The Building the Bridge survey for z=3 Ly-alpha emitting galaxies II: Completion of the survey

1.8 MB·English

by  L. F. Grove

Astronomy&Astrophysicsmanuscriptno.bridge2 (cid:13)c ESO2009
January24,2009
The Building the Bridge Survey for z = 3 Lyα emitting galaxies. II.
⋆
Completion of the survey
L.F.Grove1,J.P.U.Fynbo1,C.Ledoux2,M.Limousin1,3,P.Møller4,K.Nilsson5,andB.Thomsen6
9
1 DarkCosmologyCentre,NielsBohrInstitute,UniversityofCopenhagen,JulianeMariesVej30,2100Copenhagen,Denmark
0
2 EuropeanSouthernObservatory,AlonsodeCo´rdova3107,Casilla19001,Vitacura,Santiago19,Chile
0
3 Laboratoired’AstrophysiquedeToulouse-Tarbes,Universite´deToulouse,CNRS,57avenued’Azereix,65000Tarbes,France
2
4 EuropeanSouthernObservatory,Karl-Schwarzschild-Strasse2,D-85748Garching,Germany

n 5 Max-Planck-Institutfu¨rAstronomie,Ko¨nigstuhl17,69117Heidelberg,Germany
a 6 InstituteofPhysicsandAstronomy,AarhusUniversity,NyMunkegade,DK-8000AarhusC
J
4 Received-;Accepted-
2
ABSTRACT

]
O Context. Wehavesubstantialinformationabout kinematicsandabundancesofgalaxiesatz ≈ 3studiedinabsorptionagainstthe
lightofbackgroundQSOs.Atthesametimewehavestudied1000sofgalaxiesdetectedinemissionmainlythroughtheLyman-break
C
selectiontechnique.However,weknowverylittleabouthowtomaketheconnectionbetweenthetwodatasets.
h. Aims.Weaimatbridgingthegapbetweenabsorptionselectedandemissionselectedgalaxiesatz≈3byprobingthefaintendofthe
p luminosityfunctionofstar-forminggalaxiesatz≈3.
- Methods.Narrow-bandsurveysfor Lyman-α(Lyα)emittershaveproventobeanefficientprobe offaint,star-forminggalaxiesin
o thehighredshiftuniverse.Wehaveperformednarrow-bandimaginginthreefieldswithinterveningQSOabsorbers(adampedLyα
r absorberandtwoLyman-limitsystems)usingtheVLT.WetargetLyαatredshifts2.85,3.15and3.20.
t
s Results.Wefindaconsistent surfacedensityofabout 10Lyα-emitterspersquarearcminperunitredshiftinallthreefieldsdown
a toourdetectionlimitofabout3×1041ergss−1.Theluminosityfunctionisconsistentwithwhathasbeenfoundbyothersurveysat
[ similarredshifts.About85%ofthesourcesarefainterthanthecanonicallimitofR=25.5formostLyman-breakgalaxysurveys.In

noneofthethreefieldsdowedetecttheemissioncounterpartsoftheQSOabsorbers.Inparticularwedonotdetectthecounterpartof
1
thez=2.85dampedLyαabsorbertowardsQ2138−4427.ThisimpliesthattheDLAgalaxyiseithernotaLyαemitterorfainterthan
v
ourfluxlimit.
5
Conclusions.Narrow-bandsurveysforLyαemittersareexcellenttoprobethefaintendoftheluminosityfunctionatz ≈ 3.There
4
isaveryhigh surfacedensityof thisclassof objects. Yet,weonly detect galaxieswithLyαinemissionandhence thedensityof
8
galaxieswithsimilarbroadbandmagnitudeswillbesubstantiallyhigher.Thisisconsistentwithaverysteepslopeofthefaintend
3
oftheluminosityfunctionashasbeeninferredbyotherstudies.Thisfaintpopulationofgalaxiesisplayingacentralroleintheearly
.
1 Universe.Thereisevidencethatthispopulationisdominatingtheintergratedstar-formationactivity,responsibleforthebulkofthe
0 ionizingphotonsatz&3andlikelyalsoresponsibleforthebulkoftheenrichmentoftheintergalacticmedium.
9
Keywords.cosmology:observations–quasars:individualBRI1346−0322,BRI1202−0725,Q2138−4427–galaxies:highredshift
0
:
v
i
X 1. Introduction In 2000 we started a surveyaiming at bridgingthe gap be-
tween emission and absorption selected galaxies. The goal of
r
a Strong arguments (Fynboetal. 1999; Haehneltetal. 2000; thesurveywastodetectfaintz ≈ 3galaxiesusingnarrow-band
Schaye 2001; Rauchetal. 2008; Barnes&Haehnelt 2008) in- imagingselectionofLyman-α(Lyα)emitters(LAEs)andinthis
dicatethatthereisverylittleoverlapbetweenemissionselected way bridge the gap between the DLAs and the LBGs. During
galaxies(primarilyLyman-BreakGalaxies,LBGs,Steideletal. the1990iesandearly2000sitwasestablishedthatLAEscanbe
2003) and absorption selected galaxies (primarily the Damped usedtoselecthigh-zgalaxies(e.g.Møller&Warren1993) and
Lyman-αAbsorbers,DLAsWolfeetal. 2005).Thesimplerea- thatthismethodeasilytracessignificantlydeeperintothelumi-
sonforthisisthatLBGsamplesarecontinuumfluxlimitedand nosity function than what is possible with spectroscopic sam-
thatthecurrentfluxlimitofR≈25.5isnotdeepenoughtoreach plesofLBGs(e.g.Cowie&Hu1998;Fynboetal.2001).Inour
theleveloftypicalabsorptionselectedgalaxies.Thisisunfortu- survey we targeted the fields of QSOs with intervening DLAs
nate as we then know little about how to combine the detailed primarily to be able also to search for the galaxy counterparts
informationonabundancesandkinematicsinferredfromobser- of the DLAs, but also to anchor the fields to known structures
vations of DLAs with the informationabout coloursand lumi- atthe targetedredshifts.Thefirstpaperofthe surveywaspub-
nositiesofhigh-zgalaxiesdetectedinemission. lishedbyFynboetal.(2003)(hereafterPaperI),wherewepre-
sentedtheresultsfromtwoofthethreetargetedfields.Sincethen
Sendoffprintrequeststo:L.F.Grove,[email protected] the study of LAEs has progressedsubstantially,mainly on two
⋆ Basedonobservations collectedattheEuropean Organisationfor fronts. First, large samples covering a range of redshifts have
AstronomicalResearchintheSouthernHemisphere,Chile,underpro- been collected using wide-field imagers both on 4m and 8m
grams 67.A-0033, 267.A-5704, 69.A-0380, 70.A-0048, and 072.A- class telescopes (e.g. Gronwalletal. 2007; Ouchietal. 2008;
0073.
