III. Candidate Hα emitters and EROs in the PKS 1138–262 proto-cluster at z = 2.16

23 Pages·2008·0.82 MB·English

A&A428,793–815(2004) Astronomy
DOI:10.1051/0004-6361:20040075 &
(cid:1)c ESO2004 Astrophysics
A search for clusters at high redshift
α
III. Candidate H emitters and EROs in the PKS 1138–262
proto-cluster at z=2.16
J.D.Kurk1,L.Pentericci2,H.J.A.Röttgering1,andG.K.Miley1
1 SterrewachtLeiden,POBox9513,2300RALeiden,TheNetherlands
e-mail:[email protected]
2 Max-Planck-InstitutfürAstronomie,Königstuhl17,69117Heidelberg,Germany
Received21January2003/Accepted19February2004
Abstract. In this paper we present deep VLT multi wavelength imaging observations of the field around the radio galaxy
PKS1138−262aimedatdetectingandstudyinga(potential)proto-clustercenteredatthisradiosource.PKS1138−262 isa
massive galaxy at z = 2.16, located in a dense environment as indicated by optical, X-ray and radio observations. We had
alreadyfoundanover-densityofLyαemittinggalaxiesinthisfield,consistentwithaproto-cluster structureassociatedwith
theradiogalaxy. Inaddition, wefind40candidateHαemittersthathavenominalrestframeequivalent width>25Åwithin
1.8Mpcand2000kms−1oftheradiogalaxy.Furthermore,wefind44objectswithI−K >4.3.Thisnumberofextremelyred
objects(EROs)isabouttwicethenumberfoundinblankfieldEROsurveys,suggestingthatsomeEROsinthisfieldarepartof
theproto-cluster.ThedensityofHαemittersandextremelyredobjectsincreasestowardstheradiogalaxy,indicatingaphysical
association.FromcomparisonswithotherKband,ERO,HαandLyαsurveys,weconcludethatPKS1138−262islocatedina
densitypeakwhichwillevolveintoaclusterofgalaxies.
Keywords.galaxies:active–galaxies:clusters:general–galaxies:evolution–cosmology:observations–
cosmology:earlyUniverse
1. Introduction essentiallydevoidofthese. Thiseffectcanbe explainedif we
assume a process in which spirals lose their gas and ability
Thesearchforclustersathighredshifthastwomainincentives:
to form stars or are converted into early type galaxies in the
distant clusterscan be used to constraincosmologicalmodels
course of their evolution. Massive ellipticals, however, domi-
and they provide a reservoir of high redshift galaxies, which
natingclustercoresuptoz ∼ 1seem toformahomogeneous
canbeusedtostudygalaxyformationandevolution.
population.Evenatz=1.27,vanDokkumetal.(2001)observe
Accordingtohierarchicalclusteringtheories,clustersform
a scatter of the colour–magnitude relation of cluster galaxies
by the gravitational amplification of primordial density fluc-
similartothatinlowerredshiftclusters.
tuations.In a low densityuniverse,fluctuationscease to grow
after a redshift z ∼ (Ω−1 − 1) (Peebles 1980), resulting in a The tight colour–magnituderelation observed out to high
0
cluster populationthat evolvesveryslowly at low redshift. In redshifts can be explained by several galaxy formation sce-
anΩ = 1universedensityfluctuationscontinuetogroweven narios. De Propris et al. (1999) propose a passive luminosity
0
atthepresentepoch,implyingthattheclusterpopulationwould evolution model, where galaxies form all their stars in a sin-
still be evolving rapidly (Eke et al. 1996). The detection of gleburstatz=2afteramonolithiccollapse(e.g.,Eggenetal.
even a single distant massive cluster, such as MS 1054−03at 1962).VanDokkum&Franx(2001)showthattheobservations
z = 0.83, constrains the parameters (Ω ,σ ) of cosmological arealsoconsistentwithascenarioinwhichearly-typegalaxies
0 8
models(Bahcall&Fan1998;Donahueetal.1998). arecontinuouslytransformedfromspiralgalaxies,causingthe
The study of galaxies in nearby and distant clusters pro- progenitors of the youngest, low redshift early-type galaxies
videsstrongconstraintsontheirevolutionandformation.Ithas todropoutofthesampleathigherredshifts(progenitorbias).
been shown that clusters at high redshift can contain a differ- ModelsbyvanDokkum&Franx(2001)showthatabouthalf
ent galaxypopulationmix than those at low redshift.Butcher oftheearly-typegalaxiesweremorphologicallytransformedat
& Oemler (1984) found that at 0.1 < z < 0.5, compact clus- z<1andtheirprogenitorsmayhavehadroughlyconstantstar
ters have significant numbers of blue galaxies, the fraction formation rates before transformation. These models indicate
increasing with redshift, while at z < 0.1 cluster cores are a mean luminosity weighted formation of stars in early-type
Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20040075
794 J.D.Kurketal.:Asearchforclustersathighredshift.III.
galaxies of z = 2.0 (for ΩM = 0.3 and ΩΛ = 0.7), consis- There has long been evidence that powerful radio galax-
tentwiththe currentlyfavouredhierarchicalgalaxyformation ies at high redshift (HzRGs, z > 2) are located in the center
modelspredictingthemergingofsmallergalaxiesathighred- of (forming) clusters of galaxies. Yates et al. (1989) and Hill
shift(Kauffmann1996).Athirdexplanationisgivenbymetal- & Lilly (1991) find that the average environmentof 70 pow-
licity differences in bright and faint ellipticals. A model by erful classical double radio sources at 0.15 < z < 0.82 is
Kauffmann& Charlot(1998)whichincludeshierarchicalfor- that of an Abell 0 cluster, with some in environmentsas rich
mation of ellipticals out of disc galaxies which have formed as Abell class 1. At z ∼ 1 and higher, there is also evidence
stars at modestrates andallows forthe ejection of metalsout for galaxy over-densities associated with radio galaxies. Best
of discs by supernova explosions predicts the establishment (2000)presentsananalysisoftheenvironmentsof283CRra-
ofamass-metallicityrelationamongbothlate-andearly-type diogalaxiesat0.6<z<1.8.ThedensityofK-bandgalaxiesin
galaxies.Inthismodel,largeellipticalsaremoremetal-richbe- these field, their angular cross correlationamplitude and near
causetheyareformedfromthemergersoflargerdiscs. infrared colours correspond to the properties of low redshift
The strongest observational constraints on these models Abellrichnessclass0to1clusters.Theauthorconcludesthat
comefromthehighestredshiftdata.Bothforthestudyofclus- many,butnotall,powerfulradiogalaxiesatz∼1lieincluster
terandgalaxyevolutionasampleofclustersathighredshiftis environments.Furthermore,Nakataet al. (2001)haveapplied
thereforedesired. aphotometricredshifttechniquebasedonfiveopticalandnear
Inrecentyears,muchefforthasbeeninvestedinthesearch infraredimagesofthefieldof3C324atz = 1.2andidentified
fordistantclusters,usingbothopticalandX-rayobservations. 35objectsasplausibleclustermembers.Theevidenceextends
At z > 0.5, it becomesdifficultto identify the projected two- eventoz=3.8,whereobservationsinthefieldofradiogalaxy
dimensional over-density produced by cluster galaxies, be- 4C41.17byIvisonetal.(2000)tentativelyrevealanumberof
cause large numbers of foreground and background galaxies luminoussubmmgalaxiesover-densebyanorderofmagnitude
reduce the density contrast in the optical wavelength regime. ascomparedtotypicalfields.
