Passive galaxies as tracers of cluster environments at z~2

0.27 MB·English

by  V. Strazzullo

Astronomy&Astrophysicsmanuscriptno.refs (cid:13)c ESO2015
January23,2015
∼
Passive galaxies as tracers of cluster environments at z 2
V.Strazzullo1,2,E.Daddi1,R.Gobat1,B.Garilli3,M.Mignoli4,F.Valentino1,M.Onodera5,A.Renzini6,A.Cimatti7,
A.Finoguenov8,N.Arimoto9,M.Cappellari10,C.M.Carollo5,C.Feruglio11,E.LeFloc’h1,S.J.Lilly5,
D.Maccagni3,H.J.McCracken12 M.Moresco7,L.Pozzetti4,G.Zamorani4
1 Irfu/Serviced’Astrophysique,CEASaclay,OrmedesMerisiers,F-91191GifsurYvette,France–e-mail:[email protected]
2 DepartmentofPhysics,Ludwig-Maximilians-Universita¨t,Scheinerstr.1,81679Mu¨nchen,Germany
3 INAF–IASF,viaBassini15,I-20133,Milano,Italy
5 4 INAF-OsservatorioAstronomicodiBologna,viaRanzani1,40127Bologna,Italy
1 5 InstituteforAstronomy,ETHZu¨rich,Wolfgang-Pauli-strasse27,8093Zu¨rich,Switzerland
0 6 INAF-OsservatorioAstronomicodiPadova,Vicolodell’Osservatorio5,I-35122,Padova,Italy
2 7 DipartimentodiFisicaeAstronomia,Universita´diBologna,VialeBertiPichat6/2,I-30127,Bologna,Italy
8 DepartmentofPhysics,UniversityofHelsinki,GustafHa¨llstro¨minkatu2a,FI-0014Helsinki,Finland
n 9 NationalAstronomicalObservatoryofJapan,SubaruTelescope,650NorthAohokuPlace,Hilo,HI96720,USA
a 10 DepartmentofPhysics,UniversityofOxford,DenysWilkinsonBuilding,KebleRoad,Oxford,OX13RH,UK
J 11 IRAM-InstitutdeRadioastronomieMillime´trique,300ruedelaPiscine,38406SaintMartindHe`res,France

1 12 InstitutdAstrophysiquedeParis,UMR7095CNRS,Universite´PierreetMarieCurie,Paris,France
2

]
O ABSTRACT
C
Even10billionyearsago,thecoresofthefirstgalaxyclustersareoftenfoundtohostacharacteristicpopulationofmassivegalaxies
.
h withalreadysuppressedstarformation.Herewesearchfordistantclustercandidatesatz∼2usingmassivepassivegalaxiesastracers.
p Withasampleof∼40spectroscopicallyconfirmedpassivegalaxiesat1.3<z<2.1,wetunephotometricredshiftsofseveralthousands
- passivesourcesinthefull2sq.deg.COSMOSfield.Thisallowsustomaptheirdensityinredshiftslices,probingthelargescale
o structureintheCOSMOSfieldastracedbypassivesources.Wereporthereonthethreestrongestpassivegalaxyoverdensitiesthat
r weidentifyintheredshiftrange1.5<z<2.5.Whiletheactualnatureoftheseconcentrationsisstilltobeconfirmed,wediscusstheir
t
s identificationprocedure,andtheargumentssupportingthemascandidategalaxyclusters(likelymid-1013M range).Althoughthis
⊙
a searchapproachislikelybiasedtowardsmoreevolvedstructures,ithasthepotentialtoselectstillrare,cluster-likeenvironmentsclose
[
totheirepochoffirstappearance,enablingnewinvestigationsoftheevolutionofgalaxiesinthecontextofstructuregrowth.

