Table Of ContentMon.Not.R.Astron.Soc.000,1–9(2011) Printed6January2011 (MNLATEXstylefilev2.2)
Galaxy Environments in the UKIDSS Ultra Deep Survey (UDS)
R. W. Chuter1⋆, O. Almaini1, W. G. Hartley1, R. J. McLure2, J. S. Dunlop2, S. Foucaud3,
C. J. Conselice1, C. Simpson4, M. Cirasuolo2 and E. J. Bradshaw1
1UniversityofNottingham,SchoolofPhysics&Astronomy,Nottingham,NG72RD
2InstituteforAstronomy,UniversityofEdinburgh,RoyalObservatory,EdinburghEH93HJ
3DepartmentofEarthSciences,NationalTaiwanNormalUniversity,No.88,Section4,TingzhouRoad,WenshanDistrict,Taipei11677,Taiwan
1 4AstrophysicsResearchInstitute,LiverpoolJohnMooresUniversity,TwelveQuaysHouse,EgertonWharf,BirkenheadCH411LD
1
0
2
n Accepted2010December20.Received2010December8;inoriginalform2010August4
a
J
4 ABSTRACT
Wepresentastudyofgalaxyenvironmentstoz∼2,basedonasampleofover33,000K-band
] selectedgalaxiesdetectedintheUKIDSSUltraDeepSurvey(UDS).Thecombinationofin-
O
frareddepthandareaintheUDSallowsustoextendpreviousstudiesofgalaxyenvironment
C to z > 1 without the strong biases associated with optical galaxy selection. We study the
. environmentsofgalaxiesdividedbyrestframe(U −B)colours,inadditionto‘passive’and
h
‘star-forming’subsetsbasedontemplatefitting.Wefindthatgalaxycolourisstronglycorre-
p
latedwithgalaxyoverdensityonsmallscales(< 1Mpcdiameter),withred/passivegalaxies
-
o residinginsignificantlydenserenvironmentsthanblue/star-forminggalaxiestoz ∼ 1.5.On
r smaller scales(< 0.5Mpc diameter)we also finda relationshipbetweengalaxyluminosity
st and environment, with the most luminous blue galaxies at z ∼ 1 inhabiting environments
a comparable to red, passive systems at the same redshift. Monte Carlo simulations demon-
[
stratethattheseconclusionsarerobusttotheuncertaintiesintroducedbyphotometricredshift
1 errors.
v
Keywords: GalaxyEvolution,Environment,DeepSurveys–Infrared:Galaxies.
9
4
8
0
.
1 1 INTRODUCTION Hα (Baloghetal. 2004) found that it’s strength does not depend
0 on environment but that the fraction of galaxies with equivalent
Ithaslongbeenknownthatthepropertiesofgalaxiesdependonthe
1 width, W0(Hα) >4A˚ is environmentally dependant, decreasing
environmentinwhichtheyarelocated.Elliptical,non-star-forming
1 withincreasingdensity.Theyalsonotedthatemissionlinefraction
: galaxies occupy more dense regions of space than star-forming, appearstodependonboththelocalenvironment(∼1Mpc)andon
v disc-dominatedgalaxies,givingrisetotheso-calledmorphology-
i thelargescalestructure(∼5Mpc).
X densityrelation(Oemler1974;Dressler1980).Thephysicalorigin
Studies at higher redshifts (z ∼ 1) have used surveys such
ofthisrelationisstillsubjecttodebate,withdisagreementmainly
r as DEEP2 (Davisetal. 2003) and VVDS (LeFe`vreetal. 2005).
a centeringonwhethertherelationarisesduetointernalorexternal
DEEP2 investigations (Cooperetal. 2006; Cooperetal. 2007)
processes(naturevsnurture).
usedtheprojectedthirdnearestneighbourstatistic,studyinggalaxy
Most recent low redshift studies (Kauffmannetal. 2004;
properties and thecolour-density relation respectively. They con-
Baloghetal. 2004) utilise the Sloan Digital Sky Survey (SDSS)
cluded that there is a strong dependence on rest frame (U −B)
or the Two-degree-Field Galaxy Redshift Survey (2dFGRS)
colour, with blue galaxies occupying lower density regions but
to conduct statistical investigations of galaxy environments.
showing a strong increase in mean local density with luminosity
Kauffmannetal.(2004)constrainedthespecificstarformationrate
atz∼1.Thistheyconcludeisconsistentwiththerapidquenching
(SSFR)usingthe4000A˚ breakandfoundthattheSSFR(andnu-
ofstarformationbyAGNorsupernovafeedback,asrampressure
clear activity) depend most strongly on local density, from star-
stripping,harassmentandtidalinteractions,whichoccurpreferen-
forminggalaxiesatlowdensitiestopredominantlyinactivesystems
tially in clusters, would be insufficient to explain these findings.
athighdensities.
Cooperetal.(2007)alsoobservedthatthefractionofgalaxieson
Studies by vanDerWeletal. (2008) and Bamfordetal.
theredsequenceincreaseswithlocaldensity,asinthelocalUni-
(2008)foundthatstructure,colourandmorphologyaremainlyde-
verse,butthisweakenswithredshiftanddisappearsbyz∼1.3.
pendent on galaxy mass but that at fixed mass, colour and, to a
TheVIMOSVLTDeepSurvey(VVDS)investigatedthered-
lesserextent,morphologyaresensitivetoenvironment.Studiesof
shiftandluminosityevolutionofthegalaxycolour-densityrelation
uptoz∼1.5(Cucciatietal.2006).InagreementwithCooperetal.
