High-redshift QSOs in the FIRST survey

In a pilot search for high-redshift radio QSOs, we have obtained spectra of 55 FIRST sources (S(1.4GHz) > 1 mJy) with very red (O-E > 3) starlike optical identifications. 10 of the candidates are QSOs with redshifts 3.6 < z < 4.4 (4 previously known), six

Mon.Not.R.Astron.Soc.000,000–000(0000)Printed1February2008

A(MNLTEXstyle lev1.4)

High-redshiftQSOsintheFIRSTsurvey

C.R.Benn,1 M.Vigotti,2M.Pedani,3J.Holt,1,4K.-H.Mack,6,2,7R.Curran,1,5

S.F.S´anchez1

1Isaac

arXiv:astro-ph/0109313v1 19 Sep 2001

NewtonGroup,Apartado321,38700SantaCruzdeLaPalma,Spain

diRadioastronomia,CNR,viaGobetti101,40129Bologna,Italy

3CentroGalileoGalilei,38700SantaCruzdeLaPalma,Spain

4DepartmentofPhysics&Astronomy,UniversityofShe eld,HicksBuilding,She eldS37RH,UK

5DepartmentofPhysicalSciences,UniversityofHertfordshire,CollegeLane,Hat eld,Herts,AL109AB,UK6ASTRON/NFRA,Postbus2,NL-7990AADwingeloo,theNetherlands7RadioastronomischesInstitutderUniversit¨atBonn,AufdemH¨ugel71,D-53121Bonn,Germany

2Istituto

1February2008

ABSTRACT

Inapilotsearchforhigh-redshiftradioQSOs,wehaveobtainedspectraof55FIRST

sources(S1.4GHz>1mJy)withveryred(O-E>3)starlikeopticalidenti cations.10ofthecandidatesareQSOswithredshifts3.6<z<4.4(4previouslyknown),sixwithz>4.Theremaining45candidatescomprise:az=2.6BALQSO;3low-redshiftgalaxieswithnarrowemissionlines;18probableradiogalaxies;and23Mstars(mainlymisidenti cations).Thesuccessrate(high-redshiftQSOs/spectroscopically-observedcandidates)forthissearchis1/2forS1.4GHz>10mJy,and1/9forS1.4GHz>1mJy.Withane ectivesearchareaof4030deg2,thesurfacedensityofhigh-redshift(z>4)QSOsdiscoveredwiththistechniqueis0.0015deg 2.

Keywords:quasars:general-quasars:emissionlines-radiocontinuum:galaxies-earlyUniverse

1INTRODUCTION

Observationsofhigh-redshift(z>4)QSOsstronglycon-strainmodelsof:theformationandevolutionofgalaxiesandtheircentralblackholes(Kau mann&Haehnelt2000);thecontributionofQSOstotheionisationoftheintergalac-ticmediumathighredshifts(Steidel,Pettini,Adelberger2001);and,throughstudiesoftheLymanabsorptionforest,thechemicalevolutionoftheintergalacticmediumalongthelineofsighttotheQSO(Rauch1998,Hamann&Ferland1999).

Mostofthe～300z>4QSOsnowknownhavebeendiscoveredasaresultofsearchesforobjectswithunusuallyredopticalcolours(Fig.1).AsummaryofrecentsearchesisgiveninTable1.Kenne cketal(1995a,b)searchedforob-jectswithunusuallyredg,r,icoloursin340deg2coveredbythesecondPalomarSkySurvey(POSS-II)ofthenorthernhemisphereanddiscovered10z>4QSOs(seealsoDjor-govski2001).Similarly,SloanDigitalSkySurvey(SDSS)commissioningimageshavebeensearchedforobjectswithredu,g,r,i,zcolours.Morethan100z>3.5QSOshavebeenfoundsofar(Fanetal2000,2001;Schneideretal2001;

Email:crb@ing.iac.esc0000RAS

Zhengetal2001;Andersonetal2001,Richardsetal2001),

andthisnumberwillriserapidlyasSDSSnearscompletion(～10000deg2)overthenextfewyears.Inthesouth,Storrie-Lombardietal(2001)searchedUKSTplatesscannedwiththeAutomatedPlateMeasuringfacility(APM)inCam-bridgeforobjectswithBJ R>2.5,andfound49QSOswithz>4.