However,theJ−K colourofnearlyallgalaxiesouttoz=2is
ThereisalsoevidencethattheenvironmentsofHzRGsare
a simplefunctionof redshift,becausetheir nearinfraredlight
dense in terms of ambient gas, as expected for the centers of
isdominatedbyevolvedgiantstars.Thehighestredshiftclus- (forming)clusters.Radiocontinuumobservationsof∼70radio
ter found to date (Stanford et al. 1997, CIG J0848+4453 at galaxies at z ∼ 2 (Carilli et al. 1997; Pentericci et al. 2000b)
z = 1.27) has been discovered in a near infrared field survey show that 20–30% have large (≥1000 radm−2) radio rotation
asahighdensityregionofobjectswithveryredJ−Kcolours.
measures(RMs).TheseRMsaremostprobablyduetomagne-
Opticalspectroscopyconfirmedtheredshiftsofeightmembers
tized,ionizedgaslocaltotheradiosourcesandarecomparable
and a 4.5σROSAT X-raydetectionconfirmsthe cluster’sex-
totheRMsoflowerredshiftradiogalaxieswhichlieinthecen-
istence.AlthoughthedetectabilityofhotclustergasinX-rays
tersofdense,X-rayemittingclusteratmospheres(Tayloretal.
is severely reduced by cosmological surface brightness dim-
1994).
ming, Rosati et al. (1998)has foundwithin the ROSAT Deep
Cluster Survey a cluster at z = 1.11 (Stanford et al. 2002, Although convincing evidence for high density environ-
RDCS J0910+5422)with 9 spectroscopicallyconfirmedclus- mentsassociatedwithradiogalaxiesatz> 1hasbeendemon-
ter members and a cluster at z = 1.26 (Rosati et al. 1999, strated,theredshiftsofpossibleclustermembershavenotbeen
RX J0848.9+4452) with 6 cluster members confirmed. Most confirmedbyspectroscopyanditisthereforeimpossibletopro-
vide a velocity dispersion for these structures. In a program
of the confirmed galaxies have red colours, consistent with
passively evolved ellipticals formed at high redshift (z ∼ 5). whichiscurrentlybeingcarriedoutwiththe VLT,wearetar-
ThelatterclusterisveryclosetoCIGJ0848+4453,withwhich geting the fields of luminous radio galaxies at z > 2 and ob-
servethesewith theaim ofdetectinglineemittinggalaxiesin
it might form a superstructure and is possibly in the process
theassociatedcluster.Atz > 2theLyαlineisredshiftedinto
of merging.The recently started XMM Large Scale Structure
theopticalwavelengthregion,wherewe canusenarrowband
Survey(Refregieretal.2002),coveringover64squaredegrees
filters (∼1%) to isolate its flux from the sky background.We
of sky, should be able to detect clusters with X-ray luminosi-
tiesof2×1044 ergs−1 outtoz = 2.Despitethesuccessofthe haveselectedtenluminousradiogalaxiesat2.2< z < 5.2,for
nearinfraredandX-raytechniques,itisdifficulttopushthese which we will carry out both imaging and multi object spec-
methodstofindz(cid:3)1over-densities. troscopy.Thesurveyisprogressingverywellandhasalready
producedthediscoveryofthemostdistantstructureofgalaxies
A practical way to find clusters and groups of galax-
known(atz=4.1,Venemansetal.2002).
ies at high redshift is to study fields containing luminous
radio galaxies. These can be observed up to the epoch of We consider the Lyα imaging and spectroscopy as a first
galaxy formation and efficiently selected by their steep spec- step to the characterization of the cluster properties. The
trum in the radio regime (Röttgering et al. 1994; Lacy Lyα emitters in a cluster form only a fraction of the galaxies
et al. 1994; De Breuck et al. 2000). The most distant ra- presentandmightnotberepresentativeforsomeofthecluster
dio galaxy found to date has a redshift of 5.2 (van Breugel or galaxy properties. The program will therefore be followed
et al. 1999). The host galaxies of powerful radio sources are upbye.g.broadbandimaginginseveralcolours.Inthispaper,
amongst the most massive at any redshift (Jarvis et al. 2001; we present new optical and near infrared observations of the
De Breucketal.2002)andareassociatedwith∼109 M(cid:4) BHs fieldofPKS1138−262withtheaimofuncoveringpopulations
(Lacyetal.2001;McLure&Dunlop2002). ofHαemittinggalaxiesandEROsinthestructure.
J.D.Kurketal.:Asearchforclustersathighredshift.III. 795
TheradiogalaxyPKS1138−262ataredshiftof2.16,was redsequence,butwilllaterformclusters,willbecalledproto-
selected from a compendium of more than 150 z > 2 radio clustershere.
galaxies as the optimum object for beginning a high redshift
clustersearch.Itcombinesmostoftheabovementionedcluster
indicationswitharedshiftsuitableforbothLyαandHαimag- 2. Observationsanddatareduction
ing.Themagnitudeof1138−262isthebrightestofallknown
2.1.Opticalobservations
radiogalaxiesclosetoz = 2.Aftercorrectionforpossiblenon
stellarcomponents,theK bandmagnitudeis16.8,fromwhich TheobservationsofPKS1138−262werecarriedoutwiththe
astellarmassof1012 M(cid:4) wasinferred(Pentericcietal.1997). VLT1. With the aim of detecting Lyα emitting galaxies at
The radio galaxy possesses a giant (∼120 kpc) and luminous z = 2.16, we have observed the field of 1138−262 for half
Lyαnebula,withawealthofstructure:abrightregionassoci- anhourin Bbandandfourhoursina 2%narrowband,using
ated with the radio jet and filaments extending over >40 kpc FORS1 at Antu (UT1).Subsequentmultiobjectspectroscopy
(Pentericcietal.1997;Kurketal.2000b).Theopticalcounter- of candidate emitters was also carried out with FORS1, em-
partoftheradiogalaxyisextremelyclumpyandresolvedinto ployingthreemaskswithintegrationtimesof4to6h.Theop-
many componentsby the HST (Pentericci et al. 1998).These tical imaging and spectroscopy observations are described in
clumps have properties similar to LBGs. The morphology of detailin Paper I and II. For an overviewof both old and new
thesystemisconsistentwithhierarchicalmodelsofgalaxyfor- observations,seeTable1.