⋆ E-mail:[email protected] (2007) they found that the local colour-density relation progres-
(cid:13)c 2011RAS
2 R. W. Chuter et al.
sively weakens and possibly reverses in the highest redshift bin 1.5
(1.2<z<1.5).Thismayimplythatquenchingofstarformationwas
more efficient in high density regions. The VVDS team also ob-
1
servedthatthecolour-densityrelationsdepend onluminosityand
found that at fixed luminosity there is a decrease in the number est
of red objects as a function of redshift in high density regions. B)r 0.5
−
Thisimpliedthatstarformationendsatearliercosmicepochsfor (U
moreluminous/massivegalaxies,whichisconsistentwithdownsiz-
0
ing(Cowieetal.1996).Wenote,however,thattheVVDSsurvey
isbasedonopticalI-bandselection,andassuchwillbestrongly
−0.5
biased against red, passive galaxies at z > 1. Conclusions from 0 500 1000 −20 −22 −24
deep K-selected samples suggest that the galaxy colour bimodal- N MK (AB)
ity is present to at least z∼1.5 (e.g. Cirasuoloetal. 2007) and
Figure1.RestFrame(U−B)coloursforUDSgalaxiesbetween0.75<
may be still be present at z∼2 (Cassataetal. 2008; Krieketal.
z<1.75.ShownasahistogramandasafunctionofabsoluteK-bandmag-
2008; Williamsetal. 2009). Furthermore red galaxies have been
nitude.Thedivisionbetweenredandblueisshown,usingthered-sequence
seen to strongly cluster at z>1.5(Daddietal. 2003; Quadrietal.
slopederivedbyHartleyetal.(2010).
2007;Hartleyetal.2008;Hartleyetal.2010)whichsuggeststhat
acolour-densityrelationmayalsoexistatthesehigherredshifts.
A number of physical processes may be responsible for the
targetdepthofK=25(AB)overasingle4-pointingmosaicofthe
observed environmental trends. Mergers or tidal interactions can
Wide-fieldcamera(WFCAM,Casalietal.2007),givingtheUDS
teargalacticdiscsapartandarelikelytoplayanimportantrolein
forming the most massive galaxies observed today (Toomreetal. anareaof0.88x0.88degrees.The5σ,ABdepthswithin2′′aper-
turesfortheJ,HandK-bandsare23.7,23.5and23.7respectively
1972; Faroukietal. 1981). Other processes such as gas stripping
fortheDR3,makingitthedeepestnearinfraredsurveyoversucha
canseverelyreducethestarformationratebyremovingthecoldgas
largeareaatthetimeofrelease.Fordetailsofthestackingproce-
from galaxies falling into massive dark-matter halos (Gunnetal.
dure,mosaicing,catalogueextractionanddepthestimationwere-
1972;Dekel&Birnboim.2006).Feedbackisalsothoughttoplay
ferthereadertoAlmainietal.(inprep.)andFoucaudetal.(2007).
amajor role, either fromAGN or supernovae (e.g. Benson 2005;
The field is also covered by deep optical data in the B, V, R, i′
Springeletal. 2005). These processes may heat or eject the gas
withingalaxiesandthuseffectivelyterminateanyfurtherstarfor- and z′ -bands with depths of BAB=28.4, VAB=27.8, RAB=27.7,
mation, which can rapidly lead to the build-up of the galaxy red i′AB=27.7 and zA′B=26.7 from the Subaru-XMM Deep Survey
sequence. Finally infalling cold gas in low mass dark matter ha- (SXDS) (Furusawaetal. 2008, 3σ, 2′′ diameter). Data from the
losmayfalldirectlyontothegalaxy, whereas inhigh masshalos SpitzerLegacyProgram(SpUDS,PI:Dunlop)reaching5σ depths
the gas is thought to be heated by shocks and therefore remains of24.2and24.0(AB)at3.6µmand4.5µmrespectivelyandU-band
supported(Whiteetal.1991;Birnboimetal.2003).Akeygoalof datafromCFHTMegacam(UAB=25.5;Foucaudetal.inprep)are
alsoutilised,whichresultsinaco-incidentareaof0.63deg2 after
observational extragalactic astronomy is to disentangle which of
masking.
theseprocessesareresponsibleforestablishingthebimodalgalaxy
Thegalaxysampleusedhereisprimarilybasedonselectionin
populationsobservedinthelocalUniverse.
With the recent advent of deep, wide-field infrared imaging theK-bandimage,uponwhichweimposeacutatKAB=23tomin-
imisephotometricerrorsandspurioussourcesandalsotoensurea
in the UKIDSS UDS we can now extend studies of galaxy envi-
highlevelofcompleteness(≃100percent)andreliablephotomet-
ronmentstoz>1.Selectionintheinfraredavoidsthemajorbiases
ricredshifts(seeCirasuoloetal.2010;Hartleyetal.2010).Bright
againstdustyand/orevolvedstellarpopulations,allowingustoin-
starsaretriviallyremovedfromthecombinedcataloguebyexclud-
vestigatewhethercorrelationsobservedatlowredshiftalsooccur
ingobjectsonastellarlocusdefinedby2-arcsecand3-arcsecaper-
at high redshift and how these change over time. The large con-
tiguousareaofthissurveyalsoallowsustoprobeawiderangeof tures,whichiseffectivetoK <18.1.Thefainterstarsareremoved
environmentsusinglargesamplesofgalaxies. usinga(B−z′)−(z′−K)diagram(Daddietal.2004)andthe
The paper is structured as follows: §2 outlines the data and criterion(z′ −K) < 0.3 (B −z′)−0.5. These cuts and care-
fulmaskingofbrightsaturatedstarsandsurroundingcontaminated
selectioncriteriausedinthiswork.§3discussesthemethod used
regionsleave33,765galaxiesinoursample.