1 Keywords.galaxies:clusters:general-galaxies:high-redshift-cosmology:large-scalestructureofUniverse
v
3
2
3 1. Introduction z∼2passivesourcesarenolongerthedominantpopulationeven
5 among massive galaxies. However, the observed evolution of
0 Up to at least z∼1, passive galaxies typically with early- massiveclustergalaxiesuptoz∼1(andalsotheoreticalmodels,
. type morphology dominate the high-mass end of the galaxy e.g. DeLuciaetal. 2006) typically suggests early (z&2-3) for-
1
population, and are the best tracers of the highest density mationepochsfortheirstellarpopulations(e.g.,Meietal.2009;
0
5 peaks in the large scale structure. The evolution of pas- Manconeetal.2010;Strazzulloetal.2010).Wemightthusex-
1 sive galaxy populations at z.1 – and in particular with pect that the surge of passive galaxies around 10 billion years
: respect to environmental effects – has been explored in agooccurreddifferentlyindifferentenvironments.
v
detail also thanks to large spectroscopic campaigns (e.g., Severalrecentstudieshaveclaimedevidenceofsignificantstar
i
X Kauffmannetal. 2004; Bernardietal. 2006; Gallazzietal. formation even in central cluster regions at z&1.5 (among sev-
2006, 2014; vanderWeletal. 2008; Sa´nchez-Bla´zquezetal. eral others, Hiltonetal. 2010; Tranetal. 2010; Hayashietal.
r
a 2009; Kovacˇetal. 2014; Valentinuzzietal. 2011; Muzzinetal. 2010;Santosetal. 2011;Fassbenderetal.2011;Brodwinetal.
2012). On the other hand, spectroscopy of passive galaxies at 2013),suggestingthatindeedweareapproachingtheformation
z&1.5 has been until recently very difficult: in spite of several epoch of massive cluster galaxies. However, it is also notice-
investigationspushingspectroscopicconfirmationandmorede- ablethatmassivepassivegalaxiesareoftenfoundeveninsuch
tailedstudiestohigherredshifts(e.g.Cimattietal.2004,2008; most distant clusters, although in many cases sharing their en-
Daddietal.2005;Krieketal.2006,2009;Onoderaetal.2012; vironment with galaxies in a still active formation phase (e.g.,
vandeSandeetal. 2011, 2013; Toftetal. 2012; Gobatetal. Kurketal.2009;Papovichetal.2010;Gobatetal.2011,2013;
2012,2013;Brammeretal.2012;Weiner2012;Krogageretal. Tanakaetal. 2012, 2013; Spitleretal. 2012; Strazzulloetal.
2014; Newmanetal. 2014; Bellietal. 2014), sizable spectro- 2013;Newmanetal.2014;Andreonetal.2014).Thismaysug-
scopic samples are still rare, and studying z>1.5 passive popu- gestthat,evenatacosmictimewhenstarformationratedensity
lations mainly relies on photometric samples (e.g. Wuytsetal. is at its peak (e.g. Madau&Dickinson 2014), and star forma-
2010; Belletal. 2012; Ilbertetal. 2013; Muzzinetal. 2013a; tionisstillactiveinaconsiderablefractionofmassivegalaxies
Cassataetal. 2013). These studies show that the number den- (e.g.Ilbertetal.2013;Muzzinetal.2013a),thedensestcoresof
sity of passive galaxies rapidly falls beyond z>1, so that by
1
Strazzulloetal.:Passiveoverdensitiesatz∼2
Fig.2: Distributions of K-band magnitude, stellar mass and photometric redshift for the sample of K < 23, z > 1.3 passive
AB
galaxies.Thethreeleftmostpanelsshowthefullsample(seetext,blackline),itspBzKsub-sample(redline),andthespectroscopic
sample(scaledby×10,redshadedarea).Verticaldottedlinesshowthemagnitudeandmass(atz<2.5)completenesslimitsofthe
parentK-selectedcatalog(seetext).Therightmostpanelshowsthefullphoto-zdistributionoftheK <23passivesample(black
AB
line),andofitslog(M/M )>10.9sub-sample(shadedarea,lighter-grayatz>2.5wheresampleisbeyondmasscompleteness).
⊙
most evolved cluster progenitorsalready host a typically small scopicallyconfirmedpBzKs,selectedintherange1.3<z <2.1
spec
butcharacteristicpopulationofmassivequiescentgalaxies.For and with restframe UVJ colors (Williamsetal. 2009) consis-
thisreason,overdensitiesofpassivesourcesmightbeconsidered tentwithpassivepopulations(15and27galaxiesfromtheO12
aspossiblesignpoststoclustersatleastuptoz∼2. and VIMOS samples, respectively, including 3 24µm-detected
sources as noted below). Fig. 1 shows the performanceon this
sample of our photo-zs, estimated with EAzY (Brammeretal.