Adisadvantageofthepurelyopticalsearchesisthatcomplexcolourcriteriaareneededtoexcludethemuchlargernumberofredstars,whichmakesitdi culttocorrectforincompleteness.Alternatively,simpleone-colourcriteriacanbeused,butthenalargefractionofthehigh-redshiftQSOsismissed(compareinTable1thesurfacedensitiesat-tainedfordi erenttypesofopticalselection).StartingwithasampleofradioQSOsallowsonetoreducethenumberofcandidateswithoutresortingtocomplexcolourcriteria,andislesslikelytobiastheselectionagainstdustyobjects.E.g.Hooketal(1998)soughtredobjectsidenti edwith at-spectrumQSOswithS5GHz>25mJyover1600deg2,andfound6withz>3(nonewithz>4).Snellenetal(2001)soughtredobjectsidenti edwith at-spectrumradiosourcesS5GHz>30mJyintheCosmicLensAll-SkySur-vey(CLASS,Myersetal2001)over6400deg2,andfound4QSOswithz>4,i.e.1z>4QSOper1600deg2.

Highersurfacedensitiescanbeattainedbyobserving

In a pilot search for high-redshift radio QSOs, we have obtained spectra of 55 FIRST sources (S(1.4GHz) > 1 mJy) with very red (O-E > 3) starlike optical identifications. 10 of the candidates are QSOs with redshifts 3.6 < z < 4.4 (4 previously known), six

2C.R.Bennet

al

Figure1.ThevariationofPOSSIO Ecolourwithredshift,forQSOsfromtheFIRSTbrightquasarsurvey(crosses,Whiteetal2000),theSDSSandPOSSIIsearchesforhigh-redshiftQSOs(spots),andaselectionofradioQSOswith3.4<z<4.0fromtheNASAExtragalacticDatabase(circles).TheQSOsinthesesampleswereselectedinpartonthebasisofcolour,butnev-erthelesstracethetypicalreddeningofaQSOwithincreasingz(predictionfortypicalQSOshownassolidline)asthecontinuumredwardoftheLyαlinemovesoutoftheobservedOband(redlimit≈5000 A).Asredshiftincreasesabove4.5,thepredictedbrightnessofaQSOinEbanddropssharply,sofewarelikelytobedetected.WiththecriteriaO E>3,E<18.6usedhere,ourQSOsearchisprobablycompleteforz>4,butincompletefor3<z<4.

thecounterpartsoffainterradiosources.Inthispaperwereportonapilotsearchforhigh-redshiftQSOsamongstveryred,O E>3(seeFig.1),opticalidenti cationsofFIRSTradiosourcesS1.4GHz>1mJy(withnoselectiononradiospectralindex).

ThereisnooverlapbetweenthissearchandtheFIRSTbrightquasarsurvey(Whiteetal2000,Beckeretal2001),whoseselectioncriterionO E<2precludedetectionofmosthigh-redshiftQSOs(nonewithz>3.7werecata-logued),asdothecolourselectioncriteriaofmostlargeQSOsurveys,e.g.the2dFQSOsurvey(Croometal2001).

2SAMPLE

TheFIRSTradiocatalogue(Whiteetal1997)currentlyin-cludes722354sourcesdetectedwithpeakS1.4GHZ>1mJyover7988deg2(≈90sourcesdeg 2),mainly7<RA<17h,-5<Dec<57o.Wesoughtopticalidenti cationsofthesesourceswithobjectscataloguedbytheAPM(Irwinetal1994)onthePOSS-IO(blue)andE(red)plates.Wedidnotseekopticalidenti cationsatthemid-pointsoflikelydoubleradiosources(～10%ofthecatalogue),sincethiswouldhaverequiredasubstantialenlargementofthesearcharea.Thecolour-magnitudedistributionofthestar-likeopti-calidenti cationslyingwithin1.5arcsecofaFIRST

