosecalculatedbyadoptingtheimprovedredshiftsofSDSS quasar host. For comparison purposes we construct a con-
quasars from Hewett&Wild (2011), and found good agree- trol sample of more than 9,000 quasars without detected
ments with a dispersion of ∼300kms- 1. This dispersion is AALs,matchedinredshiftandi-bandmagnitudetotheAAL
consistent with the redshift uncertainty ∼ 350kms- 1 based quasars. We create composite spectra using the method de-
on the MgII centroid. Thereforewe adopta nominaluncer- scribedinVandenBerketal.(2001). Inshort,eachspectrum
tainty of ∆v =350kms- 1 in absorber velocities, which is wasshiftedtorestframe,rebinnedontoacommondispersion
off
dominatedby systemic redshiftuncertainties. We finally ar- of1Åperbin,andnormalized.Thefinalcompositespectrum
rivedatasampleof∼1800quasarswithAALs,amongwhich wasgeneratedbytakingthemedian(orgeometricmean)flux
∼10%showmultipleAALsinSDSSspectra(consistentwith densityineachbinoftheshifted,rebinned,andscaledspec-
Wildetal.2008). ThesampleofAALsusedinthefollowing tra. An error spectrum was generated from the 68% semi-
analysis is listed in Table 1. Additional information about interquantilerangeofthefluxdensitiesineachbinscaledby
these quasars can be retrieved from the value-added SDSS N1/2,whereN isthenumberofspectracontributingtothat
DR7quasarcatalogcompiledinShenetal.(2011). spec spec
bin.WereferthereadertoVandenBerketal.(2001)formore
Fig.2showsthedistributionofabsorbervelocityv forthe
off detailsregardinggeneratingthecompositespectra.
wholesampleinblack,andforasubsetofz<1.4quasarsfor
The left panel of Fig. 3 shows median composite spec-
whichwehaveredshiftsfrom[OII]or[OIII]inred. Positive tra for the above quasar samples. AAL quasars have col-
values indicate blueshifted relative to the systemic and neg-
ors lying between normal quasars and the so-called “dust-
ative values indicate redshifted. We found that the absorber reddened”quasarsinSDSS(withcolorexcess6∆(g- i)>0.3;
velocitydistributionusingthissystemicredshiftdefinitionis
similar(albeitslightlybroaderduetothelessaccurateMgII 6 ∆(g- i)is a goodindicator ofintrinsic quasar colors which accounts
emission line redshifts) to that using the subset of our sam- forthebandshiftingwithredshift(e.g.,Richardsetal.2003);larger∆(g- i)
valuesindicatereddercolors.
4 SHEN&MÉNARD
FIG. 4.— Left: Continuum-subtracted mediancompositesofthe[OII]emissionline,stackedinthequasarrestframe. Theblacklinesshowthelevelof
emissionforthecontrolquasars. Wedetecta70and53%excessfortheblueshiftedandredshiftedsamplesrespectively. Right: Samequantifyshownforthe
[NeV]λλ3346,3426emissionline.Inthiscasewedonotdetectanychangeinthelineflux.
Richards et al. 2003). The reddening of AAL quasars rel- tioncurve,werequireE(B- V)∼0.12toachievethelevelof
ative to the control quasars is well described by a SMC- [OII] emission enhancementrelative to the continuum, sub-
like extinction curve, with E(B- V)∼0.03, consistent with stantially larger than the inferred E(B- V)∼0.03 from the
VandenBerketal. (2008). The rightpanelshows flux ratios compositespectra.