2 L.F.Groveetal.:TheBuildingtheBridgeSurveyforz=3Lyαemittinggalaxies.II.Completionofthesurvey
Nilssonetal.2008).Second,moredetailedstudiesoftheprop-
erties have been carried through,most notably based on LAEs
in the GOODS fields (e.g. Nilssonetal. 2007; Gronwalletal.
2007;Pentericcietal.2008).
Inthisfinalpaperfromour”Buildingthebridge”surveywe
first presentthe results from the third field, namely the sample
of LAEs identified in the field of the quasar BRI1202−0725.
Second, we combine the results of the entire survey covering
three fields to derive the luminosity function of the LAEs at
z ∼ 3. We then discuss our results in the light of the substan-
tialprogressthathasbeenmadebyanumberofgroupsoverthe
lastfewyearsonthestudyofLAEsatz&2.
Throughoutthis paper,we assume a cosmologywith H =
0
70kms−1 Mpc−1,Ω = 0.3andΩ = 0.7.Wealsothroughout
m Λ
usemagnitudesontheABsystem(Oke1974).
2. ThefieldofBRI1202−0725
Fig.1.Transmissioncurvesofthenarrow-andbroad-bandfilters
2.1.Imaging usedfortheobservationsofBRI1202−0725.
The field of BRI1202−0725 (z = 4.70) was included in
qso Table 1. Log of imaging observations with FORS1 and 2 of
this survey due to the presence of a Lyman-limitsystem along
thethreesurveyedfields.The5σdetectionlimitsarecomputed
the line-of-sight towards the quasar at a redshift of z = 3.2
for a 3′′diameter aperture. The data for BRI1346−0322 and
(Storrie-Lombardietal. 1996). This field was observed in ser-
Q2138−4427arereproducedfromPaperI.
vice mode at the VLT 8.2 m telescope, unit Yepun, during the
nightsJanuary30throughFebruary62003usingtheFORS2in-
filter z ,z totalexp. PSFfwhm 5σ
strument.ThewavelengthoftheLyαtransitionattheredshiftof qso abs
time(hr) (′′) limit
z = 3.20isλ = 5105Å,whichcorrespondstothecentralwave- BRI1202−0725 4.70,3.20
lengthofthe61Åwide[OIII]VLTfilter.Thefieldwasobserved N 8.3 1.02 25.6
inthisnarrow-bandfilter(NB)andintheBesselV andSpecial Va 2.9 0.96 26.6
R filters. The transmissioncurvesof the three filters are shown R 1.6 0.95 26.1
in Fig. 1. The integration times in the V and R filters were set BRI1346−0322 3.99,3.15
bythecriterionthatthebroadon-bandV imagingshouldreach N 8.9 0.93 25.6
B 2.5 1.02 26.6
about half a magnitude deeper than the narrow-band imaging
R 1.7 0.94 26.0
so asto geta reliableselectionof objectswith excessemission
Q2138−4427 3.17,2.85
inthenarrow-bandfilter.Forthebroadoff-bandRimaging,we
N 10.0 0.96 26.5
aimed at reaching the 5σ significance level at one magnitude
B 2.5 1.04 26.9
deeper than the spectroscopic limit of R(AB)=25.5 for LBGs R 1.7 0.93 26.3
(i.e. aiming at R(AB)=26.5 at the 5σ significance level). The
totalintegrationtimes,theseeing(FWHM)ofthecombinedim-
ages and the 5σ detection limits for a 3′′diameter aperture are a Duetothehighertargetredshiftofthisfieldthewavelengthofthe
given for each filter in Table 1. For comparison with previous Lyα-linewasmatchedbytheV-bandratherthanB.
work we also list the corresponding numbers for the fields of
BRI1346−0322andQ2138−4427presentedinPaperI1.
2.2.LAEcandidateselection
The images were reduced (de-biased and corrected for
CCD pixel-to-pixel variations) using the FORS pipeline The selection of LAE candidates is based on the “nar-
(Grosbøletal.1999).Theindividualreducedimagesineachfil- row minus on-band broad” versus “narrow minus off-band
ter were combined using a code that optimizes the Signal-to- broad” colour/colour plot technique (Møller&Warren 1993;
Noise (S/N) ratio for faint, sky-dominatedsources (see Møller Fynboetal. 1999, 2000, 2002; Fynboetal. 2003). The object
&Warren1993fordetailsonthiscode). identification and photometry was carried out in SExtractor
The broad-band images were calibrated as part of the (Bertin&Arnouts 1996) using the dual-image mode having a
FORS calibration plan via observations of Landolt standard detection image and measuring the photometric properties on
stars (Landolt 1992). We transformed the zero-points to the theindividualimages.Forthedetectionweusedaweightedsum
AB system using the relations given by Fukugitaetal. (1995): of the V-band (20%) and narrow-band(80%) imagesto secure
V(AB)=V−0.02and R(AB)=R+0.17.For the calibration of the anoptimaldetectionofobjectswithexcessinthe narrowfilter.
narrow-bandimages,weusedobservationsofthespectrophoto- Beforeobjectdetectionwe convolvedthedetectionimagewith
metricstandardstarsEG274andGD71. aGaussianfilterfunctionhavingaFWHMequaltothatofpoint
sources. We used a detection threshold of 1.1 times the back-
1 Notethatduetoanerrorinthephotometriczeropointsusedinthat groundsky-noiseanda minimumareaof5connectedpixelsin
paperthelistedbroadbanddetectionlimitsareslightlydifferentfrom the filtered image. For total magnitudes we use the SExtractor
theoriginalones(byabout0.15mag).Thisisnotaffectingtheconclu- MAG AUTOandtocomputecoloursweusetheisophotalmag-
sionsofthatpaper. nitudes(MAG ISO).Inourfinalcatalogue,weincludeonlyob-
L.F.Groveetal.:TheBuildingtheBridgeSurveyforz=3Lyαemittinggalaxies.II.Completionofthesurvey 3
jects with total S/N>5 in the isophotal aperture in either the
narrow-ortheV-bandimages.Intotal,wedetect3202suchob-
jects withinthe 400×400arcsec2 field aroundBRI1202−0725.