toestimategalaxyenvironments.Theresultsarethenpresentedin
§4and§5,with§6summarisingourconclusions.Throughoutthis
paperweassumeaΛCDMcosmologywithΩm=0.3,ΩΛ=0.7and
H0=71kms−1Mpc−1. 2.2 PhotometricRedshifts
Themagnitudes from the DR3catalogue were used todetermine
photometric redshifts and stellar ages by χ2 minimisation using
a large set of templates. This was performed with a code based
2 DATAANDSAMPLESELECTION largely on the HYPERZ package (Bolzonellaetal. 2000) using
bothaveragelocalgalaxySEDsandtemplatesfromtheK20survey.
2.1 TheUKIDSSUltraDeepSurvey
Six SEDs of observed starbursts from Kinneyetal. (1996) were
This work has been performed using the third data release alsoused toimprove thecharacterisation of young blue galaxies.
(DR3) of the UKIRT (United Kingdom Infra-Red Telescope) In- This yeilded photometric redshifts with a δz/(1+z)=0.008 with a
frared Deep Sky Survey, Ultra-Deep Survey (UKIDSS, UDS; standard deviation of σ=0.034 after the exclusion of outliers (for
Lawrenceetal.2007;Almainietalinprep).TheUKIDSSproject more detail see Cirasuoloetal. 2007; Cirasuoloetal. 2010 and
consistsof5sub-surveysofwhichtheUDSisthedeepest,witha references within).Thisalsoprovided rest framemagnitudes and
(cid:13)c 2011RAS,MNRAS000,1–9
GalaxyEnvironmentsin theUKIDSS UltraDeep Survey(UDS) 3
colours,ofwhichtherestframe(U−B)andK-bandabsolutemag- regionsandthenumberofrandomswithintheaperturearecounted
nitudeareutilisedhere.Sourceswithanunacceptablefit(χ2 >15) (NAper).Thenumber of galaxieswithintheaperture isthennor-
r
areremovedfromour sampleasthesearelikelytobeunreliable. malisedtogivethefinaldensitymeasurement,ρ/ρ :
r
Thisremoves4%ofthegalaxysample,themajorityoftheseareei-
ρ NAper NTot
therQSOs(36%),cross-talk(26%)ortheminormembersofpairs = g × r (3)
ormergers(23%),withtheremainderconsistinglargelyofobjects ρr NrAper NgTot
withverylowsurfacebrightness.Thefractionofotherwiseuseful whereNTotandNTotarethentotalnumberofrandompointsand
r g
objectsrejectedistherefore<0.6%.
galaxiesrespectively,sothatρ/ρ =1correspondstoadensitycon-
r
sistentwiththatofarandomdistributionofgalaxies.Thismethod
waschosentobethebasisofthisworkasitisconceptuallysim-
2.3 PassiveSample
ple,andasconcludedbyCooperetal.(2005),thistechniquehasa
Todefineapassivegalaxysubsetwithminimalcontaminationfrom distinctadvantageinfieldsmaskedbyalargenumberofholes.The
dustystarformingobjectsweuseasubsetofgalaxiesoutlinedin nearest neighbour method would require the exclusion of a large
Hartleyetal.(2010).Templateswereusedtofiteitheraninstanta- fractionofdataclosetoholesandfieldedges.
neousburstparameterisedbyanage,oranexponentiallydecaying The nth nearest neighbour method was first employed by
star-formation rate parameterised by an age and τ, the e-folding Dressler (1980), this calculates the distance to the nth nearest
timeintheexponentiallydecliningstar-formationrate,suchthat, galaxy,D inMpcandisexpressedhereasasurfacedensity,
n
SFR=SFR0×e−age/τ (1) Σ = n (4)
n πD2
n
where SFR is the star-formation rate at the time of observation
Thesurfacedensity,Σ isthenrenormalisedsuchthat,
andSFR0 wastheinitialvalue.Wedefineaconservativepassive n
sample as galaxies that are simultaneously old (age>1Gyr) and Σ
δ = n (5)
have ongoing star formation with SFR 6 0.1% of SFR0, and n Σ¯
astarformingsamplewithSFR>10%ofSFR0,with3947and whereΣ¯ isthemediandensityofgalaxieswithinthefield.Tore-
22,158galaxiesineachsamplerespectively.
Todefinetheredsequenceweperformedaχ2minimisationto ducetheeffect of theedges, thedistancetothenearest edge was
calculatedandifthiswaslessthanthedistancetothethirdnear-
fitanequationoftheform(U−B)=a×M +b totheold,burst
K estneighbour thentheobjectwasremovedfromthesample.This
galaxies,definingtheredsampletobeallgalaxieswithin3σofthis
methodwasonlyusedinthisworktotesttheprimaryfindingsof
fit(seeHartleyetal.(2010)foramoredetaileddescription).Inthis
theaperturemethod.Theresultsarepresentedintheappendix.