source

Figure2.Distributionincolourandmagnitudeofstarlike(POSSI/APM)opticalidenti cationsofFIRSTradiosources.ObjectswithnoOmagnitudehavebeenplottedwithO E=21.7-E,andtheEmagnitudeshavebeenrandomised±0.1sothatthedensityofpointsinthisregioncanbeseen.MostoftheobjectsinthelowerpartofthediagramaretrueQSOs.Mostofthoseclusteredatupperrightaremisclassi edradiogalaxies(thedashedlineshowstheexpectedcolour-magnituderelationshipforaredshiftedgiantellipticalgalaxy),andmanyoftheotherredobjectsaremisidenti cationswithgalacticMstars.SomemayalsobeBALQSOs,whichtendtobered.Thecandidatehigh-redshiftQSOsobservedherearedrawnfromtheregionenclosedbythedottedline.

isshowninFig.2.Weselectedascandidatez>4QSOsallstarlikeopticalidenti cationswithO E>3(seeFig.1)andE<18.6(thePOSSbluelimitisO≈21.7).Trialspectroscopyshowedthat～80%oftheseareactuallyradiogalaxies,whichgreatlyoutnumberQSOsatE～18,O E～3(Fig.2).Itwaspossibleto lteroutmostofthesebyin-spectingimagesfromthedigitisedPOSS-IIsurvey,usingtheSExtractorprogram(Bertin&Arnouts1996)tocharacterisetherelationshipbetweenFWHMandintensityforstars(ineachimage),andthustodistinguishbetweenstellarandex-tendedimages.Inaddition,we lteredoutthe≈25%ofstarlikeimagesforwhichAPMrecordednobluemagnitude(i.e.impliedmagnitudefainterthantheblueplatelimit),butwhichwereeasilyvisibleontheMinnesotaAPSscansofPOSS-I(Penningtonetal1993);theseimagesweremissedbyAPMduetothecoarserscanningresolutionandgreatersusceptibilitytoconfusionwithnearbyimages.Thisselec-tionprocedureyieldedatotalof109candidatehigh-redshiftQSOs.

3OBSERVATIONSANDREDUCTION

Spectrawereobtainedofarandomsampleof55ofthe109candidateswiththeIsaacNewtonTelescope’sIDSspectro-graphon2001Mar14,andwiththeIsaacNewton(IDS),WilliamHerschel(ISIS)andCalarAlto2.2-m(CAFOS)

c0000RAS,MNRAS000,000–000

In a pilot search for high-redshift radio QSOs, we have obtained spectra of 55 FIRST sources (S(1.4GHz) > 1 mJy) with very red (O-E > 3) starlike optical identifications. 10 of the candidates are QSOs with redshifts 3.6 < z < 4.4 (4 previously known), six

High-redshiftQSOsinFIRST

Table1.Recentsearchesforhigh-redshiftQSOsSearchprogramme(1)

DPOSS-IISDSS

APM/UKST

INTwide-anglesurv.

Colourcriterion(2)

griugriz

BJ R>2.5griz

opticallimit(3)r<19.6i<20R<19.3r<24

SlimmJy(4)––––

Areadeg2(5)340180800012.5

Cands

(6)85

z>(7)4.04.04.04.0

N(8)1018493

σ

deg 2(9)0.0290.100.00610.24

Hiz/cands(10)0.11

Reference(11)

3

Kenne cketal1995a,bFanetal2001

Storrie-L.etal2001Sharpetal2001

CLASS at-spectrumO E>2E<19S5>306400274.040.000630.15Snellenetal2001GB/FIRST at-spec.O E>1.2E<19.5S5>251100504.000.00.00Hooketal1998“O E>1.2E<19.5S5>251100503.060.0050.12“FIRSTO E>3E<18.6S1.4>14030554.060.00150.11Thispaper“O E>3E<18.6S1.4>10403094.040.00100.44“

Column4gives,whereappropriate,theradio ux-densitylimitSν,forfrequencyνinGHz.Column9givesthesurfacedensityofhigh-redshiftQSOsdiscovered.Column10givestheratiobetweenthenumberofhigh-redshiftQSOsfound(column8)andthenumberofcandidatesforspectroscopy(column

6).

centredat4800 A.TheCAFOSspectrumhasdispersion8.5 A/pixel.