2. Photometricredshiftestimationforhigh-redshift 2008) and calibrated as in O12. The normalized median abso-
passivegalaxiesintheCOSMOSfield lutedeviation(NMAD)of∆z/(1+z)is2.5%onthefullsample,
or1.8%excludinggalaxieswithlessreliablez (Fig.1),with
Usingoneofthefirstsizablesamplesofz&1.4passivegalaxies spec
nocatastrophicoutliers(thus<2.5%forthissample).Allresults
in the COSMOS field for calibration, in Onoderaetal. (2012)
presentedherearebasedonthephotometriccatalogbyM101.
(hereafter O12) we could estimate more accurate photomet-
ric redshifts (photo–zs, z ) for high-redshiftpassive sources.
phot
We havenow assembled a new, independentsample of passive
3. Passivegalaxyoverdensitiesatz> 1.5
galaxiesinCOSMOSwithredshiftsmeasuredthroughUVfea-
tures using VLT/VIMOS spectroscopy.We targeted 29 IAB<25 Spectroscopic confirmation of large passive galaxy samples at
galaxiesselectedaspassiveBzKs(“pBzKs”,Daddietal.2004, highredshiftisfornowprecluded,sowerelyonpassivecandi-
plus624µm-detectedpBzKs)fromtheMcCrackenetal.(2010) dates with photo-zscalibrated as above. We select a sample of
(hereafterM10)catalog.A redshiftwasmeasuredfor34ofthe z>1.5passivegalaxiesasfollows:withaninitialBzKselection
35 targets, with a robust estimate for 29 sources. The observa- ontheM10K <23catalog,wetakeallpBzKgalaxies,aswell
AB
tions,analysis,andafullredshiftlistwillbepresentedinGobat asgalaxiesformallyclassified asstar-formingBzKs (“sBzKs”)
et al. (in prep.). Here we focuson a sub-sample of 42 spectro- buthavingaS/N<5intheB-(andpossiblyz-)band.Fromthis
firstselection,weretainallgalaxiesthatalsohaveUVJpassive
colors (assuming their z as of Sec. 2). Sources detected at
phot
24µm and satisfying the abovecriteria are retained,because of
thepossibilityofAGN-powered24µmflux.Sourceswithpossi-
blycontaminatedIRACphotometry(asintheM10catalog)were
discarded(<10%).InFig.2,blackandredlinesshowthestellar
mass, K-band magnitude and photo–z distributions for the full
retained sample of passive galaxies according to these criteria,
andforitssubsampleofpBzKsources,respectively.
The K <23 limit corresponds to a 90% completeness for
AB
point-like sources, going down to ∼22.5 for disk-like pro-
files (see M10). At z∼2.5 these limits would correspond to a
masscompletenessoflog(M/M )∼10.8-11(∼10.9inthefollow-
⊙
ing, Salpeter (1955) IMF) for an unreddened solar metallicity
Bruzual&Charlot(2003)SSPformedatz∼5.Ontheotherhand,
thecombinationof selectioncriteriaadoptedaboveis expected
1 Anevenbetterphoto-zaccuracy(aslowas1.5%,withamarkedim-
provementatz&1.8)canbeobtainedcalibratingphoto-zsonthissam-
Fig.1: Comparison of z vs. z for the full spectroscopic ple with the more recent photometry from the Muzzinetal. (2013b)
phot spec
sample of 1.3 < z < 2.1 passive galaxies. Lower quality z UltraVISTA catalog, in agreement with - and only marginally better
spec
determinations are shown in gray. Blue and red circles mark than-theresultsobtainedwithMuzzinetal.(2013b)photo-zs,aswill
bediscussedinaforthcomingpaper(Strazzulloetal.,inprep.).Herewe
sourcesfromtheVIMOSandO12samples,respectively.Orange
usetheM10catalogwhichincludesthesouthernpartofthe2sq.deg.
stars highlight MIPS-detected sources. Dotted and solid lines
COSMOSfieldwhereoneofouroverdensities(Sec.3.1)isfound,not
showthebisectorandarelativescatterof2.5,5and10%.
coveredbytheUltraVISTAsurvey(McCrackenetal.2012).