mation in which the LBG building blocks will merge into a We have complemented the original optical imaging
single massive system, such as the massive galaxiesobserved with broad band observations in R and I, using FORS2 at
at the centers of some rich clusters. The extremely distorted Kueyen(UT2)in2001.ThedetectorofFORS2wasaTektronix
radio morphology(Carilli et al. 1997) is strong evidence that thinnedandanti-reflectioncoatedCCDwith2048×2048pix-
the jets have been deflected from their originaldirection by a els and a scale of 0(cid:6).(cid:6)2 per pixel in standard resolution mode,
denseandclumpymedium.Theobservedrotationmeasuresof yielding a field size of ∼6.(cid:6)8×6.(cid:6)8. Six exposures of 5 min
the radio emission (6200 radm−1, the largest in a sample of during non-photometric conditions were taken through the
70HzRGs,seeCarillietal.1997;Pentericcietal.2000b)and R_Specialfilter,whichhasacentralwavelengthof6550Åand
its steep gradient over the radio galaxy components also tes- FWHM of 1650 Å. The R_Special filter has higher transmis-
tify that the radio source is embedded in a dense magnetized sionthanthestandardBesselRfilteranditstransmissioncurve
medium.Additionalevidenceforadensesurroundingmedium isalmostsymmetricalaroundthecentralwavelength,whilethe
comesfromChandraX-rayobservations,whichrevealthermal Bessel filter has its peak at 6000 Å and declines towards the
emission from shocked gas (Carilli et al. 2002).The pressure red. Six non-photometricexposuresof 7.5 min were taken in
ofthishotgasisadequatetoconfinetheradiosource. service mode through the Bessel I filter, which has a central
wavelength of 7680 Å and FWHM of 1380 Å. During visitor
Narrow band imaging of redshifted Lyα emission of a
time,threeweekslater,anadditionaleighteenphotometricex-
7(cid:6)×7(cid:6) region around the radio galaxy (Kurk et al. 2000a,
posuresof5minwereobserved.Theobservationsweremade
Paper I) and subsequent Lyα spectroscopy (Pentericci et al. employingajitteringpatternwithoffsets<20(cid:6)(cid:6) betweenexpo-
2000a, Paper II) revealed 14 Lyα emitting galaxies and one
surestominimizeflatfieldingproblemsandtofacilitatecosmic
QSO.Thegalaxieshaveredshiftsintherange2.16±0.02with ray removal. The seeing on the resultant images and the 3σ
a velocitydispersionsubstantiallysmaller thanexpectedfora limitingmagnitudeina1(cid:6)(cid:6)apertureasmeasuredonthecentral
random sample of galaxies selected by the narrow band fil-
squarearcminuteofthecombinedimagesislistedinTable2.
ter. In addition, the Chandra X-ray observations of the field
Image reduction was carried out using the IRAF2 reduc-
of 1138−262 have revealed at least five AGN at z ∼ 2.16
tionpackage.Theindividualframeswere biassubtracted,flat
(Pentericci et al. 2002). On the basis of the evidence from
fielded with twilight flats and cosmic rays were removed.
theradiogalaxyproperties,theLyαhaloandthegalaxyover-
The frames were combined using the DIMSUM3 package.
density,weconcludedthatthestructureofgalaxiessurrounding
DIMSUM builds a cumulativesky frame from 6 to 10 subse-
PKS1138−262is(theprogenitorof)acluster.
quentunregisteredimages.Objectsintheunregisteredframes
Thenewobservationsof1138−262arereportedinSect.2. were detected with SExtractor (Bertin & Arnouts 1996) and
Detectionandphotometryofobjectsinthe fieldof1138−262 masked during the process of background determination.
arepresentedinSect.3.Subsequently,theselectionfromthese The obtained sky frames were subtracted from the images.
objectsof K bandgalaxies,EROs, candidateHαemittersand
candidateLyα emittersis presentedin Sect. 4. The properties 1 Based on observations carried out at the European Sou-
thern Observatory, Paranal, Chile, programmes P63.O-0477(A&D),
oftheEROsandcandidatesareanalyzedinSect.5.Adiscus-
P65.O-0324(B),andP66.A-0597(B&D).
sion of the implications of these results for the nature of the
2 IRAF is distributed by the National Optical Astronomy
structurecanbefoundinSect.6,whichisfollowedbyasum-
Observatories,whichareoperatedbytheAssociationofUniversities
maryoftheresultsandconclusionsinSect.7.Throughoutthis
for Research in Astronomy, Inc., under cooperative agreement with
article,weadopta Hubbleconstantof H = 65kms−1Mpc−1
0 theNationalScienceFoundation.
andaΛ dominatedcosmology:ΩM = 0.3andΩΛ = 0.7.The 3 DIMSUMisasetofscriptstoreduceditheredimagescontributed
over-densitiesofgalaxiesathighredshift,which havenotyet to IRAF and written by P. Eisenhardt, M. Dickinson, A. Stanford,
reachedvirializationand/oracolour-magnituderelationwitha J.WardandF.Valdes.
796 J.D.Kurketal.:Asearchforclustersathighredshift.III.
Table1.Observationsandfilterproperties.
Date M Tel./Instr. Filter λc λfwhm Exp. Time P
(1) (2) (3) (4) (5) (6) (7) (8) (9)
12–4–1999 V UT1/FORS1 BBessel 429 88 300 1800 1
12–4–1999 V UT1/FORS1 NB0.38 381.4 6.5 1800 9000 1
13–4–1999 V UT1/FORS1 NB0.38 381.4 6.5 1800 5400 1
6–3–2001 S UT2/FORS2 RSpecial 655 165 300 1800 1
5–3–2001 S UT2/FORS2 IBessel 768 138 450 2700 1
27–3–2001 V UT2/FORS2 IBessel 768 138 300 5400 1
6–1–2001 S UT1/ISAAC J 1240 160 45/5 3600 1
s
17–4–2000 S UT1/ISAAC H 1650 300 13/8 2496 1
3–4–1999 S UT1/ISAAC K 2160 270 10/10 2900 1
s
4–4–1999 S UT1/ISAAC K 2160 270 10/10 1900 1
s
6–1–2001 S UT1/ISAAC K 2160 270 13/8 2496 1
s
16–4–2000 S UT1/ISAAC NB2.07 2070 26 60/5 5700 1
17–4–2000 S UT1/ISAAC NB2.07 2070 26 60/5 6300 1
19–4–2000 S UT1/ISAAC NB2.07 2070 26 60/5 5400 1
10–1–2001 S UT1/ISAAC K 2160 270 13/8 4576 2
s
11–1–2001 S UT1/ISAAC NB2.07 2070 26 75/5 9000 2
7–2–2001 S UT1/ISAAC NB2.07 2070 26 75/5 6000 2
21–2–2001 S UT1/ISAAC NB2.07 2070 26 75/5 2250 2
Notes:(1)Date:Day–Month–Year; (2)Visitor(V)orService(S)mode;(3)telescopeandinstrument;(4)ESOfiltername; (5)filtercentral
wavelengthinnm;(6)filterfullwidthathalfmaximuminnm;(7)exposuretimeforsingleframesinseconds,forIRobservationsDIT/NDIT
whereDITisintegrationtimeforsub-integrationandNDITnumberofsub-integrations;(8)totalexposuretimeinseconds;(9)pointingonF1
orF2(seeSect.2.2).
Table2.Resultantimages. non-photometric conditions was scaled to the photometric
I band data and the R band data was calibrated using an
Band T Size M Seeing olderRbandimageof1138−262fromPentericcietal.(1997).
lim
(1) (2) (3) (4) (5) Astrometriccalibrationwascarriedoutbyidentifying18stars
NB0.38 4.0 46.6 26.5 0.75(cid:6)(cid:6) intheUSNO-A2.0catalogue(Monetetal.1998),whichistied
B 0.5 46.6 27.5 0.70(cid:6)(cid:6) totheTychocatalogue(Hoegetal.1997).Theabsoluteastro-
R 0.5 46.6 26.4 0.85(cid:6)(cid:6) metricaccuracyobtainedinthiswayis∼0(cid:6).(cid:6)2.