work, to separate redand blue galaxies, weuse thered-sequence
slopefromHartleyetal.(2010)butchoosethedivisionbetweenthe
twopopulationstofittheminimumintheoverallcolourbimodality
(asshowninthehistograminFigure1).Thisleadstoadividingline 4 RESULTS
inthecolour-magnitudediagramasfollows:
Belowweexploretherelationshipbetweengalaxycoloursanden-
(U−B)=−7.09×10−3M +0.52 (2) vironmentasafunctionofredshift.Relativelybroadredshiftbins
K
areusedtominimisethecontaminationduetophotometricredshift
Thisboundarywasfoundtoseparatetheredandbluepopulations
errors.Thesesourcesofuncertaintyareexploredfurtherinsection
effectivelytoz∼1.75.Athigherredshiftthebimodalityingalaxy
5.
coloursislessclear,whichmayinpartbeduetophotometricer-
rors.Afullexaminationofthisissueandtheevolutionofthered
sequence willbe presented in Cirasuoloet al. (inprep). Previous 4.1 RedandBlueEnvironments
studieshavefoundevidenceforanevolutioninthelocationofthe
redsequencewithredshift(e.gBrammeretal.2009).Forsimplic- Figure2showshistogramsofgalaxyenvironmentswithintwoaper-
ity we choose not to model the red sequence in such detail and tures of 1Mpc and 500kpc diameter. These are divided into four
instead use the fixed colour selection boundary given above. We redshiftbinsbetweenz=0.25andz=2.25andintoredandblue
note, however, that using an evolving boundary made no signifi- populations.AKS-testwasperformedonthesamplestoassessthe
cantdifferencetoanyoftheconclusionspresentedinthiswork. difference between the red and blue populations, withall but the
Table1showstheresultingnumber ofredandbluegalaxies final redshift bin showing highly significant differences between
assignedtoeachphotometricredshiftbin,includingtheconserva- thetwopopulations.Inthefirstthreeredshiftbinswecanexclude
tivesubsamplesofpassiveandactivelystar-forminggalaxies. thenullhypothesisthat theredandbluegalaxiesaredrawnfrom
thesameunderlyingpopulationwithasignificanceinexcessof5σ
significance,fallingto<2σand<1σsignificanceatz >1.75.
Figure 3 is a plot of the mean density of red, blue, passive
3 ENVIRONMENTALMEASUREMENT
(black)andstarforming(cyan)galaxiesinbinsofabsoluteK-band
Weusedtwomethodstocalculategalaxyenvironment: Countsin magnitude.Errorbarsarederivedfromtheerroronthemeanden-
an Aperture and nth Nearest Neighbour. In both methods all the sityof galaxies within agiven bin. Sources of error are explored
galaxieswithinaphotometricredshiftbinarecollapseddownonto furtherinSection5.Thepassiveandstarforminggalaxiesarede-
a2Dplaneandtheredshiftinformationwithinthebinisnotutilised finedinSection2.Asbeforetheyareplottedinfourredshiftbins
anyfurther.Intheaperturemethodaperturesof1Mpc,500kpcand but with an additional 250kpc aperture. This plot illustrates that
250kpcdiameter(physical)areplacedoneachgalaxyandthenum- redand/or passive galaxiesresideinsignificantlydenser environ-
berofgalaxieswithinthataperturearecounted(NAper).Asample mentsthanblueand/orstar-forminggalaxiesfromthepresentday
g
of∼100,000randomgalaxiesarethenputdownintheunmasked toz ∼1.5,andthisdifferenceisapparentatallluminosities.This
(cid:13)c 2011RAS,MNRAS000,1–9
4 R. W. Chuter et al.
1.5 1.8
1.4 0.25<z<0.75 1Mpc 0.25<z<0.75 500kpc 2.5 0.25<z<0.75 250kpc
1.6
1.3
2
> 1.4
ρr1.2
ρ/
< 1.1 1.2 1.5
1
1 1
0.9
1.5 1.8
1.4 0.75<z<1.25 1Mpc 0.75<z<1.25 500kpc 2.5 0.75<z<1.25 250kpc
1.6
1.3
2
> 1.4
ρr1.2
ρ/
< 1.1 1.2 1.5
1
1 1
0.9
1.5 1.8
1.4 1.25<z<1.75 1Mpc 1.25<z<1.75 500kpc 2.5 1.25<z<1.75 250kpc
1.6
1.3
2
> 1.4
ρr1.2
ρ/
< 1.1 1.2 1.5
1
1 1
0.9
1.5 1.8 M (AB)
K
1.4 1.75<z<2.25 1Mpc 1.75<z<2.25 500kpc 2.5 1.75<z<2.25 250kpc
1.6
1.3
2
> 1.4
ρr1.2
ρ/
< 1.1 1.2 1.5
1
1 1
0.9
−18 −20 −22 −24 −18 −20 −22 −24 −18 −20 −22 −24
M (AB) M (AB) M (AB)
K K K
Blue Red Star Forming Passive
Figure3.Theaveragegalaxyoverdensity asafunctionofK-bandluminosity, displayedinfourredshiftbins(toptobottom)and
usingprojectedapertures ofdiameter1Mpc,500kpcand250kpc(threecolumns).Galaxies aredisplayedinred,blue,passiveand
star-formingsubsets,asdefinedinSection2.Notethechangeinscaleforeachcolumn.Theenvironmentsaredefinedsothatρ/ρr=1
correspondstoadensityconsistentwiththatofarandomdistributionofgalaxies.
0.25<z<0.75 0.75<z<1.25 1.25<z<1.75 1.75<z<2.25
Red 3,469 3,500 2,565 481
Blue 7,367 7,255 4,674 1,711
Passive 1,623 1,395 722 87
Star-forming 6,938 6,741 5,048 1,817
Table1.Tableshowingthenumberofgalaxiesineachsampleandineachredshiftrange,includingourconservativesubsetsofpassive
andactivelystar-forminggalaxies.