2
Strazzulloetal.:Passiveoverdensitiesatz∼2
Fig.3:ExamplesofΣ mapsinredshiftslicesofthefullpassivegalaxysampleinthe2sq.deg.COSMOSfield(Northisup,Eastis
5
left).Mapsarescaledsothatredcolorscorrespondto>5σsignificance.ThethreecandidateoverdensitiesdescribedinSec.3.1are
highlightedwithwhitecircles(2Mpcradiusproper).Ineachplot,theinsetshowsthedistributionoflog(Σ )valuesinthemapand
5
itsgaussianfit(whiteandorangelines,respectively),andthewhitearrowshowsthepeakΣ valueofthehighlightedoverdensity.
5
toresultinalargelypure(intermsofcontaminants)butnotcom- ∼50kpcand3-4Mpc),whileΣ wouldprobemediandistances
7
pletesampleofmassive(log(M/M )&10.9)passivegalaxies.For closerto2Mpcwithminimum/maximumof0.5/4.5Mpcatz∼2,
⊙
thisreason,wemaybemissingsomeoverdensitiesorreducing thus becoming less sensitive to the scales we need to probe.
theirsignificance,whichwouldaffectinaconservativewaythe Fig.3showsthreeexamplesofΣ maps.Foreachmap,weesti-
5
resultspresentedhere.TheBzK selection,andthe depthofthe matethesignificanceofoverdensitiesbyfittingthedistribution
M10 catalog, effectively limit our sample at z>1.5 and z<2.5, oflog(Σ )inthewholemapwithaGaussian(Fig.3).
5
respectively(Fig. 2, right). The fullsample of K <23 passive At the same time, we also use an independentapproach to
AB
galaxycandidatesincludes∼4500sources.Ofthese,∼3500are searchforconcentrationsofmassivepassivegalaxieswithcon-
at 1.5≤zphot≤2.5(∼70%atlog(M/M⊙)>10.9,∼50%selected as sistentphoto–zsinaverysmallcluster-coresizedarea.Inpartic-
pBzK,∼10%24µm-detected). ular,basedonobservationsofthez = 2clusterClJ1449+0857
(Gobatetal. 2011, 2013; Strazzulloetal. 2013, other exam-
ples in Sec. 1), we search our log(M/M )>10.9 passive sam-
3.1.Identificationofclustercandidates ⊙
ple forsourceswith atleast threeotherpassive galaxieswithin
We buildlocaldensitymapsforthefullCOSMOSfieldinred- |∆z|/(1+z)<7.5%(accountingforphoto-zuncertaintiesdescribed
shift slices, based on the catalog of passive galaxy candidates above)andaphysicaldistance≤150kpc2.Thisapproachinde-
describedabove.Inspiteofthequoted∼2%photo-zrelativeac- pendently retrieves three most significant (& 7σ in the Σ5 - as
curacy(bycomparisonwithspectroscopicredshifts,Sec.2),we well as Σ3 - maps) overdensitieshighlighted in Fig. 3, and de-
needtoconsiderthatthespectroscopicsampleisverybiasedto- scribedherebelow:
wardsbrighter,lower-redshiftsources(Fig.2),thusphoto-zper- •CC1002+0134at1.5.z .1.8–Aconcentrationofpas-
phot
formanceonthebulkofoursampleislikelysignificantlyworse. sive sources around RA, Dec ∼10h02m40s, +01◦34m20s, with
Weestimatedamorerealisticphoto-zaccuracyasafunctionof 4 galaxies within a radius r = 90 kpc and z within ≤1.5σ
phot
magnitude,recalculatingphoto-zson SEDsof well-fitted spec- (giveneachsourcemagnitude,basedonthesimulationdescribed
troscopicsources(|∆z|/(1+z)< 2.5%)dimmedtofaintermagni- above) from a mean z ∼1.76. However, the photo-z distri-
phot
tudes,randomlyscatteringfluxesin thedifferentbandsaccord- bution of the central sources in this candidate overdensity is
ingtophotometricerrorsinourcatalog.Thissimulationindeed quitebroadcomparedtotheexpectedphoto-zuncertainties(see
givesaphoto-zaccuracy<2.5%atKAB.21(Sec.2),butrisingto Fig. 4). This might suggest a chance superposition of passive
∼3.5%(5%)atKAB∼22(22.5),andtomorethan6%approaching galaxies,orpossiblyofdifferent,unrelatedstructures,alongthe
ourKAB ∼23limit.Theseestimatesare“model-independent”in lineofsightat1.5 < z < 1.8.Ontheotherhand,Aravenaetal.