oftheAALquasarcompositestothatofthecontrolquasars. Wenowfocusonthe[OII]emissionexcess. Toquantifyit
The“redshifted”and“blueshifted”samplesshowaprominent andestimateitssignificance,wecreatecontinuum-subtracted
excessofnarrow[OII]emission,whichisabsentinthecaseof median composites around the [OII] emission line. This is
the“intervening-like”sample.Thelackof[OII]emissionex- done by subtracting a running median filter with a width of
cess,thesimilaramountofreddeningtoclassicalintervening 20 pixels and by masking the region3710<λ<3750 Å to
absorptionsystems (e.g., York et al. 2006), and the fact that preservetheemissionline. Withinthisregionthecontinuum
theyjointheplateauofthevelocitydistribution(e.g.,Fig.2), estimateisinterpolated. TheresultsareshowninFig.4. The
suggestthatAALswithvoff>1500kms- 1 areinfactmostly centroidof the excess[OII] emission is within ∼150kms- 1
classicalinterveningabsorbers7. Wenotethatthelarge-scale from the systemic velocity (Fig. 4). Considering the typical
fluctuationsseeninthecompositeratiosareduetocorrelated redshiftuncertaintiesof individualobjectsused in the stack-
noisearisingfromvarianceintheshapeofquasarcontinua. ingthisisconsistentwiththesystemicvelocity. Wemeasure
The correlation between the presence of absorbers with the line flux by integrating the flux density over the emis-
voff < 1500 kms- 1 and [OII] emission is of great interest. sion line profile. The line flux error is estimated by boot-
WhilethecompositeMgII absorptionlinesare,byconstruc- strapping the sample of reference quasars and repeating the
tion,offsetbyhundredsofkms- 1withrespecttothequasars, procedure100times. Thisallowsusto includesample vari-
thestackedexcess[OII]emissionisfoundmoreorlessatthe ance in the errorestimation. The [OII] excess is detected at
quasarsystemic velocity(see below). Thusthe excess [OII] 8σ (7σ) forthe blueshifted (redshifted)sample, correspond-
emissionoriginatesfrommateriallyingatthequasarsystemic ing to an enhancementof 70% (53%) of the line flux of the
velocity. Thecentralengine,i.e.,theblackholeradiation,ap- controlquasars. Theenhancementincreasesto94%(63%)if
pears to be similar in AAL quasars and in normal quasars: we use mean composites. For comparison, we measure the
otherthanthereddeningandMgII absorption,nodifference properties of the [NeV]λλ3346,3426, which is entirely ex-
isseeninthecontinuumandbroademissionlinesbetweenthe citedby AGNgivenitshighionizationpotential. We follow
twoquasarpopulations. Thissuggeststhatdust-reddeningis thesame procedureandthe resultsarepresentedinthe right
theexplanationforthecolordifferenceseeninAALquasars panel of Fig. 4. In this case, we do not observe any signif-
and normal quasars. One might ask if this dust reddening icantexcessin the [NeV]λλ3346,3426. ThereforetheAGN
might also cause the apparent [OII] excess seen in the flux ionizing source of AAL quasars is similar to that of normal
ratio plot in Fig. 3. Dust reddening could occur on spatial quasars, and we conclude that the excess [OII] emission is
scalesmuchsmallerthanthe[OII]emissionregionbutmuch duetodifferenthostproperties.Interestingly,no[OII]excess
largerthanthecontinuumplusbroadlineregion. Inthiscase isseeninthecompositespectrumof∼300quasarswithbroad
the continuum is attenuated while the [OII] emission is not, MgIIabsorptionlinesselectedfromSDSS(Shenetal.2011),
causing an apparent enhancement of [OII] strength relative whichisexpectedifbroadabsorptionlines(BALs)areubiq-
totheunderlyingcontinuum. AssuminganSMC-likeextinc- uitousamongquasarsandtheappearanceofBALQSOsonly
depends on LOS. This difference reinforces our conclusion
7 Thisstatementisforthelow-zMgIIAALs,andmaynotholdtruefor thatthehostsofAALquasarsareintrinsicallydifferent.
thehigh-zandhigh-ionizationAALs(suchasCIV).
QUASARSWITHMGIIAALS 5
4. DISCUSSION AAL quasars thus have properties intermediate between
ThedistincthostpropertiesofAALquasarsintermsofex- dust-reddened and classical quasars, in terms of both [OII]
cess[OII]emission(withabsorbervoff<1500kms- 1)suggest ewmitihssAioAnLaqnudadsaursstbreeidndgemnionrge.evTohlevseedfithnadninthgesdaurestc-roendsdisetneendt
that a significantfractionof these associated absorbersmust
quasarsfollowinga gas-richmerger. Theobscuringmaterial
bephysicallyassociatedwiththequasaranditshost,andare
notfromdisconnectedexternalgalaxies8. Becausetheprop- in the immediate circumnuclearregion has largely been dis-
persed,butthehoststarformationrateisstillhigherthannor-
ertiesoftheactiveSMBHaresimilarforAALquasarsandfor
malquasars. Alternatively,AAL quasarscouldbe similarto
normalquasars,weexpectthattheamountof[OII]emission
dust-reddenedquasars,butareseenalongalessobscuredline
contributedbythequasaritselfshouldbesimilaraswell. We
ofsight. Ineitherscenario,weexpectAALquasarstoshowa
thus interpret the excess [OII] emission as due to enhanced
higherfractionofless-developedbulges,disturbedmorpholo-
star formation within the AAL quasar host galaxy, as [OII]
giesandinteractionsthanclassicalquasars.