Theerrorbarsonthecolourindicesarederivedusingthemax-
imum likelihood method of Fynboetal. (2002). For the selec-

tionofLAEcandidateswerestrictedthesampletoS/N>5inthe
narrow-bandfilterasdescribedbelow.
In Fig. 2 we show the colour-colour diagram used for the
final selection of LAE candidates. In order to constrain where
objects with no special spectral features in the narrow filter
are in the diagram, we calculated colours based on synthetic
galaxy SEDs taken from the Bruzual & Charlot 1995 mod-
els(Bruzual&Charlot2003).Wehaveusedsimplesingle-burst
models with ages ranging from a few Myr to 15 Gyr and with
redshiftsfrom0to1.5(opensquaresinFig.2)andmodelswith
agesrangingfromafewMyrto1Gyrwithredshiftsfrom1.5to
3.0 (open triangles). For the colours of high-redshift galaxies,
we included the effect of Lyα blanketing (Møller&Jakobsen

1990; Madau 1995), but for none of the models the effects
of dust are included. Fig. 2 shows the N(AB)−V(AB) versus
N(AB)−R(AB)colourdiagramforthesimulatedgalaxycolours
(leftpanel)andfortheobservedsourcesinthetargetfield(mid-
dle and right panels). The dashed line indicates where objects

withaparticularbroad-bandcolour(correspondingtoa100Myr
Fig.3. The 400×400 arcsec2 field surrounding the QSO
old galaxy at z = 3.20 and either absorption (upper right) or
BRI1202−0725as observedin the narrow-bandfilter. North is
emission(lowerleft)inthenarrowfilterwillfall.
upandEastistotheleft.TheQSOisidentifiedbya“×”atthe
Inthemiddlepanel,weshowthecolour-colourdiagramfor
fieldcentreandthepositionsofcandidateLAEsareshownwith
alloftheobjectsdetectedinthefield.Thelarge,densegroupof
boxes.
pointscorrespondtothenormalfieldgalaxypopulationwithout
specialpropertiesinthenarrow-bandfilter,inthefollowingref-
Table2.LogofspectroscopicobservationswithFORS2.
eredtoascontinuumobjects.Unfortunately,theobserveddistri-
butionofthecontinuumobjectsisorientedalongthesamedirec-
mask Exp.time Date Effectiveseeing
tionasthatexpectedforemissionlineobjects(indicatedwiththe
(hr) (2004) (′′)
dashedline).Thisis dueto thefactthatthecentralwavelength
mask1202A 6.3 March21-23 0.94
ofthenarrow-bandfilterisbluewardsofthecentralwavelength
mask1202B 6.0 March21-23 0.76
oftheon-bandbroadfilter(V).Forthisreasonweforthisfield
mask1202C 5.0 March21-23 0.96
choosearatherconservativecriterionforselectionofcandidates,
namely N(AB)−V(AB) < −1.Afterweedingoutafew spuri-
oussources(relatedtobrightstars)wedetect25suchobjectsin
theBRI1202−0725fieldwhichweconsiderasLAEcandidates
inthefollowing.Thecoloursofthecandidatesareshowninthe
at the VLT telescope, unit Yepun. The mask preparation was
rightpanelofthefigure.
done using the FORS Instrumental Mask Simulator. The field
Acontourimageofthecombinednarrow-bandimageofthe of BRI1202−0725was coveredby 3 masks. Itwas possible to
400×400arcsec2 fieldsurroundingtheQSOBRI1202−0725is
fit all candidates into the slits. The spectra were obtained with
shown in Fig. 3. The QSO is identified by a “×” at the field
theG600Bgrismcoveringthewavelengthrangefrom3600Åto
centre and the positionsof selected LAE candidatesare shown
6000Åataresolvingpowerof900.Forpossibleidentificationof
withboxes.
otheremissionlinesatthesameredshiftwealsoobtainedspec-
IntheselectionofLAEcandidatesweapplyacolourselec-
tra with the G600R grism coveringthe wavelength range from
tion which translates into the equivalent width (EW) selection
5000Å to 7500Å. The detector pixels were binned 2×2 for all
criterion illustrated in Fig. 4 given in observed quantities. The
observations through the masks. In Table 2 we give the main
selection criterion translates into candidates having observed
characteristicsofthespectroscopicobservations.
EW&100Å(orrestframeEW&25Å).
TheMOSdata were reducedandcombinedasdescribedin
Fynboetal.(2001).Theaccuracyinthewavelengthcalibration
2.3.Multi-objectspectroscopy is about±0.1pixelfora spectralresolutionof R = 900,which
translates to ∆z = 0.0002. Average object extraction was per-
The aboveselection of LAE candidatesonly selects for excess formed within a variable window size matching the spatial ex-
emission in the narrow-band filter. This is likely to originate tensionoftheemissionline.Therefore,thefluxshouldbecon-
fromtheLyαatz ∼ 3.2(Fynboetal.2003),however,interlop- served.
