Three candidate clusters of galaxies at redshift ~1.8: the "missing link" between protoclusters and local clusters?

0.34 MB·English

by  M. Chiaberge

Accepted for publicationin ApJL
PreprinttypesetusingLATEXstyleemulateapjv.03/07/07
THREE CANDIDATE CLUSTERS OF GALAXIES AT REDSHIFT ∼1.8: THE “MISSING LINK” BETWEEN
PROTOCLUSTERS AND LOCAL CLUSTERS?
Marco Chiaberge1,2, A. Capetti3, F. Duccio Macchetto1, P. Rosati4, P. Tozzi5, and G. R. Tremblay6
Accepted for publication in ApJL
ABSTRACT
We present three candidate clusters of galaxies at redshifts most likely between 1.7 and 2.0, which
0
correspondstoafundamentallyunexploredepochofclustersevolution. Thecandidateswerefoundby
1
studyingtheenvironmentaroundournewlyselectedsampleof“beacons”low-luminosity(FRI)radio
0
galaxies in the COSMOS field. In this way we intend to use the fact that FRI at low z are almost
2
invariably located in clusters of galaxies. We use the most accurate photometric redshifts available

n to date, derived by the COSMOS collaboration using photometry with a set of 30 filters, to look for
a three-dimensional space over-densities around our objects. Three out of the five FR Is in our sample
J
which possess reliable photometric redshifts between z = 1.7 and 2.0 display overdensities that
phot
3 togetherarestatisticallysignificantatthe 4σ level,comparedto fieldcounts,arguingfor the presence
1
ofrichclustersofgalaxiesintheirMpcenvironment. Thesefirstresultsshowthatthenewmethodfor

findinghigh-z clusterswerecentlyproposed,whichmakesuseoflowpowerFRIradiogalaxiesinstead
]
O of the more powerful FR II sources often used in the literature to date, is returning very promising
candidates.
C
Subject headings: galaxies: clusters: general — galaxies: active
.
h
p
1. INTRODUCTION Riley 1974), are almost invariably located in clusters of
-
o galaxies (e.g. Zirbel 1996; Owen 1996, and references
The searchfor clusters of galaxiesat z >1 has proven
r therein) and are also associated with the brightest clus-
t to be particularly difficult, mainly because of the re-
s ducedcontrastbetweenclustermembersandfieldgalax- ter members (e.g. Best et al. 2007). For z > 0.5 an in-
a creasingfractionofthemorepowerful“edge-brightened”
ies. High-z clusters have been searched for using a vari-
[
FR IIs are also found in rich environments(e.g.Prestage
ety of techniques, including the so-called“redsequence”
1 technique(Gladders&Yee2000),andtheuseofbothX- & Peacock 1988; Hill & Lilly 1991; Best 2000), but the
v ray surveys (Rosati et al. 2002, and references therein), fraction of FR II in rich clusters is still lower than that
2 of FR Is. Furthermore, the host galaxies of FR Is are
and infrared surveys (e.g. Elston et al. 2006). Recently,
3 more similar to normal “inactive” ellipticals than those
Eisenhardt(2008)presentedasampleofclustersofgalax-
2 ofFRIIs. GiantLyαemitters,possiblyrepresentingpro-
iesfromtheSpitzerInfraredArrayCameraShallowSur-
2 tocluster regions, have been discovered around powerful
vey. Of their 335 cluster candidates in their sample, 12
. radio galaxies at z > 2 (e.g. Steidel et al. 2000; Penter-
1 have confirmed spectroscopic redshift 1 . z . 1.4, and
0 a few (less than 10) have photometric redshift (obtained icci et al. 2001; Zirm et al. 2005), but the relationship
0 usingphotometryin4bands,B ,R,I,and3.6µm)inthe with today’s clusters is still a matter of debate, mainly
W
1 range 1.5 . z . 1.7. None of their candidates have because of the small sample of clusters known between
: phot z ∼1 and 2, and in particular at 1.5<z <2.
v either photometric or spectroscopic redshift higher than
Herewetakeadvantageofthespecificpropertiesofthe
i 1.7. A similar effort is being pursued by the SpARCS
X environmentofFRIstosetthe stageforfillingthatred-
project (e.g. Wilson et al. 2009), which found no candi-
shift gap. We utilize our newly selected sample of high-
r dates at 1.4<z <2.2.