I 2.0 46.6 26.8 0.65(cid:6)(cid:6)
Note that the narrow and broad band images obtained
J 1.0 7.5 24.8 0.45(cid:6)(cid:6)
Hs 0.7 7.5 23.8 0.70(cid:6)(cid:6) in 1999 (Paper I) were reduced again. This time we used
K 1.6 12.5 23.0 0.45(cid:6)(cid:6) DIMSUM and obtained a homogeneously reduced set of im-
s
NB2.07 4.8 12.5 22.8 0.50(cid:6)(cid:6) ages in all observed optical and infrared bands. To overcome
differencesin geometricaldistortion,allimageswere mapped
Notes:(1)Broadbandornarrowband(seeTable1forspecifications);
tomatchtheRbandimage.Inthisway,wehaveobtainedafirst
(2)totalexposuretime(hours);(3)fieldsizeinsquarearcminute;(4)
3σ limiting Vega magnitude in 1(cid:6)(cid:6) aperture as measured on central set of images with their original spatial resolution. A second
squarearcminuteofimage;(5)seeingonresultantimage. setwasmadein whichthe I, Bandnarrowband([O]/8000,
calledNB0.38fromnowon)imageswereconvolvedwiththe
kernel required to match their point spread functions to the
Theimageoffsetsweredeterminedbymeasuringthepositions R band image, which had the worst seeing conditions during
of a number(∼20)ofstars oneachframe.Pixelsonthe CCD observations.The pixel-to-pixelalignmentin the finalimages
which were significantly discrepant in each sky frame were isaccuratetowithinapixel(0(cid:6).(cid:6)2)overtheentireimage.
markedasbadpixelsandtheexposuretimeforeachpixelwas
computedbyDIMSUM. Thisexposuremap is later used asa
2.2.Infraredobservations
weightmapforobjectdetectionandphotometry.Inthelaststep
oftheprocess,allindividualbroadbandframeswerecombined In 2000 and 2001 Antu’s infrared camera ISAAC was em-
byaveragingwhileidentifiedcosmicraysandbadpixelswere ployed to carry out imaging of the field of 1138−262 in sev-
omitted.Theregisterednarrowbandimageswerecombinedby eralnearinfraredbroadbandsandanarrowbandthatincluded
computingtheaverageofeachpixelstackandrejectingpixels theredshiftedHαemissionline(Hefilter,NB2.07fromnow
whoseintensitylevelswere10σaboveorbelowthenoiselevel on).TheshortwavelengthcameraofISAACisequippedwith
expectedfromtheCCDgainandreadoutnoisespecifications. a Rockwell Hawaii 10242 pixel Hg:Cd:Te array which has a
For the flux calibration of the photometric data the stan- pixel scale of 0.147(cid:6)(cid:6), yielding a field of ∼2.(cid:6)5×2.(cid:6)5. This is
dard stars GD108 (Oke 1990) and LTT4816 (Hamuy et al. significantly smaller than the field covered by the optical ob-
1992, 1994) were used. The I band data obtained during servations,sowecarriedouttwopointingsintheinfrared:one
J.D.Kurketal.:Asearchforclustersathighredshift.III. 797
centered at the position of the radio galaxy as the optical ob- additionalimagesweightedtogiveahomogeneousnoiselevel.
servations(α,δ = 11h40m48s, –26◦29(cid:6)10(cid:6)(cid:6), hereafterF1) These were created by multiplication of the reduced images
J2000
andonetotheNorthEast(α,δ =11h40m57s,–26◦28(cid:6)48(cid:6)(cid:6), by their associated exposure time maps. Only for the detec-
J2000
hereafterF2)coveringsixconfirmedLyαemitters(PaperII). tion of Lyα emitters, a homogeneous noise level image con-
All infrared images were taken in jitter mode, where the volved to the R band seeing was used, where the detection
telescope isoffsetrandomlybetweenexposuresbutneverfar- sensitivity for slightly extended objects (0(cid:6).(cid:6)85) is highest. A
ther from the original pointing than 20(cid:6)(cid:6). In J , H and K in- disadvantageoftheuseofconvolvedimagesforobjectdetec-
s s
dividual frames were exposed for 100 to 225 s using sub- tion is that spurious sources (e.g. remaining cosmic rays) be-
integrationsoflength10to45stoavoidover-exposureofthe comeindistinguishablefromrealsources.Threeregionsinthe
background. The narrow band frames were exposed for 300 Lyα detection image that were badly affected by bright stars
or 375 s with sub-integrations of 60 to 75 s respectively. wereblanked(foratotalof7.75arcmin2).Thesourceextrac-
Specifications (date, mode, band, integration time and point- tionparametersweresetsothat,detectedobjectsmusthaveat
ing) of all observations are presented in Table 1. Note that least 8 connected pixels with flux in excess of 1.5 times the
only F1 was observed in J and H band. Observations in K backgroundnoise level of the detection image, exceptfor the
s s
ofF1weretakeninESOperiod63(P63,1999)andperiod66 Lyαdetectionimage,whereasourcehastohave14connected
(P66,2001).ThesensitivityinP66hadincreasedby45%com- pixels.Toensurethatthecoloursarecomputedcorrectly,object
pared to P63, amongst others due to an aluminization of the photometrywasdoneontheconvolvedimages,byemploying
main mirror. We have scaled the measurements done in P63 SExtractor’s double image mode using the apertures defined
to P66, effectively reducingthe formalexposure time in P63. ontheweightedimages.Aweightmapcreatedfromthesquare
ThetotalexposuretimeinK forF1intermsofP66timeunits rootoftheexposuretimemapwasusedtoestimatetheerrors
s
is1.6h. inthephotometry.
Theinfraredobservationswerereducedinthesamewayas Kron (1980)and Infante (1987) have shown that for stars
theopticalones.However,theatmosphericemissioninthenear andgalaxyprofilesconvolvedwithGaussianseeing,>94%of
infraredisvariableonatimescalecomparablewiththeexpo- the flux is inside the appropriately scaled Kron aperture. We
suretimeofindividualframes,causingfringingresidualsinthe havethereforeusedSExtractor’sMAG_AUTOimplementation
framesafterbackgroundsubtractionusingthemedianofsixto ofKron’sfirstmomentalgorithmto estimate the totalmagni-
ten frames. These residuals had to be removed in the K and tudesofthesources.Theresultantmagnitudeswerecorrected
s
NB2.07 frames observed in 2000 by subtracting a low order forgalacticextinctionofA =0.172(Schlegeletal.1998)and
B
polynomialfittothelinesandcolumnsofthemaskedimages. assuming an R = 3.1 extinction curve, which resulted in a
V
An overview of total exposure time, limiting magnitude and decreaseofthezero-pointsofB,RandIby0.2,0.1and0.1re-
resultantseeingcanbefoundinTable2. spectively. No changes were necessary for the infrared
Theinfraredimageswereregisteredwiththeopticalrefer- zero-points.
enceimageusingthesamepixelscale.InF137objectswere SExtractorclassifiesthelikelihoodofdetectedobjectstobe
usedforthealignmentandinF240,resultinginlessthanone starsorgalaxiesusinganeuralnetwork.Theresultantstellar-
pixeldifferencebetweenallimagesovertheentirefield.TheK icityindexhasarangefrom0.0to1.0,wherestarsshouldhave
s
andNB2.07imagesofF1andF2weremergedintoonerect- avaluenear1.0andgalaxiesavaluenear0.0.