erscausedbyotheremissionlinesingalaxiesatlowerredshifts Ofthe25targetedLAEcandidates,18are confirmedemis-
isalsopossible.Therefore,follow-upspectroscopyisnecessary sionlineobjects.Forthesevenremaningcandidatesnoemission
forconfirmingtheLyαoriginoftheexcessemission. linewasidentifiedinthespectrum.Thespectraforthe18con-
Follow-upmulti-objectspectroscopy(MOS)wascarriedout firmedcandidatesareshowninFig.5.We consideracandidate
in visitor mode on March 21–23, 2004, with FORS2 installed confirmedif thereis an emission line detected with at least 3σ
4 L.F.Groveetal.:TheBuildingtheBridgeSurveyforz=3Lyαemittinggalaxies.II.Completionofthesurvey
Fig.2. Leftpanel:Colour-colourdiagramforsimulatedgalaxiesbasedonBruzual&Charlot1995galaxySEDs(Bruzual&Charlot
2003).Theopensquaresare0<z<1.5galaxieswithagesrangingfromafewto15Gyrandtheopentrianglesare1.5<z<3.0galaxies
withagesrangingfromafewMyrto1Gyr.Thedottedboxenclosesthesimulatedgalaxycolours.Thedashedlineindicatescolours
of objects with a particular broad-bandcolour and with an SED in the narrow-bandfilter rangingfrom an absorption line in the
upperrightparttoastrongemissionlineinthelowerleftpartofthediagram.Middlepanel:Colour-colourdiagramsforallobjects
detected at S/N>5 in either the narrow band or the V band in the BRI1202−0725. In the lower left part of the diagram there
is a large number of objects with excess emission in the narrow filter. Candidate LAEs are those objects with a 1σ upper limit
on N(AB)−V(AB) below the 98% percentile. Right panel: The colours of the QSOs and of the LAE candidates are indicated.
CandidatesconfirmedtobeemissionlineobjectsbasedonourspectroscopicobservationsdescribedinSect.2.3areshownbyfilled
symbols.Candidatesthatcannotbeconfirmedbythepresentspectroscopyareindicatedbyopensymbols.Thesolidlineindicates
themedianN(AB)−V(AB)colourforalldetectedobjectsasafunctionofN(AB)−R(AB)andthedottedlinesindicatethe2%and
98%percentilesintheN(AB)−V(AB)colour.ThehorisontaldottedlinedenoteourselectionofN(AB)−V(AB)<−1.
confirmed emission line candidates are most likely to be Lyα
based on the absence of other emission lines at longer wave-
lengths (mainly NeIII, Hβand O[III], the positions of which
are allcoveredbyour spectroscopy).The limitson the fluxra-
tioswe inferaresimilartothosederivedinFynboetal.(2001)
and based on their analysis contamination from OII emitters
is very unlikely. The overall efficiency for detection and con-
firmation of LAEs is therefore #(confirmed LAEs)/#(observed
LAEs)=18/25=72%.ThisisthesameasfoundforQ2138−4427
andsomewhatlessthantheresultsofBRI1346−0322(PaperI).
Theredshiftdistributionforthe LAE sample in the field of
BRI1202−0725isshowninFig.6andcomparedwiththefilter-
curve.Itcanbeseenthatthetargetsfilloutthevolumeprobedby
thefilter.ThemeanredshiftoftheLAEsis3.203withastandard
deviationof0.013.
We have investigated the properties of the non-confirmed
candidates and find that they are in the red end of the broad-
bandcoloursofthecandidatesampleandunresolved.However,
theirlowline fluxespreventthemfrombeingconfirmedbythe
Fig.4. Ourequivalentwidth selectioncriterionis illustratedby presentspectroscopicobservations.Thisanalysiswascarriedout
plotting the line equivalent width against the Lyα flux in the forallthreefieldsandwefoundsimilarpropertiesinallcases.
isophotalapertureforthe LAEcandidates.Thecontinuousline
shows our N(AB)−V(AB) colour selection criterion converted
into equivalent width. Filled symbols indicate objects subse- 3. TheLAEpopulation
quentlyconfirmedbyspectroscopytobeemission-linesources.
In the following we combine the results of the entire Building
Open circles indicate objects that were observed, but not con-
the Bridge Survey to characterise the population of LAEs.
firmed.TheerrorbarsonEWsarederivedasdescribedinFynbo
The survey constitutes observationsof three fields centered on
etal.(2002).
the quasars BRI1202–0725,BRI1346–0322and Q2138–4427
searchingforLyαemittersatredshiftsz = 3.20,3.15and2.85,
respectively. These redshifts are targeted due the presence of
highcolumndensity absorptionsystems alongthe line of sight
significanceatthecorrectpositionintheslitletwithinthewave- towards the quasars. In the fields 25, 26 and 36 emission line
lengthrangecorrespondingtothefiltertransmission.Allofthe objects were identified through the narrow-band technique. In
L.F.Groveetal.:TheBuildingtheBridgeSurveyforz=3Lyαemittinggalaxies.II.Completionofthesurvey 5
Fig.5.16×16arcsec2 imagesand1-Dspectraof18(notethatcandidates#8and11arecoveredbythesamespectrum)confirmed
LAEs in the field of BRI1202−0725.The hatched areashave been excludedfromthe analysisdue to the overlapwith night-sky
lines.Theunitsontheordinateofthespectraarecountsin1800sec.Thename,redshiftandcoordinates(Epoch2000)areprovided
foreachobject.Foreachcandidate,weshowimagesfromtheN-,V-andR-bandfilters(fromlefttoright)withNorthupandEast
totheleft.
6 L.F.Groveetal.:TheBuildingtheBridgeSurveyforz=3Lyαemittinggalaxies.II.Completionofthesurvey
Fig.6. Redshift distribution of LAEs in the field of
BRI1202−0725 relative to the filter transmission curve.