the sense that they use observed (rather than synthetic) SEDs, (2012)alreadyreportedtheidentificationofacandidatecluster
but they still assume that SEDs of less massive and/or higher atz ∼ 1.55atthesameposition,basedonanoverdensityof
phot
redshift sources in our sample behave similarly to those of the galaxieswith 1.5<z <1.6, a radio source at a consistent red-
phot
spectroscopicsourcesusedasinputsinthesimulation.Forcom- shift, a tentativedetection of extendedX-rayemission, and the
parison,theformal68%errorsestimatedbyEAzYonthesimu- presence of a small population of passive sources. The actual
latedSEDswouldbeonaverage40-50%larger(reaching∼8% natureofthisstructureisthusstillunclear.
at KAB∼23).For thisreason,we build ourdensitymapsin red- •CC1003+0223atzphot∼1.90–Aconcentrationof4passive
shift slices of ∆z=±0.2 (correspondingto a ±1σ relative accu- galaxiesatRA,Dec∼10h03m05s,+02◦23m24s withinr<110kpc
racyof6-8%at1.5<z<2.5)withastepof0.05incentralredshift. andz within.1.4σfromthemeanz (afurthergalaxywith
We use the Σ5 (5th nearest neighbour) density estimator, zphotcpohontsistentwithin1σisfoundatr<p3ho4t0kpc).Halfofthese4
which for our sample probesa median (overthe full map) dis- sourceswereselectedasUVJ-passivesBzKs(seeSec.3).None
tance of ∼1.4 Mpc in the z∼2 slice, with minimum and maxi-
mumdistancesof100-200kpcand4-5Mpcinallredshiftslices, 2 Based on our sample of log(M/M )>10.9 passive galaxies, the
⊙
thusproperlyprobingthetypicalscalesweareinvestigating.For Poissonian probability of finding ≥ 3 neighbours at ≤ 150 kpc (or
comparison, a Σ3 estimator would also probe such scales (me- ≥ 4 sources within a radius of ≤ 150 kpc) with a |∆z|/(1+z)< 7.5%
dian distance ∼1.1 Mpc at z∼2, minimum/maximum distances is<5×10−4(<2×10−7,respectively)atallredshiftsprobed.
3
Strazzulloetal.:Passiveoverdensitiesatz∼2
Fig.4:Photo–zdistributionsforthewhole(passiveandstar-forming)populationoflog(M/M )> 10.9galaxieswithinanaperture
⊙
ofr<600kpcfromthethreepassiveoverdensitiesaslabeled(redlines,errorsasfromGehrels1986).Galaxiesandphoto–zsused
here are from Muzzinetal. (2013b) for CC1003+0223 and CC0958+0158, and from Ilbertetal. (2009) for CC1002+0134 (see
text). In each panel, the blue line and dotted area show the median and 16th −84th percentiles of the photo–z distribution (from
thesamecatalogs)in r <600kpcaperturesat100randompositionsintheCOSMOSfield,whilethegrayed-outregionshowsthe
redshiftrangewherethesampleisnolongermass-complete.Blackfilled(empty)circlesscatteredabovethehistograms(atrandom
y-axiscoordinates)showourphoto-zdeterminationsforthepassive sampleusedinthiswork within300kpc(600kpc)fromthe
overdensitycenter(larger/smallersymbolsshowgalaxiesmore/lessmassivethanlog(M/M )=10.9).