Theimagesweredebiasedand at- elded,cosmicrayswereeliminated,andthespectrawereextracted,wavelength-calibratedandintensity-calibratedintheusualway,usingtheIRAFpackage.Asummaryoftheobserva-tionsandresultsisgiveninTable2.

4RESULTS

Figure3.Colour-magnitudedistributionforthecandidatehigh-redshiftQSOs.ThesymbolsforspectraltypeareasusedinTa-ble2(Q=QSO,S=narrow-emission-linegalaxy,G=radiogalaxy, =star,?=probableradiogalaxy).LargefontindicatesS1.4GHz>10mJy;smallfontS1.4GHz<10mJy.AnominalPOSSIblueplatelimitO=21.7isindicatedbythedashedline.ObjectswithnoOmagnitudehavebeenplottedwithO E=21.7-E,andtheEmagnitudeshavebeenrandomised±0.2magtoreducecrowding.Thearrowsindicateupperorlowerlimits.

spectrographsonservicenightsbetween2000March19and2001Jan5(datesgiveninTable2).Apartfrom2000Apr5,May12and2001Jan5,thenightswerephotometric.TheINTIDSspectrographwasusedwiththeR150Vgrating,yieldingspectrawithdispersion6.5 A/pixel,usuallycen- tredat6500A.TheWHTISISdual-armspectrographwasusedwiththeR158RandR158Bgratingsintheredandbluearmsrespectively(withadichroicseparatingthelightblueandredof6100 A).Intheredarm,withaTEKCCDdetector,thisyieldedspectrawithdispersion2.9 A,usuallycentredat7400 A.Inthebluearm,withanEEVCCD,thedispersionwas1.6 A/pixel,andthespectrawereusually

c0000RAS,MNRAS000,000–000

Spectrashowingclearemissionorabsorptionfeatureswere

classi ed‘Q’(QSO,broademissionlines),‘S’(Sy2orstar-bursttypespectrum,narrowemissionlines)or‘ ’(spectralfeaturesofMstar).Theremainderhavebeenclassi ed‘G’(probableradiogalaxy,i.e.giantelliptical),wherethechar-acteristic4000- Abreakisdetected,otherwise‘?’.Mostofthe‘?’objectswillberadiogalaxies,ratherthanmisiden-ti cationswithstars,judgingfromthenumberofMstarsdetected,thetypicaldistributionofspectraltypesatthismagnitude,andthemagnitudedistributionsandthedistri-butionsontheskyofthe‘?’and‘ ’objects.Oneofthe‘?’objects,1036+42,hasa atSEDinconsistentwithitbe-ingaradiogalaxy;itmaybealow-redshiftQSO(z<2.2).Oftheother‘?’objects,nonehasanSEDconsistentwithitbeingahigh-redshiftQSO,exceptperhaps1120+34and1349+38.Theredshiftsofthehigh-redshiftQSOsweremea-suredfromemissionlinesotherthanLyα(becauseoftheasymmetricalabsorption),exceptinthecaseof0839+51.Thecolour-magnitudediagramofthecandidatesisshowninFig.3.Thespectraofthehigh-redshiftQSOsarepre-sentedinFigs.4,5.