TheredshiftsofLAEsfilloutthevolumeprobedbythefilter.
eachofthefieldsofBRI1346–0322andQ2138–4427theemis-
sionfromtwoemitterswereidentifiedwithotheremissionlines
than Lyα (in three cases correspondingto [OII] and the fourth
caseCIV, Fynboetal.2003).Therefore,thesefourobjectshave
beenexcludedfromthecurrentanalysis.Inthefollowinganaly-
sisweconsider18,18and23spectroscopicallyconfirmedLAEs
astheconfirmedsample.Theentirephotometricsampleincludes
7, 6 and 11 additional objects. These have not been confirmed
spectroscopically.ForthefieldsofBRI1202–0725andQ2138–
4427 all candidates were observed, so the non-confirmed sys-
tems were lacking an emission line at our sensitivity. For the
field BRI1346–0322three candidates were not observed leav-
Fig.7.Totalmagnitude(MAG AUTO)distributionoftheLAEs
ingthreeasspectroscopicallynotconfirmedcandidates.
inNarrow-(top)andR-band(bottom).Thecurvesareaverages
The LAE population is characterised in terms of magni-
betweenfieldsanderrorbarsarethecorrespondingstandardde-
tudes, colours and derived entities like Lyα flux, luminosity,
viations.Opencirclesanddottedlinesmarkthedistributionfor
EWs and star formation rates (SFRs). For total magnitudeswe
allphotometricallyselectedcandidates,whilefilledsymbolsand
usetheSExtractorMAG AUTOwhichareusedtocomputethe
solidlinesmarktheconfirmedLyαemitters.Thetrianglesindi-
Lyαflux,luminosityandSFR.Thecolourindicesarecomputed
cate1σupperlimitsforgalaxiesdetectedatlowerlevels.Thear-
basedonisophotalmagnitudes(MAG ISO)andareusedtoesti-
rowinthelowerplotmarkstheR=25.5limitforspectroscopic
matetheEW.Forallmagnitudesthesamedetectionareaisused
surveysofabsorptionlinesystems.Itshouldbenotedthatforthe
inallbands.Detailsofthecomputationofthederivedproperties
candidateselectiontheisophotalmagnitudeswereusedandhere
canbefoundinFynboetal.(2002).Finally,inthenextsection
we use totalmagnitudes.All magnitudeshavebeenrescaled to
wederivetheluminosityfunctionofLAEsatz∼3andcompare
z=3,thoughneglectingthesmalldifferencesink-correctionfor
this with the recent measurements by Gronwalletal. (2007);
theR-band.
Ouchietal. (2008) and Rauchetal. (2008). Table 3 gives an
overview of the LAE samples of the three fields. From the ta-
bleitcanbeseenthatthesurveycoversLAEswithluminosities
inLyαdowntoafewtimes1041 ergs s−1,makingitoneofthe
deepestsurveysofLyαemittersatz≈3.Table4summarisesthe nitudes. In this case the limits are caused by a less significant
main characteristicsof our sample and the three used for com- detection in the on-band broad (B or V) and the values corre-
parison. spondtousingthe1σlevelsforthesebands.
FromTable3itcanimmediatelybeseenthatthetotalnum-
berofobjectsiscomparableamongthethreefields.Thedetected
3.1.CharacteristicsofspectroscopicallyconfirmedLAEs
numberofLAEstranslatesintosurfacedensitiesof8,8and10
InTablesA.1–A.3wegivethemeasuredandderivedproperties per square arcmin per unit redshift, respectively for the fields
fortheconfirmedLAEs.Themagnitudesinthesetablesaretotal ofBRI 1202–0275,BRI 1346–0322andQ 2138–4427,consis-
magnitudes,andthelowerlimitscorrespondto1σsignificance tentwiththealmostsimilarfluxlimitsinthethreefields.From
levelsfortheSExtractorMAG AUTOapertures.Thesetranslate thetableitcanalsobeseenthattheotherpropertiesaresimilar
into the limits given for the fluxes and luminosities. The EWs between the fields and between the confirmed LAEs and non-
arederivedfromthecolourindicesbasedontheisophotalmag- confirmedcandidates.
L.F.Groveetal.:TheBuildingtheBridgeSurveyforz=3Lyαemittinggalaxies.II.Completionofthesurvey 7
Table3.OverviewofLAEsamplesfromthethreefieldsincludedinthepresentsurvey.
Field z N a N b N c Dens.(conf) EW EW L (Lyα)d L (Lyα)d SFR SFR
tot conf non 0,conf 0,non conf non conf non
arcmin−2∆z−1 Å Å 1041ergs−1 1041ergs−1 M yr−1 M yr−1
⊙ ⊙
BRI1202–0275 3.20 25 18 7 8 ≥27 ≥21 ≥6.06 ≥4.07 ≥0.62 ≥0.42
BRI1346–0322 3.15 24 18 6 8 ≥22 ≥26 ≥12.21 ≥9.92 ≥1.25 ≥1.02
Q2138–4427 2.85 34 23 11 10 ≥20 ≥18 ≥3.66 ≥4.58 ≥0.38 ≥0.47
a NumberofLAEsinthefield.InthefieldsofBRI1346–0275 andQ2138–4427 twoforegroundemissionlinesystemshavebeenexcluded
fromthissample.
b NumberofconfirmedLAEsinthefield.
c Numberofcandidatesnotspectroscopicallyconfirmed.
d Lowerlimitluminosityforthosecandidatesthataredetectedwithasignificanceofatleast1σinthetotalmagnitude.
Table4.Summaryofthemainpropertiesofourandthecomparisonsamples.
Ref. Type z N Area Lyαluminosity
sq.arcmin ergs s−1
Thissample Spectroscopic/photometric ∼3.0 59/83 133 &0.3·1042
Aroundabsorptionsystems
Serendipitous Spectroscopic 2.30≤z≤3.61 6 − −
Field
Gronwalletal.(2007) Photometriconly ∼3.1 162 1008 &1.3·1042
Field
Ouchietal.(2008)a Photometriconly ∼3.1 356 3538 &1042
Field
Rauchetal.(2008) Long-slitspectroscopy 2.67≤z≤3.75 27 − (0.015−1.73)·1042
Field
a Theseauthorshavealsocarriedoutspectroscopicobservationsforasubsampleof41candidates.