⊙
is24µmdetected.AllareconsistentwithbeingUVJ-passivealso determinationsfor our passive galaxy sample aroundthe over-
in the Muzzinetal. (2013b) catalog, three out of four with a densitiesinthesameapertureandmassrange.Althoughclearly
photo-zconsistentwithin1σwiththemeanz estimatedhere. affected by limited statistics, Fig. 4 shows the correspondence
phot
Theredshiftdistributionofthecentralsourcesisconsistentwith between our photo-zs of passive sources identifying the over-
thepresenceofasinglestructure.Anotheroverdensityofsimi- densities, and the excess in the redshift distribution from com-
larsignificanceataconsistentphoto-zisvisibleintheΣ mapat pletelyindependentphoto-zdeterminationsofthewholegalaxy
5
<4MpcWest(Fig.3). populationintheirsurroundings.
•CC0958+0158atz ∼2.35-Aconcentrationof4passive
phot
galaxies at RA, Dec∼9h58m53s,+01◦58m01s within r<130 kpc
4. Discussionandsummary
andaz within.0.8σfromthemeanz (onefurther,lower-
phot phot
massgalaxyatthesamez isfoundatr<290kpc).Giventheir Thisletterinvestigatesthepossibleidentificationoffirstcluster-
phot
redshift (thus faintness) all of these sources were selected as like environments using evolved galaxy populations as tracers
UVJ-passivesBzKs(seeSec. 3).Onemightbeassociatedwith of an early-quenchedcluster core. We describedthe identifica-
a 24µm detection. All are consistent with being UVJ-passive tionof3candidateoverdensitiesofpassivegalaxies,selectedin
alsointheMuzzinetal.(2013b)catalog,withphoto-zsconsis- redshift-sliceddensitymapsandwith propertiessimilarto pas-
tent within 1σ with the mean z estimated here. The photo- sive galaxyconcentrationsin z∼2 clusters. Thisstudy relieson
phot
z distribution is very compact, consistent with a single struc- accuratephoto-zdeterminationforhigh-redshiftpassivesources
ture.Anoverdensityataconsistentpositionandredshiftisalso calibratedononeofthelargestspectroscopicsamplesavailable
visible in Scovilleetal. (2013) density maps. We also note the to date. We present here only the first results on the strongest
proximity of the Spitleretal. (2012) cluster candidate at simi- candidateoverdensities.Furtherinvestigationfocusingonalter-
lar redshift, ∼14 Mpc NE. Chiangetal. (2014) also claim the native sample selections and the identification of lower-mass
presenceofseveralproto-clustercandidateswithinafewMpcof structures,withimprovedphoto-zsbasedonmorerecent,deeper
CC0958+0158atasimilarphoto-z. photometry(Sec. 2),willbepresentedinaforthcomingpaper.
InFig.4weshowforcomparisonthephoto–zdistributionof Wecurrentlyhavenoproofthatthecandidateoverdensities
a mass-limited sample of the whole (passive and star-forming) weidentifiedarerealstructures.Eveniftheywereactuallyclus-
galaxy population in the surroundings (r < 600 kpc) of each ters, their expected mass and redshift would put them beyond
passive overdensity, with respect to the distribution in same- reachoftheChandraC-COSMOS(Elvisetal.2009)andXMM-
size apertures at 100 random positions in the COSMOS field. Newton(Hasingeretal. 2007; Cappellutietal. 2009) programs
These distributions in Fig. 4 are based on public galaxy cata- in COSMOS, which place 3σ limits of 4-9×1043 erg s−1
logsandphoto-zdeterminations,totallyindependentfromthose on their X-ray luminosity, thus 5-7×1013M⊙ on their mass
we use in this work. For CC1003+02233 and CC0958+0158 (Leauthaudetal.2010).Thiswouldbeconsistentwiththeirsim-
weusetheMuzzinetal.(2013b)UltraVISTAcatalog,whilefor ilaritieswiththepassiveconcentrationinClJ1449+0857(M∼
CC1002+0134-notcoveredbytheUltraVISTAsurvey-weuse 5×1013M⊙,Gobatetal.2011,2013;Strazzulloetal.2013).As
theIlbertetal.(2009)catalog(notethatthisisani-selectedcat- areference,inaWMAP7(Komatsuetal.2011)cosmologywe
alog, thus not optimal in this redshift range, as shown by the expecttofind ∼2-8structuresmoremassivethan 5-7×1013 M⊙
grayregionin Fig. 4, left).We alsoshow inFig. 4ourphoto-z inthe1.7<z<2.5rangeina2sq.deg.field(orafactor∼2higher
withaPlanckcosmology,PlanckCollaborationetal.2014).