a z low luminosity radio galaxies (Chiaberge et al. 2009)
Radio galaxies (e.g. Hall et al. 2001; Best et al. 2003;
(hereinafterPaperI)as“beacons”forclustersinthe un-
Venemansetal.2007)andquasars(e.g.Haasetal.2009)
explored range of redshift. In this Letter, we present
have also been extensively used to find high-z clusters
first results that returned three very promising cluster
(see alsoMiley & De Breuck 2008,for a review), as they
candidates, most likely located at z ∼1.7−2.0.
are associated with massive elliptical galaxies and clus-
We use the following cosmological parameters (H =
tersinthelocaluniverse. However,itisknownthatonly 0
71 Km s−1 Mpc−1, Ω = 0.27, Ω = 0.73, Hinshaw
lowpowerradiogalaxies,mostofwhichpossessan“edge- M vac
et al. 2009).
darkened” radio morphology, namely FR I (Fanaroff &
1SpaceTelescopeScienceInstitute,3700SanMartinDrive,Bal- 2. THESAMPLEOFFRI“BEACONS”
timore,MD21218
2INAF-IRA,ViaP.Gobetti 101,Bologna,I-40129 Details of the selection of the high-z (1<z <2) FR I
3INAF-OsservatorioAstronomicodi Torino,ViaOsservatorio sample are given in Paper I. Here we briefly summarize
20,I-10025PinoTorinese,Italy the relevant steps.
4EuropeanSouthernObservatory,Karl-Schwarzschild-Strasse2, We proceedinfour steps,under the basic assumptions
D-85748,GarchingbeiMu¨nchen
5INAF - Osservatorio Astronomico di Trieste, Via Tiepolo 11, that 1) the FR I/FR II break in radio power per unit
I-34143Trieste,Italy frequency(usuallysetatL1.4GHz ∼4×1032ergs−1Hz−1
6RochesterInstituteofTechnology,OneLombMemorialDrive, Fanaroff & Riley 1974) does not change with redshift,
Rochester,NY14623USA andthat2)themagnitudeandcolorofthehostsofFRIs
2 Chiaberge, M., et al.
Fig. 1.—RGB“color”imagesofthethreecandidateclustersaroundCOSMOS-FRI03(left),05(center), and226(right). Theimages
are obtained using Spitzer 3.6µm, Subaru r and B band images for the R, G and B channels respectively. White circles indicate objects
with1.6<zphot<2.3. Theprojectedsizeofthefieldsare180′′×180′′ (∼1.5×1.5Mpc2 atz∼1.8),Northisup.
at 1 < z < 2 are similar to those of FR IIs within the
TABLE 1
same redshift bin, as in the case of local radio galaxies Photometric redshifts
(e.g. Donzelli et al. 2007). Note that the photometric
redshift is not a selection constraint.
Source za 99%rangea 68%rangea zb
1) We select FIRST (Becker et al. 1995) radio sources phot phot
COSMOS-FRI03 1.96 1.55-2.32 1.88-2.15 1.59
in the COSMOS field (Scoville et al. 2007), whose 1.4
COSMOS-FRI05 1.84 1.72-2.06 1.82-1.87 2.08
GHz fluxes correspond to the range of fluxes expected COSMOS-FRI22a 1.79 1.69-2.08 1.74-1.82 1.50
for FR Is at 1<z <2 (1<F <13 mJy). COSMOS-FRI226 1.76 1.62-2.37 1.71-2.03 2.04
1.4 COSMOS-FRI228a 1.88 1.30-2.63 1.81-2.05 1.45
2) Sources with FR II radio morphologyare excluded.
3) Bright (m > 22) galaxies are rejected since they Note. —(1)=sourcename,(2)=bestfitphoto-zestimate,(3)
I
are most likely low−z galaxies with faint radio emission and(4)99%and68%confidencelevelrangeofphoto-z,a=Ilbert
et al. (from 2009). Column (5) = photo-z, b = from Mobasher
(e.g. nearby starbursts).
etal.(2007)
.
4) U-band dropouts are rejected as they are likely to
be at z >2.5. aNotshowninthisLetter (seetext)
We are left with a sample of 37 FR I candidates, all
of which are identified in the COSMOS catalog (Capak AGNs. ThegalaxytemplatesusedbyIlbertetal.(2009)
et al. 2007). are therefore adequate for our analysis.