(cid:13)c 2011RAS,MNRAS000,1–9
GalaxyEnvironmentsin theUKIDSS UltraDeep Survey(UDS) 5
0.12
0.1 0.25<z<0.75 1Mpc 0.25<z<0.75 500kpc 1.3 0.25<z<0.75 1Mpc 1.6 0.25<z<0.75 500kpc
0.08 >10σ >10σ 1.2 1.5
f0.06 ρρ</>r 1.4
1.1 1.3
0.04
1.2
0.02
1
1.1
0.102
0.1 0.75<z<1.25 1Mpc 0.75<z<1.25 500kpc 1.3 0.75<z<1.25 1Mpc 1.6 0.75<z<1.25 500kpc
0.08 ∼5σ ∼8σ 1.2 1.5
f0.06 ρρ</>r 1.4
1.1 1.3
0.04
1.2
0.02
1
1.1
0.102
0.1 1.25<z<1.75 1Mpc 1.25<z<1.75 500kpc 1.3 1.25<z<1.75 1Mpc 1.6 1.25<z<1.75 500kpc
0.08 ∼5σ ∼4σ 1.2 1.5
f0.06 ρρ</>r 1.4
1.1 1.3
0.04
1.2
0.02
1
1.1
0.102
0.1 1.75<z<2.25 1Mpc 1.75<z<2.25 500kpc 1.3 1.75<z<2.25 1Mpc 1.6 1.75<z<2.25 500kpc
0.08 <2σ <1σ 1.2 1.5
f0.06 ρρ</>r 1.4
1.1 1.3
0.04
1.2
0.02 1
1.1
0
0 1 2 3 4 0 1 2 3 4 0.4 0.6 0.8 0.4 0.6 0.8
ρ/ρ ρ/ρ (U−B) (U−B)
r r rest rest
Figure2.Histogramsofthedensityofgalaxieswithin1Mpcand500kpc −22.5<KMag<−21.5 −24<KMag<−22.5
diameteraperturescomparedtoarandomsampleforred(dashedline)and
blue(thickline)galaxies.TheσvaluesareobtainedbyperformingaKS
Figure4.Theaveragegalaxydensityina1Mpcand500kpcdiameteraper-
test,representing thesignificance inrejecting thenull-hypothesis thatthe
turescomparedtoarandomsampleasafunctionof(U −B)restframe
samplesaredrawnfromthesameunderlyingpopulation.
colour.Weseparatehigh(black)andlow(green)luminositysubsets.Note
thechangeiny-axisvaluesbetweenthetwodifferentaperturesizes.
iscomparabletowhathasbeenfoundinthelocaluniversebyother
studies(Kauffmannetal.2004;vanDerWeletal.2008).
nosityforbluegalaxiesatz∼1.Ourresultsappeartoextendthese
Figure 3 also shows that passive galaxies (shown in black)
findingstohigher redshift,suggesting thattheepoch 1 < z < 2
follow a similar density profile to red galaxies but are on aver-
representsakeyperiodoftransformationofmassivegalaxiesfrom
ageinslightlydenser environments. Thissupportstheconclusion
thebluecloudontothepassiveredsequence.
thatpassivegalaxieswithintheredpopulationareresponsiblefor
theenhancedenvironmentscomparedtobluestar-formingobjects.
The environments of galaxies that were red but not in the strict
4.2 Colour-DensityRelation
‘passive’samplewerealsoinvestigatedandthesewerefoundtolie
in-betweentheredandbluegalaxyenvironments,aswouldbeex- InFigure4wedisplayaveragedensityasafunctionof(U −B)
pected(thesearenotshownforclarity).Theactivelystar-forming rest-framecolour, withfaint (-22.5< M <-21.5) andluminous
K
galaxies exhibit the same environmental dependence as the blue (-24<M <-22.5)objectsshowningreenandblackrespectively.
K
galaxies,followingthesameluminosity-densityprofile. Thisshowsacleartrendforgalaxiesbelowz∼2,withreddergalax-
In addition to the clear separation of red and blue galaxies, iesoccupyingdenseraverageenvironments.Inthelowestredshift
wealsofindageneral trendofincreasinggalaxydensitywithlu- binthischangeoccursveryabruptlyattheboundaryinthecolour
minosity, particularly for blue galaxiesand on smaller scales. In- bimodality at (U −B) ∼0.7 (see Figure 1). There is also a
rest
specting the twointermediate redshift bins in Figure3, on scales general trend for more luminous galaxies to occupy denser envi-
below500kpcwefindthatthemostluminousbluegalaxiesappear ronments.
to inhabit environments approaching those of red/passive galax- In summary, we have found strong evidence that red galax-
ies.Theseresultsareconsistent withthefindingsof Cooperetal. ies are on average found in denser local environments than blue
(2007),whoobservedastrongincreaseinlocaldensitywithlumi- galaxies, extending the comparison to a higher redshift than any