3 Thisoverdensityisattheedgeoftheareaprobedbyourcatalog(see A final confirmation necessarily relies on spectroscopic
Fig.3).Forthisreason,ther<600kpcapertureisnotfullycoveredby follow-up, which is not available yet. For the time being, just
thecatalog;thefielddistributioninFig.4isthusscaledaccordingly,by for the most distant of our candidate overdensities, covered
theeffectivelycoveredarea. by the outer part of the zCOSMOS-deep survey (Lillyetal.
4
Strazzulloetal.:Passiveoverdensitiesatz∼2
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available spectroscopic redshifts – all of star-forming galaxies Ilbert,O.,McCracken,H.J.,LeFe`vre,O.,etal.2013,A&A,556,A55
Kauffmann,G.,White,S.D.M.,Heckman,T.,etal.2004,MNRAS,353,713
– to tentatively probe the redshift distribution in its surround-
Komatsu,E.,Smith,K.M.,Dunkley,J.,etal.2011,ApJS,192,18
ings. This small, a-posteriori-assembled spectroscopic sample, Kovacˇ,K.,Lilly,S.J.,Knobel,C.,etal.2014,MNRAS,438,717
is hampered by poor sampling of the central, densest cluster- Kriek,M.,vanDokkum,P.G.,Franx,M.,etal.2006,ApJ,649,L71
candidateregion,andsub-optimaltargetselection.Nonetheless, Kriek,M.,vanDokkum,P.G.,Labbe´,I.,etal.2009,ApJ,700,221
apossibleredshiftspikeappearsatz∼2.19with5(3)galaxiesat Krogager,J.-K.,Zirm,A.W.,Toft,S.,&whatever,x.2014,ApJ,797,17
Kurk,J.,Cimatti,A.,Zamorani,G.,etal.2009,A&A,504,331
2.18.z .2.2within∼1600(500)kpcfromtheoverdensity
spec Leauthaud,A.,Finoguenov,A.,Kneib,J.-P.,etal.2010,ApJ,709,97
center,plustwomorespikesofgalaxieswithin2Mpcofthecen- Lilly,S.J.,LeFe`vre,O.,Renzini,A.,etal.2007,ApJS,172,70
ter,atz∼2.17andz∼2.44(6and5galaxieswithin∆z∼±0.01, Madau,P.&Dickinson,M.2014,ARA&A,52,415
respectively,Fig.5).Althoughthespikescontainasimilarnum- Mancone,C.L.,Gonzalez,A.H.,Brodwin,M.,etal.2010,ApJ,720,284
McCracken,H.J.,Capak,P.,Salvato,M.,etal.2010,ApJ,708,202
ber of galaxies, their spatial distribution(Fig. 5) mightsuggest
McCracken,H.J.,Milvang-Jensen,B.,Dunlop,J.,etal.2012,A&A,544,A156
that the passive overdensity is more likely at z ∼ 2.2. Even if Mei,S.,Holden,B.P.,Blakeslee,J.P.,etal.2009,ApJ,690,42
somewhatlowerthantheestimatedphoto-zofthepassivegalax- Mei,S.,Scarlata,C.,Pentericci,L.,etal.2014,ArXive-prints
ies,thiswouldbeanywayconsistentwithintheuncertainties. Muzzin,A.,Marchesini,D.,Stefanon,M.,etal.2013a,ApJ,777,18
Muzzin,A.,Marchesini,D.,Stefanon,M.,etal.2013b,ApJS,206,8
Dedicatedfollow-upworkisobviouslystillneededtoverify
Muzzin,A.,Wilson,G.,Yee,H.K.C.,etal.2012,ApJ,746,188
whether these candidate overdensities are indeed signposts for
Newman,A.B.,Ellis,R.S.,Andreon,S.,etal.2014,ApJ,788,51