We nd10high-redshiftQSOs(z>3.6)outof55candi-dates.Thesecandidateswereselectedfrom109forthewholeFIRSTcatalogue,covering7988deg2,sothee ectiveareacoveredbythissearchis～4030deg2,i.e.we nd0.0025high-redshiftQSOsdeg 2withz>3.6,or0.0015deg 2withz>4.0.ForS1.4GHz>1mJy,E<18.6,thez>4searchwillbenear-complete(Fig.1).Thesurfacedensityontheskyiscomparedwiththatfromothersearchesforhigh-redshiftQSOsinTable1.Thesurfacedensityofz>4radioQSOsdiscoveredhereistwiceashighasthatfromtheSnellenetalsearch,becauseofthelower ux-densitylimit.Thee ciencyofthesearch(numberofhigh-redshiftQSOs/numberofcandidatesforspectroscopy)is0.44forS1.4GHz>

In a pilot search for high-redshift radio QSOs, we have obtained spectra of 55 FIRST sources (S(1.4GHz) > 1 mJy) with very red (O-E > 3) starlike optical identifications. 10 of the candidates are QSOs with redshifts 3.6 < z < 4.4 (4 previously known), six

4C.R.Bennet

al

Figure4.Spectraof8ofthe10high-redshiftQSOs(seeFig5fortheremainder).Spectralfeaturescorrespondingtothequotedredshift,assumingrest-framewavelengthsin Aof1216(Lyα),1240(NV),(SiIV/OIV]blend),1549(CIV).For0941+51,themetal-lineabsorptionsystemsatz=3.80,3.83arealso

A(AandBnotbeencorrectedfortheterrestrialatmosphericabsorptionbands,notablyat7594and6867 arelabelledatwavelengths

1302(OI/SiIIblend),1400indicated.Thespectrahavebands).

c0000RAS,MNRAS000,000–000

In a pilot search for high-redshift radio QSOs, we have obtained spectra of 55 FIRST sources (S(1.4GHz) > 1 mJy) with very red (O-E > 3) starlike optical identifications. 10 of the candidates are QSOs with redshifts 3.6 < z < 4.4 (4 previously known), six

High-redshiftQSOsinFIRST

Table2.High-redshiftQSOcandidatesRADecJ2000J2000(1)(2)

High-redshiftQSOs:

S1.4mJy(3)

R-O′′(4)

E(5)18.317.215.318.517.616.517.918.118.518.417.118.618.218.618.418.618.618.618.018.118.518.018.518.518.217.018.017.118.418.418.418.6

O E

(6)>3.4>4.53.9>3.2>4.13.93.3>3.6>3.2>3.33.9>3.13.1>3.13.3>3.1>3.13.03.2>3.6>3.23.8>3.2>3.2>3.53.03.73.53.0>3.3>3.4>3.1

Tel(7)WIIIWIIWIIWIIIIIIIIIIIIIIIIIIIII

Dateobs.(8)010105010314010314010314000319000405010314000319010314010314000718010314010314010314010314010314010314010314010314010314010314010314010314010314010314010314010314000405010314010314010314010314

Tsec(9)90020020030012003001200300120030010003003001003002003003003003001300300300300300300300300300300300300

ID(10)QQQQQQQQQQQG??S?G??G?S?G??S???GG

z(11)4.334.173.874.423.854.113.6944.2744.3162.5903.99

σz(12).01.02.03.02.01.02.005.005.005.005.015

Notes(13)

5

072518.29370517.926.60.6074711.16273903.61.50.8083141.68524517.10.90.7083946.11511203.041.60.8094119.36511932.32.71.3105756.27455553.01.40.5122027.93261903.635.00.4130940.61573309.111.51.1142308.19224157.535.40.9151601.55430931.31.50.9163950.51434003.323.80.5Low-redshiftQSOsandgalaxies:091441.31093255.83101058.24103549.91103647.80104622.91104801.61104958.45105719.80105750.00111411.21111557.91111707.05112028.10115230.18121532.16124450.93130604.97134951.93140816.68143749.26Mstars:

295621.4353439.1283247.0300732.1420852.7345436.0260032.3411043.4342807.0353300.4293244.4352757.1411736.1345829.4271808.4250956.6353906.7352603.9382334.9352205.8325917.8