(cid:13)c 2011RAS,MNRAS000,1–9
6 R. W. Chuter et al.
1.8 1.8
1.6 4
1.7 1.25<z<1.75 500kpc 1.7 1.25<z<1.75 500kpc 0.25<z<0.75 1Mpc 2 0.25<z<0.75 500kpc 3.5 0.25<z<0.75 250kpc
1.6 Q1 1.6 Q2 1.4 1.8 3
ρρ</>r111...345 111...345 ρρ</>r1.21 111...2461 12..552
1
1.2 1.2 1.6 4
1.1 1.1 0.75<z<1.25 1Mpc 2 0.75<z<1.25 500kpc 3.5 0.75<z<1.25 250kpc
1.4 1.8
1 1 3
11..46 1.25<z<1.75 500kpc 11..46 1.25<z<1.75 500kpc ρρ</>r1.2 11..46 2.52
1.2 Q4 1.2 Q3 1.2
1 1.5
1 1 1 1
ρρ</>r 1.6 1.25<z<1.75 1Mpc 2 1.25<z<1.75 500kpc 3.54 1.25<z<1.75 250kpc
1.6 1.6 1.4 1.8
3
11..24 11..24 ρρ</>r1.2 11..46 2.52
1 1.61 1 1.2 1.5
Q1 1
Q2 1.4 1.25<z<1.75 500kpc 1
1.6 4
20 1.2 1.75<z<2.25 1Mpc 2 1.75<z<2.25 500kpc 3.5 1.75<z<2.25 250kpc
y (1000 pixles) 1105 ρρ</>r11..461 ρρ/r11..241 1111....24681 12..5523
5 1
Q4 Q3 1.2 −22 −22.5 −23 −23.5 −24 −22 −22.5 −23 −23.5 −24 −22 −22.5 −23 −23.5 −24
M (AB) M (AB) M (AB)
1 K K K
5 10 15 20 −18 −20 −22 −24
x (1000 pixels) MK (AB) Figure6.AsFigure3,butusingonlythebrightestgalaxieswithM 6-22
K
(AB)astracersofgalaxydensity.Notethechangeiny-axisvaluesbetween
Figure5.ShowingthedensityofredandbluegalaxiesagainstK-bandlu-
thethreedifferentaperturesizes
minosityinfourquadrantsoftheUDSfieldshowninthebottomleft,using
medianvaluesofxandytocalculatethepositionofthecentre.Theoverall
resultforthefieldat1.25<z<1.75isshowninthebottomrightwiththe
errorbarscalculatedfromtheerroronthemeanofthefourquadrants,to
5.1 CosmicVariance
giveanestimateofthecosmicvariance.
To attempt to quantify the effects of cosmic variance on our re-
sultswedividetheUDSfieldintofourquadrants.Themedianval-
ues of the x and y positions of the galaxies were used to divide
previousstudy.Cooperetal.(2007)probeduptoz ∼1.3,finding the field, to ensure that the number of galaxies in each quadrant
thatredandbluegalaxiesatthisredshiftoccupyindistinguishable wascomparable. Theresultsforthegalaxiesintheredshiftrange
environments.Incontrastwefindthatthereisasignificantdiffer- 1.25<z<1.75 are shown in Figure 5. The average density versus
ence between red and blue galaxies between 1.25 < z < 1.75 K-bandabsolutemagnitudeisshownforthefourquadrants, with
(see Figures 2 and 3). This difference is likely to be due to our thebottomleftpanelshowingtheprojecteddistributioninthefield.
methodofselectinggalaxiesfromdeepinfraredimaging.Veryred, ThelowerrightpanelshowstheoriginalresultfromFigure3(red
passive galaxies willhave been missed from the R-band selected and blue only) but with the error bars calculated from the error
candidates used inthe DEEP2study. Thisselection effect can be onthemeanofthedensitiesinthefourquadrantsineachmagni-
quantified using our UDS sample. Considering only the most lu- tudebin. Itcan beseen that thedistinctionbetween redand blue
minousgalaxies(M 6-22.5)intheredshiftrange1<z<1.3,ifwe galaxyenvironmentsremains,althoughthereareclearlysignificant
K
imposetheselectioncriteriaofDEEP2(R 624.1) wefindthat variations across the field. We conclude that while field variance
AB
75%ofredgalaxieswouldbemissedcomparedtoonly20%ofthe isclearlypresentinthesedatatheprimaryfindingsremainrobust.
bluesample.Atfainterluminositiesvirtuallynored/passivegalax- Similar conclusions were drawn from other redshift bins and on
iesareselected. otherscales,whicharenotshownintheinterestsofbrevity.
Asacautionaryremark,however,wenotethatstrictlyspeak-
ingwehaveonlytestedtheinternalfieldvariance,sincedespitethe
relativelywidefieldoftheUDS(50×50comovingMpcatz∼1)
wemaynevertheless bepronetolarge-scalecosmicvariancedue
5 INVESTIGATINGSOURCESOFERROR tounusualsuperstructures(Somervilleetal.2004).Testingagainst
sucheffectswillrequirefurtherwide-areainfraredsurveys,suchas
Inthissectionweconductanumberofteststoinvestigatevarious
theforthcomingVIDEOandUltra-VISTAsurveys.
source of systematic uncertainty. Weattempt toquantify thecos-
micvarianceinourdatabydividingthesurveyintofourquadrants.