We search the Ilbert et al. (2009) catalog for the 37
3. USINGFRITOFINDHIGH-Z CLUSTERS sources presented in Paper I and we find all of them,
ToconfirmthattheFRIsourcesresideinaclusteren- but one. Four have 1.4 < z < 1.7 and 5 have 1.7 ≤
phot
vironment at z >1, spectroscopic redshifts of the target z < 2.0. Here we focus on the latter objects, since
phot
itself, and eventually of a significant number of galaxies they are among the highest redshift cluster candidates
inthe surroundingregionofthe sky wouldbe ideal. Un- known to date.
fortunately, the z-COSMOS catalog (Lilly et al. 2007) The five objects with 1.7≤z <2.0 are objects 03,
phot
does not include our sources. 05,22,226and228inthe catalogofPaperI.Wereferto
Inabsenceofspectroscopicdata,we takeadvantageof these objects as “COSMOS-FR I nn”. The Ilbert et al.
the recently published catalog of photometric redshifts (2009) catalog gives a “best fit” z and a range of
phot
obtained using 30 bands photometry by Ilbert et al. probableredshifts, at 68%and 99%confidence level(see
(2009) to identify the best candidates. The catalog, Table 1, where we also compare these photo-z with the
which includes objects with I < 25 in the deep Subaru previous estimates by Mobasher et al. (2007)).
area of the COSMOS field (Taniguchiet al. 2007)repre- Two out of the five objects (COSMOS-FR I 03 and
sents a significant improvement with respect to the list COSMOS-FR I 05) are detected in the X-ray band (as
of Mobasher et al. (2007), providing a factor of ∼ 3−5 point sources) both in the XMM-COSMOS (Hasinger
higheraccuracy. Theauthorsestimatetheredshiftaccu- et al. 2007) and in the Chandra-COSMOS data (Elvis
racy to be σ ∼0.04 for galaxies of i∼25 at z <1.25. et al. 2009).
∆z
For1.5<z <2.5andi∼25(similartoourfaintestFRI In this Letter, we focus on three objects, namely
hosts) the accuracy is σ ∼ 0.19, still well suitable for COSMOS-FR I 03, COSMOS-FR I 05, and COSMOS-
∆z
the purpose of this Letter. Note that the spectral en- FRI226,whichdisplaysignificantoverdensitiesofgalax-
ergydistributionofFRIshostgalaxiesisnotdominated ies in the Mpc-scale environment.
by strong emission lines7, contrary to those of all other InFig.1weshowRGBimagesofaregionof180×180
square arcsec, corresponding to ∼ 1.5×1.5 Mpc2 at a
7 We estimate a maximum contamination from the strongest redshift z ∼ 1.8, centered on each of the three radio
opticalemissionline(Hα)of∼1%. Thenarrowbandfiltersmight galaxy hosts. The images are obtained using Spitzer
beaffected byUVemissionlines(CIV 1549 andMgII2800), but
3.6µm, Subaru r and B band images for the R, G and
theircontributionisnotexpectedtoexceed10%(Buttiglioneetal.
2009). B channels respectively. The white circles indicate ob-
Cluster candidates at z ∼1.8 3
jects with 1.6<z <2.3 (see Sect. 4). The “beacon” large(∆z =0.2,similartothephoto-z accuracyforfaint
phot
FR I radiogalaxiesare at the center of eachofthe three objects at 1.5<z <2.5).
fields. Note that the host galaxy of COSMOS-FR I 03, The figure shows the overdensities of the cluster can-
which in Paper I we reported as “non-detected” in the didateswithrespecttothe controlfields,arguingfor the
HST-COSMOS I-band images (Koekemoer et al. 2007), presence of clusters. The space density enhancements
isinfactdetectedinthedeeperSubaru-COSMOSi-band appear to be centered between z ∼ 1.8−2.0, thus very
data. close to the FR Is’ best fit z (indicated by the solid
phot
lineineachplot). ForthefieldsofCOSMOSFRI 03and
COSMOSFRI226thedistributionofthe“extra”sources
is spreadacross a largerredshift range (∼1.6−2). This
is not entirely surprising, because of the uncertainty of
4. THECLUSTERCANDIDATES the photo-z estimates.
Inordertofindmoresupportforthepresenceofcluster We checked that the overdensities at zphot ∼ 1.8 are
ofgalaxiesaroundour“beacon”radiogalaxies,weagain not “artificially” generated by foreground clusters cou-
searchthe Ilbert etal.(2009)catalogfor sourcesthat lie pled with large photo-z errors at faint magnitudes. In
withinaradiusof90′′fromtheFRIhost. Werestrictthe fact, they are unrelated to the presence of any concen-
searchtoabinofphotometricredshift1.6<z <2.3. tration of lower-z objects. On the other hand, in a few
phot
The choice of that particular redshift rangeis motivated controlfieldsweobserveoverdensitiesatzphot .1,which
by the uncertainties on the estimate of z outlined do not artificially produce any overdensities at higher
phot
above. We find 58, 51 and 53 objects in the selected 3- redshifts.