angularmosaicimage.TheoverlapinF1andF2givesriseto ToderivealistofHαemittingcandidates,objectdetection
aregionofaboutonesquarearcminuteinthemosaicwherethe andaperturedefinitionneededtobe doneonthe infrarednar-
noiselevelislowest.Asafinalstepasecondsetofimageswas row band image. For our first set of catalogues, the apertures
madematchingtheresolutionofthereferenceimageusingsix werethereforedefinedontheunconvolvedhomogeneousnoise
starstoestimatethedifferenceinpointspreadfunction. level image associated with the NB2.07 image. In this way,
mostremainingcosmicraysandCCDdefectsaretoosmallto
beincludedinthelistandwedonotintroduceapreferencefor
3. Objectdetectionandphotometry
afixedspatialfrequency.Photometrywiththeseapertureswas
subsequentlycarriedoutonalleightconvolvedimages.There-
3.1.Cataloguesets
sultingcataloguecontains479objects,ofwhichthirteenwere
Theresultsofthe observationsandreductiondescribedabove eitherspuriousornotsuitableforthedetectionoflineemitters
areasetofFORS7(cid:6)×7(cid:6) imagesin NB0.38, B,RandI anda (e.g.brightstars,afewremainingcosmicraysandsomeimage
setofISAAC2.(cid:6)5×2.(cid:6)5imagesin J , H, K (2×)andNB2.07 boundarydefects).
s s
(2×). From these, we have created catalogs of detected and WearealsointerestedinthepopulationofEROsinthefield
flux-calibratedobjectsinordertoselect(i)objectsinK band, ofPKS1138−262.Theseobjectsareoldellipticalordustystar-
s
(ii)EROs,(iii)candidateHαemittersatz = 2.16and(iv)can- burst galaxies at z > 1 and may also be present in the proto-
didateLyαemittersatz=2.16. cluster structure. EROs have extreme I − K colours, i.e. they
We have used the SExtractor software (v2.2.1, Bertin & aredetectedin K bandbutareveryfaintintheoptical.Asec-
Arnouts1996)for objectdetection and photometry.Since the ondsetofcatalogueswasthereforebasedontheunconvolved
background noise level varies across the images as a result homogeneous noise level image associated with the K band
s
of the dithering technique employed, object detection was image.Fromthisset,wederivetheK bandcountsandEROs.
s
not performed directly on the final reduced images, but on Theresultingcataloguecontains550objects.
798 J.D.Kurketal.:Asearchforclustersathighredshift.III.
240
Table3.K bandgalaxycounts.
s
220
Klim n N∗ N∗ >4 >5 ee
s Sco gr
(1) (2) (3) (4) (5) (6) de 200
18.0 19 2.5±0.4 1 0 are
u 180
111899...505 123794 368...829±±±000...678 46..33±±00..34 1969 112 00) per sq 160
0
20.0 58 13.5±1.0 8.4±0.4 26 5 × 1 140
20.5 69 19.0±1.2 11.3±0.5 37 7 y (
21.0 59 23.8±1.4 16.1±0.6 47 11 nsit 120
e
21.5 84 30.5±1.6 59 16 D
100
22.0 55 34.9±1.7 64 19
0 50 100 150 200
Notes: (1) Limiting K magnitude; (2) differential counts between
s DistanceinarcsecondfromPKS1138-262
Klim − 0.5 and Klim; (3) cumulative number arcmin−2 of galaxies
brsighterthanlimitisngKlim,Poissonerrorisindicated;(4)sameas(3) Fig.1.Galaxycountsasafunctionofdistancefromtheradiogalaxyin
s circularbinsof20(cid:6)(cid:6)width.Theerror-barsrepresentPoissonianerrors.
fromblankfieldsurveybyScodeggio&Silva(2000);(5)numberof
objectsbrighterthanKlimwithI−K >4;(6)sameas(5)forI−K >5 Themean density (146×1000) per square degree inside the largest
s s s circle(210(cid:6)(cid:6),binnotshown)isindicatedbythehorizontalline.
(seeSect.4.2).
AlthoughcandidateLyαemitterswereselectedinPaperI, find 4.5±0.6 galaxiesarcmin−2 up to this limit. From Fig. 2
we have derived a new list of Lyα candidates based on the inDaddietal.(2000a),itisclearthatthereexistsconsiderable
newly reduced B and NB0.38 images using selection criteria scatterintheobservationsbydifferentauthors,whichtheyin-
consistent with the criteria for selecting the Hα emitters pre- terpretedasduetocosmologicalfield-to-fieldvariationsofup
sentedinthispaper.Forthispurpose,athirdsetofcatalogues toafactortwo.OurKs bandcountsareneartheupperlimitof
was constructed, based on the convolved homogeneousnoise theobservedvariations.
level image associated with the NB0.38 image. This set con- We have analyzed the surface density of K band se-
tains1027sources. lected galaxies as a function of distance from the radio
galaxy by counting the number of galaxies in circular areas
around1138−262.Figure1showsthenumberofgalaxiesper
4. Numbercountsandclustercandidates arcsecond2 in circular bins of 20(cid:6)(cid:6). It is clear that the counts
4.1.K numbercounts showanexcesswithin50(cid:6)(cid:6)or0.45Mpcfromtheradiogalaxy.
s
Thedeviationfromthemeandensityofthejoinedfirsttwobins
Kbandnumbercountswerederivedfromthecataloguesbased is2.2σ.
on the Ks image. Table 3 lists the number of sources per The richness of clusters can be assessed by counting the
half magnitudebinand thecumulativenumberof sourcesper numberofclustergalaxiesfoundwithinaradiusof0.5Mpcof
squaredegree.Best(2000,B00fromnowon)showsthatselect- thecentralgalaxywithmagnitudesbetweenm andm +3,m
1 1 1
ing K band objects with SExtractor’s stellaricity index below beingthe magnitudeof thecentralgalaxy.Thisvalue, N0.5, is
0.8efficientlyselectsgalaxiesasopposedtostars.Thisconclu- definedbyHill&Lilly(1991)andbasedonanearlierdefinition
sionisbasedontheJ−Kcolourofthedetectedobjectswhich by Abell (1958). The K magnitude measured for 1138−262
is in general bluer for stars. Because F2 is not imaged in J, is 16.1. We can correct this value for line emission from Hα
wedeterminedthegalaxycountsfromthetotalnumbercounts andN asmeasuredin the NB2.07band,bysolvingforline
(550 objects) by selecting only those objects with stellaricity andcontinuumcontributionsinthebroadandnarrowband.We
indexlowerthan0.8(470objects). obtain
The cumulative counts were compared with observations
b−n
of a blankfield ofsubstantialsize (43arcmin2) byScodeggio c= , (1)
∆λ −∆λ
& Silva (2000). Although these authors also make a distinc- bb nb
tion between stars and galaxies based on SExtractor’sstellar- where c, b and n are the continuum flux and flux measured
icity index (0.85), it is not clear whether the number counts in broad and narrow band and ∆λ ,∆λ are the FWHM
bb nb
in their tables are total counts or galaxies only. We assume of the broad and narrow band, respectively. The K magni-
here that galaxy counts are listed. We observe on average tude in the NB2.07 band is 15.8 and the resulting magnitude
1.5±0.1timesthenumberofobjectsexpectedfromtheblank of the continuum flux therefore 16.2. Note that this differ-
field survey, as illustrated by the observed surface density of ence of 0.1 units of magnitude is less than the 0.3 units de-
galaxies brighter than K = 20 of 13.5±1.0 arcmin−2, com- rivedinPentericcietal.(1997)basedonanassumedLyα/Hα
s
pared with 8.4 ± 0.4 arcmin−2 determined by Scodeggio & ratio of 1. In addition to line flux, we have to correct for
Silva. Less deep, but using a much larger field are the obser- othernonstellarcontributions.Pentericcietal.(1997)attribute
vation of Daddi et al. (2000a), which determine a K galaxy 0.2magnitudeunitstothepointsourcecontributionandamax-
s
countof3.29±0.07objectsarcmin−2 upto K = 18.7(which imum of 0.2 units to the resolved contribution. We conclude
s
is slightly higher than Scodeggio & Silva’s value), while we that the continuum K magnitude representative of the stellar
J.D.Kurketal.:Asearchforclustersathighredshift.III. 799
population in the brightest cluster galaxy is close to 16.5.