InFig.7weshowthemagnitudedistributionfortheLAEs. with our survey being deeper than the comparison samples. It
The lines correspond to candidates with measured magnitudes canalso be seen thatthe Ouchietal. sample hasa higherfrac-
while the triangles indicate systems with 1σ upper limits. The tion of emitters with high EW than the Gronwall et al sample.
observed magnitudes are translated to correspond to a redshift OursampleitismoreconsistentwiththeOuchietal.thanwith
z = 3 by correcting by the difference in the distance modulus theGronwalletal.sample.
between the observed redshift and z = 3. For the R-band we Table 3 also lists the range of SFR derived from the Lyα
donotapplyanyK-correctionsinceitisanegligibleeffectover luminosities(followingFynboetal.2002).TheSFRsarefound
such small redshift changes. The errorbars are standard devia- tobeintherange0.38M yr−1to31M yr−1assumingnegligible
⊙ ⊙
tionsamongthefieldsandindicatethateventhoughtheresults extinction.
inthethreefieldsareconsistent,smallnumbersandcosmicvari-
ance leadsto significantdifferencesbetweenthe fields. For the
4. LuminosityfunctionofLAEsatz ∼ 3
narrow-banditisclearthatallthebrightobjectshavebeencon-
firmed to correspond to Lyα emitters while the non-confirmed Awidelyuseddiagnosticfordescribinggalaxysamplesistheir
casesareingeneralfoundinthefaintendofthedistribution. luminosity function (LF). Here, we derive the LF of LAEs at
FromthemagnitudedistributionoftheR-banditcanbeseen z = 3.0 combining the data from the present work and from
thatalltheLAEcandidateshaveveryfaintmagnitudes.Forab- Paper I. The complete survey encompasses three fields with a
sorptionlinesystemsthemagnitudelimitofachievingaredshift totalof59spectroscopicallyconfirmedLAEs.Foreachfieldwe
is R∼25.5 indicated by the arrow in the figure. Therefore, we derive the LF independently to take into account the different
notethatoftheconfirmedemittersonly6outof59or.10%are narrow-band filters and incompleteness functions. Finally, the
brighterthanthislimit.Thefractionissimilarforthecomplete individualresultsarecombinedtoprovidetheLFatz=3.0.
candidatesample.Thisisconsistentwiththenon-confirmedcan-
didates being spread over all R-band magnitudes. The faint R-
4.1.Estimatingtheincompletenesscorrection
band magnitudes of the galaxies emphasises their importance
fortracingthefaintendoftheluminosityfunction,possiblycon- When constructing the sample of LAEs the main source of in-
tributingasignificantfractionofthetotalstarformation(seealso completeness comes from the photometric selection of candi-
Reddy&Steidel2008). dates. The following process is carried out separately for each
Fig.8showsthedistributionofrestframeequivalentwidths fieldto takeintoaccountthespecific propertiesofthedifferent
derived from the colour index “narrow band minus on-band data sets. To estimate the detection completenessof the photo-
broad”isophotalmagnitudes.Wehavenotcorrectedthebroad- metricsampleswedistribute100artificialobjectsintheimages
band magnitudes for the narrow-band contribution. Our mea- andassignbroad-andnarrow-bandmagnitudesina rangecov-
surementsarecomparedwiththoseofGronwalletal.(2007)and ering the observedvalues as well as an additionalmargin.The
Ouchietal.(2008).Itcanbeseenthatthesampleofconfirmed magnitudeswere assigned based on the off-broadband. In this
LAEsforwhichwecouldmeasuretheEWreliablyisconsistent bandmagnitudesaredistributeduniformlyintheinterval5to35.
8 L.F.Groveetal.:TheBuildingtheBridgeSurveyforz=3Lyαemittinggalaxies.II.Completionofthesurvey
Fig.9.Completenesslevelsas functionof Lyαluminosity.The
dotted line correspondsto the field of BRI 1202–0725;dashed
line to BRI 1346–0322 and triple dot-dashed line to Q 2138–
4427.
ofanon-squareband-passorthephotometricerrorfunction(see
Gronwalletal.2007).
A possible concern is whether the extension of the Lyα
emitters have a significant effect on the estimated complete-
nessfunctions.Totestthiswehavecarriedoutacomplementing
testwheretheoriginalbackgroundsubtractedimageswerefirst
Fig.8. Rest frame equivalent width distribution for the con-
dimmed.Thenwecorrectedthebackgroundnoisetotheoriginal
firmed (solid symbols and line) and entire (open symbols and
levelandtriedtorecoverthealreadydetectedcandidateemitters.
dashed line) samples (upperpanel). In the lower panelthe dis-
We dimmedthe imagesby0.25,0.5and0.75magnitudes.The
tribution for the confirmed sample is compared with that of
completenessfunctionsestimatedinthiswayweresimilartothe
(Gronwalletal. 2007, dot-dashed line) and (Ouchietal. 2008,
ones described above, but suffer from poor statistics (only 80
dashedline). The trianglesindicate cases forwhich onlylower
sourcesintotalwerefoundintheoriginalimages)andonlytrac-
limits(atthe1σlevel)couldbedetermined.
ing the magnitudesand colour indices of the detected sources.
Thismakesthepreviouslydescribedfunctionsmorereliablefor
correctionpurposesandaretheonesusedinthefollowing.
Forthe narrow-andon-broadbandsmagnitudeswereassigned
4.2.Lyαluminosityfunction
basedonauniformcolourindex(“narrow−off-broad”and“on-
broad−off-broad”)distributionintheintervalminusfivetofive. Wederivethedifferentialluminosityfunctionfortheconfirmed
This results in uniform magnitude distributions in the narrow- LAEs with a simple classical approach that has been used in
andon-broadbandsinthemagnitudeinterval10to30dropping manypreviousworks(e.g. Ouchietal.2003;Ajikietal.2003;
offandvanishingatmagnitudesofzeroand40.Theobjectsare Huetal. 2004; Malhotra&Rhoads 2004; Ouchietal. 2008).
modeled with a two-dimensional gaussian shape with the size Thevolumeusedtoconverttheobservednumberofsystemsto
correspondingtotheseeingmeasuredintheindividualimages. volume densities is computed based on the field size and hav-
Each artificial object is added to each image scaling the gaus- ingadepthcorrespondingtotheFWHMofthenarrow-bandfil-
siantotheappropriateluminositydependingonthemagnitudes ter. Furthermore, we correct the observed luminosity function
computedindividuallyforeachband.Thisprocessensuresgood by the completenessfunctionestimated in the previoussection
coverageforbothmagnitudesandcolourindices.Theprocedure toaccountfortheincompletenessofthesampleduetoourtarget
is repeated 1000 times for each field, thus a total of 105 artifi- selection.TheobtainedluminosityfunctionisshowninFig.10.