dimensionalregionsaroundthethreeFRIs,respectively. Finally,wenotethatthetwoothersourcesintherange
In Fig. 1 we mark with white circles the galaxies with of photo-z of interest (namely COSMOS-FR I 22 and
1.6<z <2.3. TheK-bandmagnitudeofthemarked COSMOS-FRI228)donotshowsignificantoverdensities
phot
objectsisintherangeK ∼20.5−25(Ilbertetal.2009). with respect to the control fields, at least in the range
s
Notethattheexpectedmagnitudeofapassivelyevolving 1.6<zphot <2.3. One possibility is that the photo-z for
L⋆ galaxy at z ∼ 1.8 is K ∼ 21 (e.g. Strazzullo et al. these objects is significantly offset.
s
2006). The “central” radio galaxy host (always located
at the center of the fields shown in the figures) has a 5. CONCLUSIONS
distinct “red” color i-Ks & 3 and the Ks magnitude is Weidentifiedthreeclustercandidateswithzphot ∼1.8,
in the range 20.5-21.9 (Ilbert et al. 2009). In Paper I according to the photometric redshift catalog of Ilbert
we showed that our FR I hosts lie on the K-z relation et al. (2009). The possible existence of clusters is sup-
formorepowerfulradiogalaxyhostswithevolvedstellar ported by the significant over-density measured on the
populations. Mpc scaleenvironmentofthe three selectedradiogalax-
Note that a large number of “red” objects are not ies. These first results show that the method of using
marked in the figure. A significant number of those ob- low luminosity radiogalaxies as “beacons” for clusters is
jects (e.g. the galaxies∼30′′ NW of COSMOS-FRI 03) returning promising candidates. A systematic study of
are indeed only bright in the infrared, and do not make thewholesampleof37radiogalaxiesisunderway. That
theopticalselectioncuttobeincludedintheIlbertetal. will allow us to determine the typical environment of
(2009) catalog. These red objects are as bright as or these high-z sources, and firmly establish whether they
slightly fainter than the radio galaxy host in K-band. aremorelikelytobeassociatedwithclustersthanFRIIs
Their color is i-Ks & 3, consistent with evolved stellar in the same range of redshifts.
populations, and their magnitude is in the range Ks ∼ We have submitted observational proposals to spec-
20.5−22. A few other galaxies, which appear slightly troscopically measure the redshift of the candidates. If
less “red” than the radio galaxy host (e.g. the group in confirmed, these would represent extremely important
the South-East corner of the field of COSMOS-FR I 05, structures that would contribute to fill the existing gap
are simply foreground (zphot ∼0.8−1) galaxies with an between z ∼1.5 and z ∼2 in cluster studies.
old stellar population. A study of the fields with high accuracy photometry
We also perform the same search on 6 randomly se- anddeephighangularresolutionimages(withthe HST)
lected fields inside the COSMOS survey area. For 3- is mandatory to investigate the spatial distribution and
dimensionalregionsofthesamesizeastheonesdescribed themorphologyoftheclustergalaxies,toderivethepho-
above,wefind28to35objectsinthe6controlfields,with tometric properties of a significant number of member
a mean value of 32±2 and a median value of 32. This candidates, and assess the existence (or the absence) of
implies an over-densityfactor of ∼1.7 for the Mpc envi- the color-magnitude relation at these high redshifts.
ronments of the three selected FR Is, statistically signif- Insummary,follow-upobservationsofthesefieldsfrom
icantatthe ∼4σ level,whenthe three candidate cluster theradiothroughtheX-rays,willallowustotacklesome
fields are combined. We also checked that doubling the ofthe mostimportantquestions onthe originof clusters
number of control fields does not alter the results. of galaxies and cluster members, to investigate the link
In Fig. 2 we show histograms representing the photo- between protoclusters at z >2 and well formed clusters
′′
metric redshift distribution of the sources within a 90 in the nearby universe and to study the effects of the
radius around our radio galaxies (red histograms), com- evolution of the intracluster gas on the radio structures.