At z = 2.16, 0.5 Mpc is equivalent to 56(cid:6)(cid:6), and the angular 7
area occupied by a disc with 0.5 Mpc radius is 2.7 arcmin2.
The number of galaxies between 16.5 and 19.5 in this area
around 1138−262 is 31. From Table 1 in B00, we find that 6
blankfieldscontain∼2.06×104galaxiespersquaredegreebe-
tween magnitude 16.5 and 19.5 (references to the blank field
datacanbefoundinthecaptionofFig.6inB00),or15galax- 5
ies within a circular area of 56(cid:6)(cid:6) radius. The net excess count
around1138−262istherefore16±6.FromTable4in Hill&
K
Lilly (1991) we read that for clusters of richness 0, 1, 2, the I - 4
mean N0.5 values determined by Bahcall (1981) are 12 ± 3,
15 ± 5, 29 ± 8. The mean value of N0.5 in 3C radio galaxy
fields(Yatesetal.1989)is11±2,similartotheaveragenum- 3
ber of 11 galaxies in the 28 z ∼ 1 radio galaxy fields found
byB00.ThemeasuredN0.5 suggeststhat1138−262islocated
2
inanenvironmentwithadensitycomparabletorichnessclass0
or 1 clusters. During the evolution of this structure, however,
more galaxiesmight fall in, furtherincreasing the richnessof
1
thecluster.
12 14 16 18 20 22
4.2.Extremelyredobjects K
Fig.2.Colour–magnitudeplotofI−K vs.K forthe544sourcesde-
Since the discovery of extremely red objects (EROs, Elston tected on the K band image. The sources within 40(cid:6)(cid:6) of the radio
s
et al. 1988, 1989), there has been considerable interest for galaxyareindicatedbycircles.
these high redshift galaxies because their properties can con-
strainmodelsofgalaxyformationandevolution.Theyarenow
z = 5 Burst
generallybelievedtobeeitherevolvedellipticalsordustystar- 6 f
burstsatz>1andhavebeenshowntoclusterstrongly(Daddi zf = 4
etal.2000a).Usingourmultibandobservations,wecansearch
z = 3
for EROs which could form a population of galaxies associ- f
ated with the radio galaxyat z ∼ 2.16.Clusters are knownto 5
possessapopulationofellipticalgalaxieswhichformaredse-
z = 2.6
quenceinacolour-magnitudediagram(e.g.Boweretal.1992). f
Theevolutionoftheellipticalgalaxypopulationinclustershas 4
beenshownbynumerousauthors(e.g. Stanfordetal.1998)to
K
besimpleandhomogeneous,indicatingthatthestellarpopula- I -
tionthatmakesuptheredsequenceisformedathighredshifts
3
(z > 2). Gladders & Yee (2000) show that the red sequence
f
can be exploitedto find clusters of galaxiesup to z ∼ 1.4 us-
ing optical imaging. Basically, the cluster red sequence is as
red asorredderthanothergalaxiesata givenredshiftandall 2
lowerredshiftsifproperlychosenfiltersstraddlingthe4000Å
breakareused(Gladders&Yee2000).Fora clusterelliptical
at z = 2.2 the 4000 Å break is redshifted to 12800 Å, in the
1
infrared J band.WehaveselectedEROsbasedontheirI−K
s
colour,whichalsotargetsgalaxiesattheredshiftoftheproto-
0 1 2 3 4
cluster.Thesamplesselectedwiththesebandscanbecompared
J-K
withliteraturedata.
Fig.3. I − K vs. J − K colour–colour plot for the 320 sources de-
tectedinthecentralK bandimage.Non-detectionsinIorJandthere-
s
4.2.1. EROsinthefieldofPKS1138−262 forelowerlimitsonthemagnitudeareindicatedbyarrows.Theradio
galaxy(indicatedbyadiamond) has J−K = 2.8and I −K = 4.7.
Figures2and3showplotsofI−K vs.K andI−K vs. J−K Thetrackmarksthecolourofastellarpopulationatz = 2.2formed
bya100Myr singleburstfollowedbypassiveevolutionforagesof
for the sources with apertures defined on the K band image.
s
0.5 Myr to 2.4 Gyr after the burst. Four formation redshifts of the
Theseplotsshowthatthereisaconsiderablenumberofobjects
populationareindicated.PKS1138−262isindicatedbyadiamond.
withveryredcolours(I−K >4)inourfield.Anenlargement
s
of Fig. 3 is shown in Fig. 4 for the range in I − K > 3.75.
800 J.D.Kurketal.:Asearchforclustersathighredshift.III.
6.0
Ellipticals Starbursts fainterfluxlevelsis asstrongasatbrightlevels,butthemea-
surementsbyDaddietal.(2000b)showthattheclusteringam-
z = 4
f plitude of EROs with K < 18.5 is twice as high as of EROs
s
with K < 19.2.Equations(8)and(9)inDaddietal. (2000b)
s
5.5 prescribethermsfluctuationofEROcountsduetocosmicvari-
ance,giventheclusteringstrengthofEROs.Wehavemadethe
z = 3 conservative assumption that K = 21 EROs are clustered as
f
strongly as K = 19 EROs, resulting in an uncertainty on the
numberofEROswith I −K > 4 (5)of11 (4).Therefore,we
5.0
K tentativelyfindanover-densityofaboutafactorof1.5,which
I - might be due to a population of EROs in the proto-cluster at
z∼2.2ontopofafieldpopulationofEROsatlowerredshift.
Animportantargumentforthepropositionthatpartofthe
4.5
ERO populationconsists of proto-clustermembersat z = 2.2
I - K = 4.3 z = 2.6 isthegradientinthespatialdistributionofEROs,asshownin
f
Fig. 5, which is similar to the distribution of K band counts.
s
The density within a 40(cid:6)(cid:6) radius of the radio galaxy is more
4.0 than four times higher than the mean density outside a 60(cid:6)(cid:6)
(0.5Mpc)radius.Thismeansurfacedensityof8×103galaxies
per square degree would imply a number of 28 EROs in the
1.0 1.5 2.0 2.5 3.0 3.5 4.0
field, roughly consistent with the number expected from the
J-K
blank field observed by Scodeggio & Silva (2000). In Fig. 2,
Fig.4. Thisclose-up ofFig.3showstheextremelyredobjects. The the objects within 40(cid:6)(cid:6) from the radio galaxy are indicated
horizontallinedenotesourEROselectioncriterionI−K>4.3.Error-
by circles. It is clear that many of the reddest objects lie
barshavebeenreducedinsizebyafactortwotoincreasethereadabil-
near the radio galaxy: there are 17 EROs within a 40(cid:6)(cid:6) ra-
ityoftheplot.Thesolidline,diamondandarrowsaredescribedinthe
dius of 1138−262 (12 arcmin−2) and 27 outside this radius
captionofFig.3.Threeformationredshiftsforthestellarpopulation
areindicated. (2.4 arcmin−2). The over-density of EROs in the field of
1138−262 is therefore mostly due to the red galaxies near
theradiogalaxy,whichisconsistentwiththeobservedexcess
ForI−K > 4.3,thereareveryfewbrightobjects:themedian
ofEROsbeingduetoaclusterpopulationassociatedwiththe
K band magnitude in the range 4.3 < I − K < 5.1 is 20.9
radiogalaxy.