isagoodindicatorofstarformationrateinregionswithlow
An archival search in the HST database resulted
ionizationparameters, andhasbeenappliedforquasarhosts
in three AAL quasars in our sample that have deep
(e.g.,Ho 2005) aftersubtractingthe [OII] contributionfrom
(> 10min) imaging data at z ∼ 0.6, all observed in
the quasaritself. Interestingly, Ménardetal. (2011) showed
serendipitous programs and the data is publicly avail-
a similar connection between MgII absorbers and star for- able. Two of them, SDSSJ085215.65+492040.8 and
mation in intervening galaxy absorbers. The measured ex-
SDSSJ130952.89+011950.6, were observed with WFC3-IR
cessof[OII]luminosityoftheAALquasarhostsisfoundto
be L=1.9 and 1.2×1041ergs- 1 forthe blueshiftedand red- (∼17 min and ∼13 min) and WFC3-UVIS (∼28 min and
∼ 25 min). The third object, SDSSJ093210.96+433813.1,
shiftedsamplesrespectively.FollowingMénardetal.(2011),
was observed with WFC3-IR with a total exposure time of
weconvertthesevaluestostarformationratesusingtheaver-
∼ 40min. In Fig. 5 we show the retrieved WFC3-IR im-
agescalingcoefficientforgalaxieswith0.75<z<1.45given
ages for these three objects, and their SDSS spectra around
byZhuetal.(2009). WethenfindAALquasarhoststohave
amedianexcessstarformationrateof∼7(5)M yr- 1forthe the MgII region. J0932 and J1309 show clear tidal features
⊙
and disturbed host morphologies even before subtraction of
blueshifted(redshifted)sample.
the quasar light, while J0852 appears to be in a small in-
A less likely interpretation for the [OII] excess would in-
teracting group where the immediate neighborto the lower-
vokedifferentISMconditionsorelementabundanceforAAL
right has a SDSS photometric redshift consistent with the
quasarhosts. Eitherway,ourresultsshowthatAALquasars
quasar redshift. Sluse et al. (2007) reported a serendipi-
representadistinctpopulationandmayindicateaspecialevo-
tously discovered AAL system in the gravitationally lensed
lutionary phase. The clear absence of [OII] emission at the
quasarRXSJ1131-1231,whichalsoshowsevidenceofinter-
redshift of the associated absorbers in the composite spec-
actions and disturbed morphology in the reconstructed host
tradisfavorsmajormergers(inwhichcaseenhancedstarfor-
image (Claeskensetal. 2006). Thus the frequency of dis-
mation for the companionwould also have been seen). Our
turbedmorphologiesand close companionsof AAL quasars
results thus imply that associated MgII absorbers could ei-
ismuchhigherthanevolvedquasars(e.g.,Bahcalletal.1997;
therbeminormergers,orgalactictohalo-scalegasoutflows
Dunlopetal. 2003; Veilleuxetal. 2009), and is more in
and infall associated with the quasar host galaxy. We still
line with the results for heavily dust-reddenedquasars (e.g.,
expect some AALs from the clustering of galaxies around
Urrutiaetal. 2008). This is fully consistent with our evolu-
quasars (e.g., Wildetal. 2008). Quantifying their contribu-
tionaryscenario,butmoredataareneededtodrawfirmcon-
tionrequiresbetterknowledgeonthestrengthofthegalaxy-
clusions. We defer a detailed analysis of the HST data and
quasar cross-correlation function and pairwise velocity dis-
follow-upstudiesofthesesystemstofuturework.
persion (for the specific type of absorbers), as well as the
Eventhoughourresults suggestan intrinsic origin forthe
sizeofthequasarproximityzone(e.g.,Hennawi&Prochaska
bulkoftheAALs, thisisastatisticalstatementandthereare
2007),andisbeyondthescopeofthispaper.