cial objects are used in the estimate. The images with artificial Theerror-barsarederivedfromtheerrorsintheindividualfields
objects are now treated exactly as the original images and the bystandarderrorpropagation.
recoveryrate yieldsthe detectioncompleteness.Thecomplete- WefitourderivedLFwiththeSchechterfunction(Schechter
nessfunctionsestimatedforthethreefieldsinthenarrow-band 1976) using a χ2-minimisation. We carry out three fits in
filtersareshowninFig.9.Fromthatfigureitcanbeseenthatthe total. First, we fit the sample of confirmed candidates with
data of the field of BRI 1346–0322are slightly shallower than L∗ >= 1042ergss−1. Second, we fit the entire photometric
theothertwofields,whichareverysimilar.Thesecompleteness sample in two luminosity intervals: L∗ >= 1042ergss−1 and
functionsareusedtocorrectthemeasuredluminosityfunctions L∗ >= 1041.6ergss−1. The brighterlimit correspondsto the re-
below. Note that we have notcorrected our data for the effects gionwhereallfieldsareestimatedtobeclosetocomplete,while
L.F.Groveetal.:TheBuildingtheBridgeSurveyforz=3Lyαemittinggalaxies.II.Completionofthesurvey 9
Fig.11. FORS1 narrow-bandimage centered on Q2138−4427
(leftpanel).Northis upandEastisto the left.Thewavelength
at peak transmission of the (Heii) narrow-band filter used cor-
responds to Lyα emission at z = 2.85. The PSF of the QSO
was carefullysubtractedwhichrevealsa sourceonly1′.′4 away
fromtheQSOline-of-sight.Thedifferentorientationsoftheslits
duringtheMOSobservations(PaperI)andthecomplementary
deep long-slit spectroscopic follow-up are shown by, respec-
tively,dottedanddashedlines.Thearrowintheupperpartofthe
image marks the location of the closest LAE (LEGO2138 36)
that was selected by comparing the narrow-band image with
theFORS1 BandRbroad-bandimages.Intherightpanel,the
B−bandimageisshownwiththesame scale andorientationas
thenarrow-bandimage.
bright objects may be caused by enhanced clustering of bright
LBGsaroundQSOabsorberfields(Bouche´&Lowenthal2004;
Bouche´etal.2005).Comparingthedensityofgalaxiesbrighter
than the limit of Gronwalletal. (2007) of 13·1041ergss−1 we
find only a marginal excess of a factor of ∼ 1.25. It is also
interesting to compare these environments with the results of
Venemansetal.(2007)studyingradiogalaxies.Theradiogalax-
Fig.10.ThederiveddifferentialLyαluminosityfunctionforthe
ies traces the largest overdensities, likely representing proto-
sample of confirmed (upper) and all (lower) LAEs. The points
clusters. The overdensity of LAEs around radio galaxies are
with open symbolsare notcorrectedforincompleteness,while
found to be of the order 2-5. Thus the overdensities found in
thosewithsolidsymbolsare.Thethinsolidlineindicateourfit
ourfieldsare,asexpected,smaller.InthefieldofQ2138–4427
totheconfirmedcandidates.Thethicksolidanddottedlinesin
the redshifts are concentrated relative to the width of the filter
thelowerpanelmarkthefitstothedeepandbrightersampleof
function (Paper I, see also Monacoetal. (2005)). In this field
allcadidates,respectively.Theerrorbarsareobtainedfromthe
theoverdensityisonlyafactorof1.1withrespecttotheresults
1σ-errorbarsoftheindividualfieldsbystandarderrorpropaga-
ofGronwalletal.(2007),thuswedonotconsiderthisgoodev-
tion.
idenceforaproto-cluster.
Thefitsaredescribedindetailinthetext.Ourresultiscomparedwith
theluminosityfunctionforemittersatz=3.1byOuchietal.(2008)in
triple-dotdashedline,Gronwalletal.(2007)indot-dashedlineand
5. GalaxycounterpartsoftheQSOabsorbers
Rauchetal.(2008)indashedline.
One of the goals of our survey was to bridge the gap between
emission and absorption selected objects. The three studied
fieldsarecenteredonQSOswithhighcolumndensityabsorption
the second limit is where the shallowest field is essentially not systemsattheredshiftforwhichLyαfallsinthenarrow-bandfil-
contributinganything(seeFig.9).Theresultsofthefitsaresum- ters.InthefieldsofBRI1346−0322andBRI1202−0725theab-
marisedinTable5.InFig.10weincludethefittotheconfirmed sorbersareLyman-limitsystemsandinthefieldofQ2138−4427
candidatesasthethinsolidlineinbothpanelsandthefittothe itisaDLA.TheobservedfluxofQ2138−4427isreducedbyap-
deep sample of all candidatesis includedin the lower panelas proximately1.5magnitudeinthenarrow-bandimageasshown
thethicksolidline.Thefittoallcandidateswiththebrighterlu- in Fig. 3 of Paper I. This is due to the strong DLA line that
minositylimitisincludedasthedottedline.Itcanbeseenthat absorbsthe QSO lightmakingit mucheasier to search for any
despitethesmalldifferencesinthebestfitparametervaluesthe faintemissionclose to the QSO in thenarrowbandthanin the
functionsareconsistent. broad-bandimages. Moreover, if the galaxy counterpart of the
WecompareourresultwiththoseofGronwalletal.(2007); z = 2.852 DLA system has Lyα emission, it would be rela-
abs
Ouchietal.(2008)andRauchetal.(2008).Atthefaintendwe tivelybrighterinthenarrow-bandthaninthebroad-bandimages.
findagoodagreement,inparticularifallthenon-confirmedcan- TosubtracttheimageoftheQSOinthenarrow-bandimage,we
didatesareLAEs.Atthebrightendwefindamarginalexcessof usedPSF-subtraction.WemodelledthePSFoftheQSOfromten
objectscomparedtotheothersurveys,eventhoughthenumbers bright,yetunsaturated,starspresentin the field.Theextension
are consistent within the error bars. The (marginal) excess of ALLSTAR of the DAOPHOT-II software (Stetson 1987, 1994)
10 L.F.Groveetal.:TheBuildingtheBridgeSurveyforz=3Lyαemittinggalaxies.II.Completionofthesurvey
Table5.ResultsoffittingaSchechterfunctiontotheLF.