whileitis1.3maglowerintherange3.5 < I−K < 4.3,with
the soleexceptionoftheradiogalaxy(I −K = 4.8, J −K =
4.2.2. EROsintheproto-clusteratz =2.2
2.8).Thisincreaseinmagnitudesuggeststhata largefraction
oftheobjectsredderthanthislimitaredistantgalaxiesandwe What coloursdo we expectfor galaxiesat z = 2.2? We have
considerthereforethe44objectswith I−K > 4.3inthefield computedevolutionarytracksforseveralstellarpopulationus-
of 1138−262as EROs for the remainder of this paper. These ingtheGalaxyIsochroneSynthesisSpectralEvolutionLibrary
objects are listed in Table A.3. However, since other authors (GISSEL93,Bruzual&Charlot1993).TheIMFwehaveused
usedifferentcriteria,wehavelistedthenumberofredobjects inthe modelsisa Salpeter(1955)law with lowermasscutoff
accordingtoseveralselectioncriteriainTable4. at 0.1 M(cid:4) and uppermass cutoff at 125 M(cid:4). In Fig. 3 a track
WewillcomparethenumberdensityofEROsinthefieldof isindicatedfora stellar populationat z = 2.2 formedbypas-
1138−262withthedensityobservedina23.5arcmin2 areaof sive evolution after a 100 Myr single burst model. The track
the ChandraDeep Field byScodeggio& Silva (2000),which startsatanageof5Myrat I −K = 0.5andleavestheplotat
agreeswellwiththedensityofI −K > 4.0objectsmeasured an age of 2.4 Gyr (I −K = 6.5). We have also computedthe
s
byCowieetal.(1996).InTable3,thenumberofobjectswith coloursforaconstantstarformationmodel,andanexponential
I−K > 4and>5foundinthefieldof1138−262isshownper star formationmodelwith τ = 1 Gyr,but these did not reach
K magnitudelimit.UptoK = 21,oneexpects33±4(6±2) I −K > 4 evenafter4Gyr,themaximumagefora galaxyat
s s
EROswithI−K >4(5)inablankfieldofthissize,whilewe z = 2.2inthiscosmology.TheEROsinthefieldof1138−262
observe47(11).Note,howeverthatDaddietal.(2000b)claim havecoloursconsistentwithgalaxieswhichhaveundergonea
that the density of EROs with R−K > 5 and K < 19 in the starburst0.5Gyrago(z=2.6)orlongerandevolvedpassively
s
43 arcmin2 CDFS, derivedbyScodeggio& Silva (2000),is a thereafter. However, dust reddening is not taken into account
factor of five smaller than the one derivedin the 700arcmin2 inthemodelspresentedhere.Reddeningofactivelystarform-
survey by Daddi et al. (2000b). This large discrepancy is not inggalaxiesbydustcanalsoshifttheirI−K coloursintothis
unexpectedgiventhestrongclusteringofEROs.Althoughwe regimeasshownbye.g.Pozzetti&Mannucci(2000).TheI−K
are using different selection criteria and most EROs detected colouroftheirellipticaltemplateincreasesto∼7atz>2,while
in our comparativelysmall 12.5arcmin2 field are fainter than thecolouroftheirdustystarbursttemplatestaysconstantat∼6.
K = 19, we should be careful drawing conclusions from this Weobservenoobjectswith I−K ∼ 7,buttherearethreeob-
comparison. It is unclear whether the clustering of EROs at jectswithlowerlimitstotheirI−Kcolourof∼6,whichcould
J.D.Kurketal.:Asearchforclustersathighredshift.III. 801
Table4.ExtremelyredobjectscountsnearPKS1138−262.
Σ=3 Σ=2
2.0
I−K # #C
(1) (2) (3)
4.3–5.3 36 27 1.5
>5.3 8 6
4.5–5.5 25 22
>5.5 7 5
1.0
cNoovteerse:d(1b)yIb−othKKcoblaonudr;im(2a)gneus;m(b3e)rnoufmEbRerOosfiEnR1O2s.5inar7c.m1ianr2cmfieinld2 mnarrow
fieldcoveredbycentralKsbandimageonly. m-broad 0.5 EW0=50A
50 EW=25A
0
ee 40 0.0
gr
e
d
e
ar
u 30
q
er s -0.5
p
00) 20
0
× 1 14 16 18 20 22
y ( Narrow band magnitude
sit 10
en Fig.6. Colour–magnitude diagram for 467 sources detected in the
D
NB2.07image.Thedot-dashedlinesarelinesofconstantexcesssig-
0 nalΣ(seetext).AlsoshownarelinesofconstantEW (forz=2.16).
0 50 100 150 200 0
DistanceinarcsecondfromPKS1138-262 CandidateHαemittersareobjectswithEW0 >25or50ÅandΣ>2
or3(seeTable5).
Fig.5.Extremelyredobject(I−K >4.3)countsasafunctionofdis-
tance from the radio galaxy. The EROs are counted in circular bins
of 20(cid:6)(cid:6) width. The error-bars represent Poissonian errors. The mean
equivalentwidth(EW )of25and50Åandtwocurvesindicat-
density(8 × 1000)persquaredegreeoutside60(cid:6)(cid:6) (0.5Mpc)isindi- 0
ing Σ equal to 3 and 2. The curves of constant Σ have been
catedbythehorizontalline.
computedformediannarrowbandandbroadbanderrors;the
actualΣofindividualsourcesdepends,amongstothers,onthe
be z ∼ 2 ellipticals and a few objects with I − K ∼ 6. We aperturesizeandlocalbackgroundnoise.
concludethattheexcessof∼10–15oftheEROsinthefieldof We find 17 candidate emitters with rest-frame equivalent
1138−262is caused mostly by galaxies at z = 2.2 which are widthEW0 > 50ÅandΣ > 3,allofwhichhavenarrowband
magnitudes ≤21.3. One of these objects is the radio galaxy,
reddenedbydust.
whileasecondiswithintheextentoftheLyαhalooftheradio
galaxy.If we lower the selection criteria to EW > 25 Å and
0
4.3.CandidateHαemitters Σ>2,wefind40candidates.AllofthesehavemNB2.07 ≤21.6.