Avolumelimitedstudyofthebrightergalaxiesisalsoperformed,
5.2 Volume-LimitedSample
toallow aclearer comparison betweendifferent redshift bins. Fi-
nallyweinvestigatetheeffectsofphotometricredshifterrorsusing Ourmeasurementofenvironmentaldensityisbasedonafluxlim-
MonteCarlosimulations. ited survey, so by definition we are using fainter galaxies to de-
(cid:13)c 2011RAS,MNRAS000,1–9
GalaxyEnvironmentsin theUKIDSS UltraDeep Survey(UDS) 7
3
1.35
0.75<z<1.25
2.52 5<z<1.25 5<z< 1.75 5 <z< 2.25 11.2.35
7 2 7
0. 1. 1. pc 1.2
M
1
o >
Phot1.5 ρρ/r1.15
z <
1.1
1
1.05
0.5
1
−19 −20 −21 −22 −23 −24
0 M (AB)
0 0.5 1 1.5 2 2.5 3 K
z
Spec
Figure8.MonteCarlosimulationshowingtheeffectofthephoto-zerrors
Figure 7. Plot of the reliable spectroscopic redshifts against photomet- onourmeasurementofgalaxyoverdensity,basedon100simulations.The
ric redshifts, highlighting the three highest redshift bins, 0.75<z<1.25, thickblacklineshowstheaverageofthesimulationsandthedashedblack
1.25<z<1.75and1.75<z<2.25,withAGNandradiogalaxiesremoved. lineshowstheoriginalresultfor0.75<z<1.25fromFigure3.
Theredsquaresandbluecirclesindicatetheredandbluegalaxiesrespec-
tively,asdefinedinsection2.2.
1.35
1.25<z<1.75
fine environments at low redshift. To investigate the influence of
1.3
this systematic effect we reduced the sample to only the bright-
estgalaxieswithM 6-22(AB)andrepeatedtheenvironmental
K 1.25
analysisusingthisvolume-limitedsample.Thisallowsthegalax-
ies to be used as tracers in the measure of environment over the pc 1.2
samerangeinluminosityatallredshifts,toallowafairercompar- 1M
>
isonbetweenepochs.TheresultsareshowninFigure6,wherethe ρρ/r1.15
averagedensityversusK-bandluminosityisshownforcomparison <
withtheflux-limitedstudyinFigure3.Asexpected, thevolume- 1.1
limitedstudyismuchnoisier(particularlyatlowredshift)butthe
resultsareconsistent withtheprimaryfindingsofSection4.Red 1.05
and/or passive galaxies are found tooccupy denser environments
onaveragetoz∼1.5. 1
−19 −20 −21 −22 −23 −24
M (AB)
5.3 MonteCarloSimulations K
Monte Carlosimulations wereconducted toinvestigatetheeffect Figure9.AsFigure8butforthe1.25<z<1.75bin.
ofphotometricredshifterrorsonourresults,explicitlyallowingfor
the effects of outliers and catastrophic errors. Such errors would
generallydilutedifferencesingalaxyenvironments,butcouldpo- δz<0.25atz ∼1(Cirasuoloetal.2010).Theresultingfractions
tentially introduce fake overdensities if a large fraction of low- were then randomly applied to the full sample, shifting galaxies
redshift structure is incorrectly assigned to a higher redshift bin. intonewbinstosimulatetheeffectofphotometricredshifterrors
We note, of course, that the effects of photo-z errors are already (‘normal’and‘catastrophic’).
presentinthedata,sothesesimulationscanonlyprovideanindi- This was repeated 100 times per redshift bin. Results are
cationofthemagnitudeoftheshiftsduetotheseeffects. displayed for the redshift bins 0.75 < z < 1.25 (Fig.8) and
The distribution of spectroscopic versus photometric red- 1.25<z <1.75(Fig.9).Thesesimulationsdemonstratethatpho-
shiftsusedinthisfieldareshowninFigure7,withspectroscopic tometric redshift errors tend to dilute the distinction between the
redshifts derived from a variety of sources (Yamadaetal. 2005; redandbluepopulationsratherthanintroducefakedifferences.The
Simpsonetal. 2006; Akiyama et al., in prep; Simpson et al., in resultingadditionalsourcesoferroringalaxydensityarecompara-
prep; Smailet al.,in prep) asoutlined in Cirasuoloet al. (2010). bletotheerrorsonthemeandisplayedpreviously.Onlyinhighest
Weusethedistributionineachredshiftbin(higherredshiftbinsin- redshiftbin(z>1.75)dothesimulatedpopulationsintroducesig-
dicatedinFigure7)toestimatethefractionofgalaxiesincorrectly nificantextrascatter(notshown).Giventhesefindings,andthelack
reassignedfromoneredshiftbintoanother.Forsimplicitytwocat- ofspectroscopicredshiftsatz∼2,weurgecautionininterpreting
egories were adopted: ‘catastrophic’ errorswere defined as those our preliminary results in this highest redshift bin. At lower red-
withδz > 0.25,whileothersareclassifiedas‘normal’.Intheab- shifts,however,theenvironmentaldifferencesappearrobust.
senceofcatastrophicerrorsweexpect>99.9%ofgalaxiestoshow Notingthatwehaveonly97spectroscopicredshiftsinthered-
(cid:13)c 2011RAS,MNRAS000,1–9
8 R. W. Chuter et al.
onrestframe(U−B)colours,aswellasusing‘passive’and‘star-
1.35
forming’galaxiesfromtemplatefitting.Weattempttoquantifythe
0.75<z<1.25
effects of cosmic variance, photometric redshift errors and flux-
1.3
limitbiasesontheresultingenvironmentalmeasurements.Wefind
thefollowingprincipleresults:
1.25
(i) We find a strong relationship between rest-frame (U −B)
Mpc 1.2 colour andgalaxy environment toz ∼ 1.5, withredgalaxiesre-
1
ρρ/>r1.15 ssicdailnegsbinelosiwgn1ifiMcpacn.tlTyhdeesnesreersuenltvsiraornemroebnutsstthtoanthbelueeffegcatlsaxoifefiseolnd
<
variance,flux-limitbiasesandphotometricredshifterrors.Theen-
1.1
vironmentsappearindistinguishablebyz ∼2,butthecurrentlack
ofspectroscopicredshiftsatthisepochdoesnotallowarobusttest
1.05
ofthistantalisingsignalatpresent.