early cluster environments. If successful, this approach would Onodera,M.,Renzini,A.,Carollo,M.,etal.2012,ApJ,755,26
provide a further option to extend the investigation of distant Papovich,C.,Momcheva,I.,Willmer,C.N.A.,etal.2010,ApJ,716,1503
cluster-like structures to the z ∼ 2 − 2.5 range. In compari- PlanckCollaboration,Ade,P.A.R.,Aghanim,N.,etal.2014,A&A,571,A16
Salpeter,E.E.1955,ApJ,121,161
son with most other (proto-)cluster search techniques at these
Sa´nchez-Bla´zquez,P.,Jablonka,P.,Noll,S.,etal.2009,A&A,499,47
redshifts,e.g.the“IRACselection”(e.g., Papovichetal.2010; Santos,J.S.,Fassbender,R.,Nastasi,A.,etal.2011,A&A,531,L15
Stanfordetal. 2012), targeted searches around radio-galaxies Scoville,N.,Arnouts,S.,Aussel,H.,etal.2013,ApJS,206,3
(e.g.,Venemansetal.2007;Wylezaleketal.2013),3Dmapping Spitler,L.R.,Labbe´,I.,Glazebrook,K.,etal.2012,ApJ,748,L21
Stanford,S.A.,Brodwin,M.,Gonzalez,A.H.,etal.2012,ApJ,753,164
(withspectroscopicorphotometricredshifts,e.g., Dieneretal.
Strazzullo,V.,Gobat,R.,Daddi,E.,etal.2013,ApJ,772,118
2013;Scovilleetal.2013; Chiangetal. 2014;Meietal. 2014),
Strazzullo,V.,Rosati,P.,Pannella,M.,etal.2010,A&A,524,A17
or – closer to what done here – overdensities of optically red Tanaka,M.,Finoguenov,A.,Mirkazemi,M.,etal.2012,ArXive-prints
galaxies (e.g., Andreonetal. 2009; Spitleretal. 2012), the ap- Tanaka,M.,Toft,S.,Marchesini,D.,etal.2013,ApJ,772,113
proachdiscussed in this work is likely to favour, by definition, Toft,S.,Gallazzi,A.,Zirm,A.,etal.2012,ApJ,754,3
Tran,K.,Papovich,C.,Saintonge,A.,etal.2010,ApJ,719,L126
themostevolvedenvironments,allowingabetterprobeofthedi-
Valentino,F.,Daddi,E.,Strazzullo,V.,etal.2014,ArXive-prints
versityofclusterprogenitorsatacrucialtimefortheformation Valentinuzzi,T.,Poggianti,B.M.,Fasano,G.,etal.2011,A&A,536,A34
ofbothclustersandtheirmassivegalaxies. vandeSande,J.,Kriek,M.,Franx,M.,etal.2013,ApJ,771,85
vandeSande,J.,Kriek,M.,Franx,M.,etal.2011,ApJ,736,L9
Acknowledgements. WethankM.PannellaandA.Saroforhelpfulinputs.VS, vanderWel,A.,Holden,B.P.,Zirm,A.W.,etal.2008,ApJ,688,48
ED,RGandFVweresupportedbygrantsERC-StGUPGAL240039andANR- Venemans,B.P.,Ro¨ttgering,H.J.A.,Miley,G.K.,etal.2007,A&A,461,823
08-JCJC-0008.ACandMMacknowledge grants ASIn.I/023/12/0andMIUR Weiner,B.J.2012,ArXive-prints
PRIN2010-2011”ThedarkUniverseandthecosmicevolutionofbaryons;from Williams,R.J.,Quadri,R.F.,Franx,M.,etal.2009,ApJ,691,1879
currentsurveystoEuclid”.BasedonobservationsfromESOTelescopesunder Wuyts,S.,Cox,T.J.,Hayward,C.C.,etal.2010,ApJ,722,1666
programIDs086.A-0681,088.A-0671,LP175.A-0839,and179.A-2005. Wylezalek,D.,Galametz,A.,Stern,D.,etal.2013,ApJ,769,79
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