stillcaseswheretheAALsareofexternalorigins. Moreover,
A number of studies have revealed that dust-reddened
the nature of these intrinsic AALs is still unknown. Never-
quasarshavedifferentpropertiesfromclassicalquasars: heav-
ily reddened (with E(B- V)&0.5) type 1 quasars, selected theless, theyprovidea uniquewindowto probethe physical
conditions and dynamical processes during the evolution of
in the near infrared and radio, usually show strong [OII]
quasarsandtheirmassivehosts,anddeservein-depthstudies,
emission (e.g., Glikman et al. 2007). In addition, these
eitherinastatisticalmannerorforindividualsystems. More
systems display a large fraction (approaching unity) of dis-
than half of these associated absorbersshow blueshifted ve-
turbedmorphologiesandinteraction(e.g.,Urrutiaetal.2008).
DustreddenedquasarsfromtheSDSS(withE(B- V)∼0.1) locity, among which some could be feedback (either stellar-
orquasar-driven)atwork,andpossiblyleadingtoasuppres-
also show an excess of [OII] emission compared to classi-
sionofthehoststarformation. Thetypicalvelocityofthese
cal quasars (Richards et al. 2003). This is in line with the
absorbers (i.e., a few hundred kms- 1) is much smaller than
Sanders & Mirabel picture that dust-reddened quasars are
thoseseeninBALsandintrinsichighvelocitynarrowabsorp-
in the early stage of evolution after the merger and have
tion lines (NALs), hence these AAL absorbers are likely to
higher star formation rate and more dust obscuration in the
be on scales much larger than nuclear scales9, and thus are
nuclei. It is interesting to note that the reddest quasars also
more likely to impactthe host galaxy. Gravitationalinflows
show a higher occurrence of associated absorbers than nor-
mayalsobepresentduringtheearlystageofquasarandhost
mal quasars (e.g., Aldcroftetal. 1994; Richardsetal. 2003;
Hopkinsetal.2004),butthebulkofAALquasarshavemuch
9Associatedabsorptionsarethereforeexpectedtoshowlittlevariabilityon
bluercolors(Fig.3).
multi-yeartimescalescomparedwithBALsandhigh-velocityintrinsicNALs,
whichcanbetestedwithmulti-epochspectroscopyoftheseabsorptionline
8 Although theseexternal galaxies maystill beaffected bytheionizing systems(Shenetal.,inpreparation).
fluxfromthequasar.
6 SHEN&MÉNARD
FIG. 5.—ThreeAALquasarsinoursamplewithdeepHSTimages. Upper: WFC3-IRimageswithasinhscaling. Fromlefttoright: J0852+4920(F125W),
J0932+4338(F110W)andJ1309+0119(F125W).Thequasarsarelocatednearthecentersoftheimages.Bottom:TheMgIIregionoftheopticalspectrumfrom
SDSSforthethreeobjects.
formation,eitherin theformof accretedgasoras partofan to improvement of the manuscript. We also thank Michael
infalling galaxy companion. While intrinsic absorbers with Strauss, Tim Heckman, Gordon Richards, Norm Murray,
infalling velocities greater than several hundred kms- 1 are MartinElvis,andYuvalBirnboimforusefuldiscussions. YS
likelyto be in thevicinityof the BH, the natureof themore acknowledges support from the Smithsonian Astrophysical
slowlymovingabsorbersneedstobeexplored. Inparticular, Observatory(SAO)throughaClayPostdoctoralFellowship.
infalling absorbers are expected in galaxy formationmodels Fundingfor the SDSS and SDSS-II has been providedby
(e.g., Kerešetal. 2005; Dekeletal. 2009) but almost never theAlfredP.SloanFoundation,theParticipatingInstitutions,
seen in galaxy spectra (Steidel et al. 2010). The possibil- theNationalScienceFoundation,theU.S.DepartmentofEn-
itythatsomeoftheseinfallingabsorbersmightbeassociated ergy,theNationalAeronauticsandSpaceAdministration,the
withcoldflowswillbeexploredinfuturework. JapaneseMonbukagakusho,theMaxPlanckSociety,andthe
HigherEducation Funding Council for England. The SDSS
WebSiteishttp://www.sdss.org/.
We thank the anonymous referee for suggestions that led
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