(ii) Selecting ‘passive’ and ‘star-forming’ galaxies using tem-
1
platefittingyieldsconsistentresults,withthepassivesubsetoccu-
−19 −20 −21 −22 −23 −24 pyingslightlydenserenvironmentsthantheglobalredpopulation
M (AB)
K atallepochs.Weconcludethatthepassivesubsetoftheredgalax-
iesareresponsiblefortheenhancedenvironmentscomparedtoblue
Figure10.TheresultsofMonteCarlosimulations basedontheextreme
galaxies.
scenarioinwhichallincorrectlyassignedphotometricredshiftsleadtoblue
(iii) Onsmallscales(< 0.5Mpc)wefindevidenceforaposi-
galaxies being incorrectly classified as red, and vice versa. The original
measuresofenvironmentforredgalaxiesandbluegalaxiesasafunctionof tivecorrelationbetweengalaxyK-bandluminosity(agoodproxy
luminosityareshownasthickredandbluelines.Blackdashed(dot-dashed) forstellarmass) andlocaldensity.Thistrendisparticularlyclear
linesshowtheresultsofMonteCarlosimulationsforthered(blue)popu- for star-forming and blue galaxies, with the most luminous blue
lationsrespectively. Thecorrespondinggreenandmagentalinesillustrate galaxiesatz∼1−2showingaverageenvironmentscomparableto
theinfluenceofdoublingandtriplingthenumberofincorrectly assigned passiveredgalaxies.Thesefindingsappearinverygoodagreement
galaxies. withthefindingsofCooperetal.(2007)atz∼1.Theseresultsare
consistentwiththeidentificationofhigh-zgalaxiesintransitionto
theredsequenceinthedensestenvironments.
shiftbin1.25< z <1.75(with64between1.25< z <1.75),
photo Toimproveonthisstudy,wenotethattheUDSimagingpro-
whereenvironmentaldifferencesappeartobeverysignificant,we
gramme will continue at UKIRTuntil 2012 and ultimatelyprobe
ranadditionalsimulationsusingonlyhalfofthespectroscopicsam-
∼ 1 magnitude deeper than the dataused inthiswork. Thiswill
ple.TheresultswereverysimilartoFigure9,indicatingthatweare be sufficient to detect L∗ galaxies to z ∼ 5 in addition to tens
notsufferingfromlownumberstatistics. ofthousandsofsub-L∗galaxiesatlowerredshift.Furthermore,an
Asanadditionaltest,weconductfurtherMonteCarlosimula-
ongoing ESO Large Programme (UDSz) will soon transform the
tionstoinvestigatetheeffectsofcontaminationofthebluegalaxies
UDSprojectwiththeadditionof∼3000galaxyspectra.Thiswill
bytheredpopulationandviceversa.Thesesimulationsweremo-
increasethenumberofUDSspectroscopicredshiftsatz >1.5by
tivated by the worry that incorrect redshifts may inevitably lead
anorder of magnitude, whichwe expect todramaticallyimprove
to errors in the resulting restframe (U − B) colours. We adopt
thereliabilityofphotometricredshiftsandallowustoextendour
anextremeapproach bysimulating100%misclassification,using
studyofgalaxyenvironmentsandlarge-scalestructuretothecru-
the contaminating fractions outlined above but moving only blue
cialepochwhenthegalaxyredsequenceisfirstestablished.
galaxiesintotheredsampleandviceversa.Wefindthatthesesim-
ulationshavetheeffectofdilutingtheenvironmentforbothredand
bluesimulations,butonceagainareinsufficienttoinfluenceanyof
theresultspreviouslypresented.Toinvestigatefurther,thecontam- ACKNOWLEDGEMENTS
inatingfractions areartificiallyincreased todouble andtriplethe
Weare grateful tothestaff at UKIRTfor operating thetelescope
maximalcontaminationindicatedbyspectroscopicredshifts.These
withsuchdedication.WealsothanktheteamsatCASUandWFAU
only servetofurther reduce theaveragegalaxydensities. Results
forprocessingandarchivingthedata.RWCwouldliketothankthe
forthe0.75<z <1.25binareillustratedinFigure10,andsimi-
STFCfortheirfinancialsupport.JSDacknowledgesthesupportof
larresultsarefoundforallotherredshiftbins.
the Royal Society via aWolfson Research Merit award, and also
Weconcludethatphotometricredshifterrors(includingcatas-
thesupportoftheEuropeanResearchCouncilviatheawardofan
trophic errors) have only served to dilute the differences in envi-
AdvancedGrant.
ronment,butanunrealisticproportionofmisclassifiedgalaxiesare
requiredtosignificantlyinfluenceourprimaryfindings.
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APPENDIX:NEARESTNEIGHBOURMETHOD
In this appendix we use the nth nearest neighbour method as an
alternative environmental estimator, to provide a test of the re-
sultspresentedearlierandtoallowcomparisonwithpreviouswork
(e.g.Cooperetal.2007).Thenthnearestneighbourwascalculated
forallthegalaxiesasafunctionofabsolutemagnitude,usingthe
methodoutlinedinSection3.Thevalueofnischosentoprobethe
samemeanscaleastheaperturesmethod.Theresultsareshownin
Figure11),whichappeargloballyconsistentwiththosederivedby
theaperturemethod.
(cid:13)c 2011RAS,MNRAS000,1–9