4.81.92.39.02.21.011.21.35.42.31.13.40.76.54.21.01.51.11.79.61.3

0.30.90.40.30.40.70.50.50.00.71.01.40.80.80.40.70.20.90.41.10.7

Irwinetal(1998),z=3.91Snellenetal(2000),z=4.41Sternetal(2000),z=4.12

0.587.002

3727,5007?blueSED

0.279.00137274861495950076563

possiblehigh-redshiftQSO

0.282

.001

3727486149595007

possiblehigh-redshiftQSO

072359.95523949.40.80.817.6>4.1I010314300*072801.49380344.21.60.717.33.7I010314300*074144.39333549.3136.80.618.4>3.3W010104400*075127.64272736.41.21.218.0>3.7W010104400*084407.26280740.46.20.517.53.0W0101051200*085700.54275540.00.80.818.5>3.2W010105400*090556.33224419.12.41.517.5>4.2I010314200*091953.76340906.71.20.817.13.0I010314200*100125.11370629.41.10.816.83.2I010314200*105102.41314914.59.00.417.8>3.9I010314300*111212.69431015.51.40.917.23.7C000512900*112518.16495158.83.01.117.83.4I010314300*113043.07514134.14.20.517.73.1I010314300*120849.72392907.10.81.215.33.2I010314300*132903.23323031.55.81.418.13.4I010314300*133833.44531642.10.90.717.93.3I010314300*135357.42343659.798.01.217.83.9I010314300*150859.95271431.01.00.417.93.4I010314300*150938.97434649.81.90.818.4>3.4I010314300*161150.75434412.41.01.017.83.4I010314300*163535.34352415.95.20.618.13.4W000718300*164217.03402230.86.40.217.43.1W000718300*164401.60420047.44.21.217.63.5W000718300*

Thecolumnsgive:(1-2)rightascensionanddeclination,(3)FIRST1.4-GHz(integrated,sometimes<peak) uxdensity,(4)radio-opticaldisplacement,(5-6)APMPOSSIEmagnitudeandO Ecolour,(7)telescopewithwhichspectrumobtained(W=WilliamHerschel4.2-m,I=IsaacNewton2.5-m,C=CalarAlto2.2-m),(8)dateofobservation,(9)exposuretime,(10)spectraltype(Section4),(11-12)redshiftandrmserroronredshift.Forlow-redshiftemission-lineobjects,therest-framewavelengthsofdetectedlinesaregivenin Ainthelastcolumn:3727OII,4861Hβ,4959/5007OIII,6563Hα.Forhigh-redshiftQSOs,seeFigs.4,5.

c0000RAS,MNRAS000,000–000

In a pilot search for high-redshift radio QSOs, we have obtained spectra of 55 FIRST sources (S(1.4GHz) > 1 mJy) with very red (O-E > 3) starlike optical identifications. 10 of the candidates are QSOs with redshifts 3.6 < z < 4.4 (4 previously known), six

6C.R.Bennet

al

Figure5.Spectraofthetworemaininghigh-redshiftQSOs(seecaptionofFig.4);andof4otheremission-lineobjectsdetectedinthissearch(seetext).1516+43wasundetectedonthePOSSblueplate,butthe eldisopticallyconfusedanditstruecolourmaybebluerthanO E=3.

10mJy,higherthanforprevioussearches,probablybecauseofthecareful lteringofthecandidates.

Fourofthe10high-redshiftQSOswereprevi-ouslyknown:0747+27(M.Irwin,privatecommunication),0831+52(Irwinetal1998),0839+51(Snellenetal2001)and1057+45(Sternetal2000).0831+52isunusuallybright(E=15)andtheLyαforestbeenstudiedindetail(Ellisonetal1999).327-MHz uxdensitiesareavailableforthreeofthehigh-redshiftQSOsfromtheWENSSsurvey(Rengelinketal1997).The0.3-1.4-GHzspectralindicesα(Sν∝να)areall at:-0.1for0725+37,0.0for0839+51,and-0.4for