paredwiththeaveragedistributionofanidenticalareain
the control fields. Clearly, this assumes that the “best”
values of z are accurate to better than the width of We acknowledge the effort of the entire COSMOS
phot
redshift bins, which therefore is chosen to be sufficiently team. We thank A. Zirmfor insightful comments on the
4 Chiaberge, M., et al.
Fig. 2.— Photo-z distributions of the ∼ Mpc scale environment of the three candidates (red histograms; COSMOS-FR I 03, 05 and
226areshownintheleft,centerandrightpanels,respectively)ascomparedtotheaveragedistributionofthecontrolfields(blackshaded
histogram). Solid lines indicate the estimated photo-z of each of FR I “beacon”. Dashed and dot-dashed lines show the 68% and 99%
rangeofconfidence level,respectively(seeSect.3).
manuscript. Wealsoacknowledgetheanonymousreferee forvaluablesuggestionsthatgreatlyimprovedthepaper.
REFERENCES
Becker,R.H.,White,R.L.,&Helfand,D.J.1995,ApJ,450,559 Hinshaw,G.,etal.,ApJS,180,225
Best,P.N.2000,MNRAS,317,720 Ilbert,O.,etal.,ApJ,690,1236
Best, P.N.,Lehnert, M.D.,Miley,G.K.,& Ro¨ttgering, H.J. A. Koekemoer,A.M.,etal.,ApJS,172,196
2003, MNRAS,343,1 Lilly,S.J.,etal.,ApJS,172,70
Best,P.N.,vonderLinden,A.,Kauffmann,G.,Heckman,T.M., Miley,G.,&DeBreuck,C.2008, A&ARev.,1
&Kaiser,C.R.2007, MNRAS,379,894 Mobasher,B.,etal.,ApJS,172,117
Buttiglione, S., Capetti, A., Celotti, A., Axon, D. J., Chiaberge, Owen, F. N. 1996, in IAU Symposium, Vol. 175, Extragalactic
M.,Macchetto, F.D.,&Sparks,W.B.2009,A&A,495,1033 RadioSources,ed.R.D.Ekers,C.Fanti,&L.Padrielli,305–+
Capak,etal.,ApJS,172,99 Pentericci,L.,McCarthy,P.J.,Ro¨ttgering,H.J.A.,Miley,G.K.,
Chiaberge,M.,Capetti,A.,&Celotti,A.2000, A&A,355,873 vanBreugel,W.J.M.,&Fosbury,R.2001,ApJS,135,63
Chiaberge,M.,Tremblay,G.,Capetti,A.,Macchetto,F.D.,Tozzi, Prestage,R.M.,&Peacock, J.A.1988,MNRAS,230,131
P.,&Sparks,W.B.2009,ApJ,696,1103 Rosati,P.,Borgani,S.,&Norman,C.2002,ARA&A,40,539
Donzelli, C. J., Chiaberge, M., Macchetto, F. D., Madrid, J. P., Scoville,N.,etal.,ApJS,172,1
Capetti, A.,&Marchesini,D.2007,ApJ,667,780 Steidel, C. C., Adelberger, K. L., Shapley, A. E., Pettini, M.,
Eisenhardt,P.R.M.e.a.2008,ApJ,684,905 Dickinson,M.,&Giavalisco,M.2000,ApJ,532,170
Elston,R.J.,etal.,ApJ,639,816 Strazzullo,V.,etal.,2006,A&A,450,909
Elvis,M.,etal.2009,ApJS,184,158 Taniguchi,Y.,etal.,2007,ApJS,172,9
Fanaroff,B.L.,&Riley,J.M.1974,MNRAS,167,31P Venemans,B.P.etal.2007, A&A,461,823
Gladders,M.D.,&Yee,H.K.C.2000,AJ,120,2148 Wilson,G.,etal.2009,ApJ,698,1943
Haas,M.,etal.2009,ApJ,695,724 Zirbel,E.L.1996, ApJ,473,713
Hall,P.B.,etal.2001,AJ,121,1840 Zirbel,E.L.1997, ApJ,476,489
Hardcastle,M.J.2004,A&A,414,927 Zirm,A.W.,etal.,2005,ApJ,630,68
Hasinger,G.,etal.,ApJS,172,29
Hill,G.J.,&Lilly,S.J.1991, ApJ,367,1