Associated absorption in radio quasars. I. C IV absorption and the growth of radio sources

1.Introduction

Quasarabsorptionlinesprovideauniqueandpow-erfultoolforprobingtenuousgascloudsthroughouttheuniverse.Thephysicalregimesprobedinabsorp-tiondi erfromthosewhichdominatetheemissionofbothgalaxiesandActiveGalacticNuclei(AGN),andhenceabsorptionprovidesacomplementaryviewofthematerialcompositionofgalaxiesandAGN.Con-sequently,absorptionlinesoccurringveryclosetothequasaremission-lineredshift—associatedabsorbers—arevaluableprobesoflocalquasarenvironments,potentiallyinterceptinggasfromarangeoflocationsalongthesightline.Forinstance,absorptioncanariseingas owsnearthequasarcentralengine,theinter-stellarmedium(ISM)ofthequasarhostgalaxy,ortheISMofneighbouringgalaxieswhichmaythemselvesbeinclustersorgroups.

Associatedabsorptionsystems(usuallyde nedbyanabsorptionredshift,za,lyingwithin5000kms 1oftheemission-lineredshift,ze)seemtobemorenumerousthancanbeaccountedforbyrandomlydistributedgalaxiesalongthequasarsightline(Wey-mannetal.1979;Foltzetal.1986,1988;Richardsetal.1999,2001andreferencestherein;butseee.g.Young,Sargent&Boksenberg1982).Theabsorptionsystemsoccurmostfrequentlyinradio-loudquasars,especiallythosewithsteepradiospectra(Andersonetal.1987;Foltzetal.1988;Richardsetal.1999,2001),favouringadirectlinkwithAGNactivityandalso,perhaps,theprocessesofradio-jetformation.Manyza≈zeabsorbersalsodi ersubtlyinioni-sationorvelocitypro lesfromthoseseenalongsight-linestraversingnormalgalaxyhalos,suggestingza≈zeabsorbingcloudsexperiencedirectlythehardra-diation eldandkinematicsoftheAGN(Petitjean,Rauch&Carswell1994;Hamann&Ferland1999;Hamannetal.2001).Inthemostextremecases,verybroadabsorption-lines(BALs;FWHMsofthousandsofkms 1)areseen(seesummariesbyTurnsheketal.1988;Turnshek1995;Weymann1995).Thesebroadtroughsareprobablyduetoout owing,densewindsclosetothenucleus.Interestingly,BALsystemsoc-curalmostexclusivelyinradio-quietquasars(butseeBeckeretal.1997,2000;Greggetal.2000).Thispaper,however,willfocusexclusivelyonthenarrowassociatedabsorption-linesystems(FWHMstenstohundredsofkms 1).

Amongradio-loudquasars,thetrendforza≈zeabsorptiontooccurpreferentiallyinsteep-spectrum

andlobe-dominatedquasars(Barthel,Tytler&Vester-gaard1997)canbeinterpretedwithinthebasicframe-workofthe‘uni edschemes’forAGNasanorienta-tione ect.Steep-spectrumradiosourcesarethoughttobeviewedatlargeangles,θ,totheradio-jetaxis,andthepresenceofabsorbingmaterialisconsistentwiththebasicgeometryofanobscuringtoruswhichblocksthenuclearlightfromtheobserverbeyondaviewingangleofaboutθ～45 (seeBarthel1989;An-tonucci1993;Urry&Padovani1995).Inpractice,considerableamountsofobscuringmaterialmustalsoliewithinthetorusopeningangle,tocausetheop-ticalreddeningseeneveninsteep-spectrumquasars(Baker&Hunstead1995;Baker1997).

AllpropertiesoftheAGNpopulationcannot,how-ever,beexplainedintermsofviewingangledi er-encesalone.Forexample,20–30%ofcompactradiosources(angularsizes<2′′)havesteepratherthan atspectra,implyingtheirradiojetsarenotbeamedstronglytowardsus.Therefore,theprojectedsizesofcompact,steep-spectrum(CSS)sourcesarenotsmallsimplybecausetheirjetsareorientedclosetoourlineofsight,butrathertheyareintrinsicallysmall(seeFantietal.1990).Infact,asaclass,CSSsources(seereviewbyO’Dea1998)doobeytheorientation-uni cationpicture(e.g.Saikiaetal.2001),buttheycompriseintrinsicallysmallversionsofthelargerradio-sourcepopulation,theirluminousradioemis-sionemanatingfromregionswithintheenvelopeofthehostgalaxy.ThemajorityofCSSsourcesarethoughttobeyoung,andpresumablywillgrowintolarge-scalesources(Readheadetal.1996;Owsianik&Conway1998;Tayloretal.2000;Saikiaetal.2001;deSilvaetal.,2001,inpreparation).There-foresourceevolutionmustalsobefoldedintoAGNstudies.However,apartfromtheevolutionofthera-diocomponents,whichhasbeenextensivelymodelled,littleisknownaboutevolutionofindividualquasarsatotherwavelengths.

Tobuildupapictureofhowtheabsorbingma-terialisdistributedaroundquasars,oritsrateofoc-currenceamongdi erenttypesofquasar,necessitatessamplingmanysightlinesoverawiderangeofviewinganglestothenucleus.Therefore,itiscrucialtousecompleteandunbiasedsamplesforsuchaninvestiga-tion.Unfortunately,mostpreviousworkwasbasedonheterogeneoussamples.Inthispaper,weinvesti-gateassociatedCivabsorptioninacompletesampleoflow-frequencyselectedquasars.Usingnewopti-calspectraatmoderateresolution(1–3 A),wefocus

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