Type log(L∗) α φ χ2
(ergs s−1) h−3Mpc3
Confirmed,log(L∗)>=42 43.55+0.05 −1.80+0.08 1.5+0.3·10−4 0.32
−0.10 −0.06 −0.3
All,log(L∗)>=42 43.30+0.05 −1.74+0.06 3.2+0.5·10−4 0.28
−0.05 −0.04 −0.5
All,log(L∗)>=41.6 43.15+0.05 −1.58+0.06 5.8+0.8·10−4 2.06
−0.05 −0.04 −0.8
FORS1long-slitspectrumwitha1′.′0-wideslitandtheG600B
grismcoveringthecandidateunderverygoodseeingconditions
(FWHM=0′.′71inthecombinedspectrum).Thepositionofthe
longslitisshownwithdashedlinesinFig.11.The2-Dlong-slit
spectrumisdisplayedintheuppertwopanelsofFig.12.Weused
the spectral PSF II optimal extraction algorithm (Mølleretal.
2000)tosubtracttheemissionfromtheQSOinthewingsofthe
DLAtrough.NoLyαemissionfromthecandidateDLAgalaxy
counterpartisdetected.
We concludethat the source seen in Fig. 11 is not an LAE
at z = 2.85.Possible Lyα emission from this source should be
fainterthan the 3σ verylow detectionlimit of ∼ 4×10−18 erg
s−1 cm−2. Within 1′′ from the QSO line-of-sight,our detection
limit for Lyα emission is even lower (by less than a factor of
twothough)duetothecombinedlimitsfromboththeMOSand
long-slitspectra.ThenearestconfirmedLAEisLEGO2138 36
(Paper I) and it is situated at a distance of 8′′ from the QSO
line-of-sight. This means that if the z = 2.852 DLA galaxy
abs
counterpartisanLAEitshouldactuallybefainterthanallofthe
LAEsdetectedinoursurvey.
WhatcouldbetheoriginofthesourceseeninFig.11?One
possibility is that this source is associated to the galaxy coun-
Fig.12. (a) portion of the 2-D long-slit spectrum of terpart of the lower redshift, z = 2.383 DLA system toward
abs
Q2138−4427centeredontheLyαtroughoftheDLAsystemat Q2138−4427.Inthiscasetheemissiondetectedinthenarrow-
z =2.852(firsttoppanel).Inthesecondtoppanel,thePSFof bandimagewouldbecontinuumemission.Makingthisassump-
abs
theQSO hasbeensubtractedusingtheSpectralPSFIIoptimal tion,weestimateabroad-bandmagnitudeofB(AB)≈ N(AB) =
extractionalgorithm(Mølleretal.2000).(b)sameasabovefor 25.7forthisobject.InthelowertwopanelsofFig.12,weshow
the DLA system at z = 2.383 toward Q2138−4427(bottom the 2-D spectrum of the QSO around the corresponding DLA
abs
twopanels).ThehostgalaxiesoftheDLAsystemsaredetected trough. No Lyα emission is detected at more than the 5σ con-
inneithercase.Theobjectseeninthenarrow-bandimageshown fidencelevel.Therearetwo3σpeaksinthe spectrumhowever
inFig.11mustbeacontinuumsource. butweconsiderthesetoouncertaintoallowfurtherdiscussion.
Another interesting possibility is that the source is part of the
hostgalaxyofQ2138−4427.
wasusedtoperformthefinalPSFmodelfittingandsubtraction.
In none of the other two fields do we see any evidence for
AftercarefulPSFsubtraction,wedetectedanextendedsourceat
a narrowbandsourceclosetotheQSO positionafterPSFsub-
animpactparameterof1′.′4(correspondingto12kpcatz=2.85)
traction.Note,however,thatforthesesightlinestheintervening
from the QSO line-of-sight at a position angle of 136.5o (see
absorbersare only Lyman-limitsystems so the fluxes from the
Fig. 11). The proximityofthis sourceto the QSO line-of-sight
QSOsaremuchlessreducedbytheabsorptionsystemsthanfor
makesitagoodcandidatefortheDLAgalaxycounterpart.
Q2138−4427.
ToestablishwhetherthesourceseeninFig.11isthegalaxy
counterpartofthez =2.852DLAsystem,wegatheredspectra
abs
ofQ2138−4427usingbothFORS1Multi-ObjectSpectroscopy 5.1.SerendipitouslydetectedLAEs
(MOS) and long-slitspectroscopy.The details of the MOS ob-
servationsweregiveninPaperI.ThepositionsofthetwoMOS ThecombinedFORS12-Dlong-slitspectrumcoversasolidan-
slitlets that covered the QSO are marked with dotted lines in gle of 370×1 arcsec2 and the wavelength range from 3600 Å
theleftpanelofFig.11.Oneofthe1′.′4-wideMOSslitletscov- to 6000 Å. This spectrum is therefore sensitive to LAEs with
ered both the QSO and the candidate DLA galaxy counterpart redshiftsintherange2.0<z<3.9.Assumingaconstantsource
while the second MOS slitlet was oriented at a position angle densityofapproximatelytenLAEsperarcmin2perunitredshift,
about70◦ smaller. Noneofthese spectrashowedLyαemission asisfoundinoursurveyforz ∼ 3LAEsdownto afluxdetec-
from the DLA absorber. However, the atmospheric conditions tionlimitofN(AB) = 26(correspondingtoaLyαfluxofabout
duringtheMOSobservationswerequitepoor(seePaperI)and 7×10−18 erg s−1 cm−2), we expect to serendipitously observe
weconsequentlyobtainedadeeper(∼5hrtotalintegrationtime) aboutsixLAEsinthelongslit.Thisnumberisofcourseonlya