In addition to the two objects identified above, there is one
4.3.1. Selectionprocedure
moreobjectin this list within the radiogalaxyLyα halo.The
Candidate line emitting objects were selected on the basis of Hαhalooftheradiogalaxyisinterestinginitself,especiallyin
their excessnarrowversusbroadbandflux, followingthe cri- comparisonwith theLyαhaloandare studiedinanotherarti-
teriaofBunkeretal.(1995)andMoorwoodetal.(2000).The cle(Kurketal.2004).Table5liststhenumberofcandidatesin
selectedcandidatesfulfilltwocriteria:first,theyhavesufficient both fields for several selection criteria, while Table A.1 lists
equivalent width (EW) and second, their broad band flux is the K magnitude and emission line properties, for the candi-
significantly lower than expected for a flat spectrum source. dates with EW0 > 25 Å and Σ > 2. The NB 2.07 narrow
Having measured the narrow band flux for each source, we band filter used also includes the [NII]λλ6548,6584Å lines
computethe expectedbroadbandflux and its standarddevia- atz= 2.16,butinwhatfollows,wewillrefertothecombined
tionassumingaflatspectrum.TheerrorparameterΣisdefined Hα+NIIfluxandequivalentwidth,asHαfluxandequivalent
asthenumberofstandarddeviationsthemeasuredbroadband widthrespectively,unlessotherwisenoted.
fluxdeviatesfromtheexpectedbroadbandfluxofaflatspec-
trum source(see also Bunkeretal. 1995).Note that Σ is well 4.3.2. NumberdensityofcandidateHαemitters
definedforobjectsnotdetectedinthebroadband.ANB2.07–
K bandversusNB 2.07magnitudeplotforthe466bonafide InrecentyearsthenumberofsuccessfulsearchesforHαemit-
s
objects in the Hα selection catalog (see Sect. 3) is shown in ters in the infrared has grown. Bunker et al. (1995) have ob-
Fig.6.Alsodrawnaretwohorizontallinesindicatingrestframe tainednearinfrarednarrowbandimagesina4.9arcmin2field
802 J.D.Kurketal.:Asearchforclustersathighredshift.III.
Table5.PropertiesofthesamplesofHαcandidates. 100
EW Σ m # #H n n
0 NB 1138
(1) (2) (3) (4) (5) (6) (7)
50
50 3 – 17 2 1.4±0.3
25 3 – 23 3 1.8±0.4
25 2 – 40 3 3.2±0.5
50 1 – 48 2 3.8±0.6 0
25 1 – 60 3 4.8±0.6
75 2 19.3 1 0 0.1±0.1 0.61±0.35a
100 3 19.5 2 1 0.2±0.1 0.12±0.05b -50
Notes:(1)Restframeequivalentwidthlowerlimit;(2)signaltonoise
lowerlimitasdefinedbyBunkeretal.(1995);(3)additionalselection
criterion: narrow band magnitude upper limit; (4) number of candi-
dates in 12.5 arcmin2 area; (5) number of candidates within the ra- -150 -100 -50 0 50
dio galaxy Lyα halo; (6) surface density of candidates in the field Fig.7. Position plot for the 40 (17) Hα candidates with EW > 25
0
of1138−262(arcmin−2);(7)surfacedensityinotherfields:a Bunker (50)ÅandΣ>2(3)indicatedbyopen(filled)squares.Largesquares
etal.(1995),bvanderWerfetal.(2000). indicate candidates for which Lyα emission has been detected (see
Sect.5.1).Axesareinarcseconds,theradiogalaxyisattheorigin.The
dottedboxesindicatethebordersofthetworeducedISAACfields.
towardsthequasarPHL957,inanattempttodetectHαemis-
sionfromadampedLyαabsorberatz=2.313.Theyfind3can-
didate Hαemittersat 2.29 < z < 2.35with EW > 75Å and also weakly on the K band sensitivity, but we do not expect
0 s
Σ > 2, brighter than a narrow band magnitude limit of 19.3. this to have a large influence on the density per bin and cer-
More recently, van der Werf et al. (2000) presented a survey tainlynotontheconclusionsfromthisplot.Thedensitywithin
for Hα emission at redshifts from 2.1 to 2.4. They have ob- a 40(cid:6)(cid:6) distance of the radio galaxy is 5.0 ± 0.9 times higher
served several fields containing known damped Lyα systems than the mean density outside a 60(cid:6)(cid:6) (0.5 Mpc) radius. The
(also including the field of PHL 957) and radio galaxies as highnumberof candidatesnear the radio galaxyis consistent
well as random fields for a total area of 55.9 arcmin2. They with1138−262beinglocatedinaregionthatisover-densein
detect two radio galaxies and a damped Lyα absorber in the Hαemittinggalaxies.
field of PHL 957 in Hα emission accompanied by two close
emitterswhichhadnotbeenobservedbefore.Inaddition,they
4.3.4. Contaminationbyotherlineemitters
find another candidate in the field of a radio galaxy, but at
a large distance from it (81(cid:6)(cid:6)), which seems to be a merging Wedonotexpectthatasignificantfractionofouremissionline
galaxy.Intotal,theyfindsevenHαemitterswithEW0 >100Å candidates are line emitters at lower redshift, since there are
and Σ > 3 down to an area weighted narrowband magnitude no strong emission lines redward of Hα at 6563 Å (Paα with
limit of 19.5. The narrow band fluxes of these candidates are a strength ∼0.2 times the strength Hα would imply z = 0.1).
>2.0×10−16ergcm−2s−1. It is possible that we have selected galaxies at z ∼ 3.13 with
Our VLT near infrared imaging is much deeper and most strong [O]λ5007 Å emission or galaxies at z ∼ 4.55 with
of our candidates have fluxes below this limit. To compare [O]λ3727 Å emission. The luminosity distance for objects
thenumberdensityofHαemittersinourfieldwiththeabove at z ∼ 4.6 is 2.5 times larger than for objects at z ∼ 2.16. If
mentioned authors, we have done the selection according to any of the emitters is indeed an [O] emitter at this redshift,
their limits, including the lower narrow band magnitude lim- its intrinsic luminosity is six times higher than that assumed
itsimposedbytheirshallowerobservations.We findonlyone forHαinTable A.1.Itisthereforeunlikelythattheseobjects
and two candidates (see Table 5), resulting in number densi- contaminateoursamplesignificantly.Morelikelyisthecontri-
ties (with largePoisson errors) slightly lowerand higherthan bution of [O] emitters to our sample, illustrated by follow-
Bunkeretal.’sandvanderWerfetal.’ssurveys,respectively. up spectroscopy to narrow band observations similar to ours.
All six galaxies suspected to be Hα emitters at z ∼ 2.2 from
low signal-to-noise near-infrared spectroscopy by Moorwood
4.3.3. Spatialdistribution

et al. (2000) were found to be [O ] emitters at higher red-
Figure 7 shows the spatial distribution of candidate Hα emit- shift when observed at high enough signal-to-noise to detect
ters. This distribution is not homogeneous over the observed the [O]λ4959 Å line (P. van der Werf, private communica-
fields. To quantify this inhomogeneity, we have counted the tion). Although on the basis of the present data, we cannot
numberofcandidateswithEW > 25ÅandΣ > 2incircular separate Hα emitters at z ∼ 2.2 from line emitters at higher
0
binsof20(cid:6)(cid:6)radiusaroundtheradiogalaxy.Wehavetakeninto redshift, the spatial distribution of the candidate Hα emitters
account the variation in sensitivity per pixel using the weight indicatesthatatleast∼12areassociatedwiththeradiogalaxy
map associated with the NB 2.07 image. This alters the den- andarethereforebonafideHαemitterssurroundingthecluster.
sity per bin by less than 20% in all bins compared with the Thereis,however,onecandidatewhichstandsoutbyitsbright-
unweightedcomputation.TheHαcandidateselectiondepends ness(K = 17.7)anditssize(∼3(cid:6)(cid:6))fromtheothers(K ≥ 19.3).