1639+43.Theother7QSOsaretoofainttobedetectedinWENSS,orliebelowtheWENSSdeclinationlimit.TheSEDsofthehigh-redshiftradioQSOsreportedhereareasvariedasthosefoundinopticalsearches.Atleast0941+51and0831+52showmetal-lineabsorptionredwardofLyα,probablyassociatedwithsaturatedLyαlinesintheforest.One,1309+57,showsverynarrowLyαemission,FWHM20 A.1639+43hasalmostnoLyαemission,whichisunusual(only～1/60ofSDSSQSOs),anditissimilarinthisrespecttothez=4.2radioQSO,0918+06,discoveredbySnellenetal.SnellenetalsuggestedthattheunusuallystrongabsorptionoftheLyαlinemightbeduetothehost

c0000RAS,MNRAS000,000–000

In a pilot search for high-redshift radio QSOs, we have obtained spectra of 55 FIRST sources (S(1.4GHz) > 1 mJy) with very red (O-E > 3) starlike optical identifications. 10 of the candidates are QSOs with redshifts 3.6 < z < 4.4 (4 previously known), six

galaxy(inbothcasesthisisprobablyagiantelliptical).AfewoftheQSOshaveprobabledampedLyαsystems(DLAs,seenin～20%ofhigh-redshiftQSOs).0941+51hasaDLAatz=3.52,andthishasnowbeenobservedathighspectralresolutionwithWHTISIS(Centurionetal,inpreparation).

Theother45candidatesareamixedbag(seeMaglioc-chettietal2000forthetypicaldistributionofidenti cationtypesofallcoloursat1mJy).1516+43isaz=2.59broad-absorption-line(BAL)QSOwithbroad,deepSiIVandCIVabsorptiontroughsbluewardoftheemissionfeatures(Fig.5).1035+30(probably),1114+29and1215+25arelow-redshiftgalaxiesexhibitingnarrowemissionlines(Fig.5).18haveSEDsconsistentwiththoseofgiantellipticalgalax-iesatz～0.3.23havespectraoflate-typeMstars.Thisnumberisconsistentwiththatexpectedforchancecoinci-dences,butafewmaybetrueradiostars(Helfandetal1999describeasearchforcounterpartsofbrightstarsinFIRST).

5CONCLUSIONS

Wehaveconductedapilotsearchforhigh-redshiftradioQSOs,obtainingopticalspectraof55FIRSTradiosources(S1.4GHz>1mJy)withred(O-E>3)starlikeopticaliden-ti cations.10ofthecandidatesareQSOswithredshifts3.6<z<4.4(4previouslyknown).Sixhavez>4.There-maining45candidatesareamixtureoflow-redshiftgalaxies(misclassi edasstellar)andMstars(mainlymisidenti ca-tions).Thesuccessrate(high-redshiftQSOs/candidates)forthissearchis1/2forS1.4GHz>10mJy,and1/9forS1.4GHz>1mJy.Withane ectivesearchareaof4030deg2,thesurfacedensityofz>4QSOsdiscoveredwiththistechniqueis0.0015deg 2.

Acknowledgments

WearegratefultoAlessandroCaccianiga(ObservatoriodeLisboa)fortakingspectrawiththeCalarAlto2.2-mtelescope.RachelCurranandJoannaHoltwere1-yearplacementstudentsatINGin1999-2000and2000-2001respectively.KHMwassupportedbytheEuropeanCom-mission,TMRProgramme,ResearchNetworkContractERBFMRXCT96-0034“CERES”.TheWilliamHerschelandIsaacNewtonTelescopesareoperatedontheislandofLaPalmabytheRoyalGreenwichObservatoryintheSpan-ishObservatoriodelRoquedelosMuchachosoftheInstitutodeAstro sicadeCanarias.TheCalarAltoobservatoryisop-eratedbytheMax-Planck-InstituteforAstronomy,Heidel-berg,jointlywiththeSpanishNationalCommissionforAs-tronomy.TheAPSCatalogofPOSSI(http://aps.umn.edu)issupportedbyNASAandtheUniversityofMinnesota.TheNASA/IPACExtragalacticDatabase(NED)isoperatedbytheJetPropulsionLaboratory,CaliforniaInstituteofTech-nology,undercontractwithNASA.

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