Triplets of Quasars at high redshift I: Photometric data

1.3 MB·English

by  M. Victoria Alonso

Mon.Not.R.Astron.Soc.000,1–??(0000) Printed4February2008 (MNLATEXstylefilev2.2)
Triplets of Quasars at high redshift I: Photometric data
M. Victoria Alonso1, Georgina V. Coldwell1, Ilona S¨ochting2, Carlos Bornancini1,3,
Malcolm G. Smith4, Diego Garc´ıa Lambas1 & Armin Rest4,5
8 1 IATECONICET-Observatorio Astron´omico de C´ordoba, C´ordoba, Argentina.
0 2 Universityof Oxofrd, Astrophysics, DenysWilkinson Building, Keble Road, Oxford OX13RH, UK
0 3 Fellow of Secretar´ıa de Cienciay T´ecnica de la Universidad Nacional de C´ordoba, C´ordoba, Argentina.
2 4 Cerro Tololo Inter-AmericanObservatory (CTIO), Colina elPino S/N, La Serena, Chile
n 5 PhysicsDepartment, Harvard University,17 Oxford Street,Cambridge, MA 02138
a
J

6 Accepted 2008January21.Received2007December 3
2

] ABSTRACT
h
Wehaveconductedanopticalandinfraredimagingintheneighbourhoodsof4triplets
p
- of quasars.R, z′, J and Ks images were obtained with MOSAIC II and ISPI at Cerro
o TololoInteramericanObservatory.Accuraterelativephotometryandastrometrywere
r obtained from these images for subsequent use in deriving photometric redshifts. We
t
s analyzed the homogeneity and depth of the photometric catalog by comparing with
a
results coming from the literature. The good agreement shows that our magnitudes
[
arereliabletostudylargescalestructurereachinglimiting magnitudesofR=24.5,z′

1 = 22.5, J = 20.5 and Ks = 19.0. With this catalog we can study the neighbourhoods
v of the triplets ofquasarssearchingfor galaxyoverdensitiessuch as groupsand galaxy
1 clusters.
9
0 Key words: galaxies: quasars - general, galaxies: photometry
4
.
1
0
8 1 INTRODUCTION At low redshifts (z ≤ 0.3), existing cross-correlation
0 analysis of quasar environments show that quasars in-
: Our present understanding of galaxy formation and evo-
v habit environments similar to those of normal galaxies
lution can be improved significantly through studies of
i (Smith et al. (1995), Coldwell et al. (2002). However, on
X quasars.Intheunifiedscenario, merger-drivenprocessesin- smallscales(projecteddistance,r ≤1h−1 Mpc;andradial
p
r cludetheoriginofaquasarbyself-regulating,supermassive, velocity difference, ∆V = 500km s−1), the quasar environ-
a black-hole growth (Hopkinset al. 2005). Thus, most mas-
mentisoverpopulatedbybluediskgalaxieshavingastrong
sivespheroidsshouldhostanoptically-brightquasarduring
star-formation rate with respect to typical galaxy neigh-
a brief period of its evolutionary history.
bourhoods (Coldwell & Lambas 2003, 2006). This result is
There is well-established evidence indicating that sev- in agreement with those of S¨ochtinget al. (2002, 2004) in
eral galaxy properties depend on the environment where
the sense that low-redshift quasars follow the Large Scale
the galaxies formed and evolved. Several processes - such
Structuretracedbygalaxyclustersbuttheyarenotlocated
as stimulated or truncated star formation, tidal stripping, near the centerof clusters. These quasars are mainly found
merging, etc. - can determine the nature of galaxy popu-
intheperipheryofsuchstructuresorbetweentwo,possibly
lations and dynamics. Also to be considered is the possi-
merging, clusters.
bility thatAGNfeedback processes could inducesignificant
At higher redshifts, quasars are often associated with
changes in the evolution of galaxies near to thequasar (see
rich environments (Hall & Green 1998; Djorgovski 1999)
forinstanceCroton et al.(2006)).Bystudyingquasarenvi-
where the comoving density of galaxies is higher than that
ronments we may deepen our understanding of therelation
expected for the general field. This should be interpreted
of quasar fueling and galaxy interactions and merging; and
as the core of future rich clusters. Some results at z ∼ 1
as a consequence, theformation of bright galaxies.
suggest that quasars reside on theperipheries of clusters or
The availability of large quasar samples has increased
in cluster mergers (Haines et al. 2001, 2004; Tanaka et al.
significantly in recent years, mainly due to the advent
2000, 2001). Tanakaet al. (2001),for example, investigated
of large surveys such as the 2dF Galaxy Redshift Sur-
a group of 5 quasars tracing a ∼ 10 Mpc structure of 4–5
vey (Colless et al. 2001) and the Sloan Digital Sky Survey
clusters, with only a single radio-loud quasar appearing to
(Stoughton et al.2002).Thesesamplesprovideanopportu-
bedirectly associated with one of theclusters.
nity for more robust statistical studies of the quasar phe-
nomenon and its link with galaxy formation. In general, the existing studies of quasar environments
2 Alonso et al.
suggest their association with forming structures, marking 293 pairs of quasars, only 7 triplets and no systems with
merging clusters and filaments. Consequently, groupings of more members. To increase the sample of triplets, we ap-
quasars can be expected to trace regions of extraordinary plied the same criteria to the V´eron-Cetty & V´eron (2003)
activity. Pairs of quasars have been strongly studied and catalogue.
theyareusedtoidentifyclusterofgalaxiesathighredshifts. Figure 1 shows the redshift distribution of the fraction
Zhdanov & Surdej(2001)suggestedthatquasarpairscanbe of the quasar number normalized to the total quasar sam-
usedastracersofhighredshiftlarge-scalestructures.More- ple, and that corresponding to pairs and triplets (systems)
over, Djorgovski et al. (2003) found a quasar pair at z ∼ 5 of quasars found with the adopted selection criteria. The
associated with large scale structure, possibly a protoclus- redshifts of each of the twelve triplets in the total sample
teratthatredshift.Morerecently,Boris et al.(2007)found of triplets are marked at the top of the figure. For systems
that quasarpairsat z∼1are excellent tracers ofhigh den- of quasars, there are two peaks at z ∼ 0.2 and z ∼ 1.8,
sity environments suggesting that they can be used to find which are more concentrated than thetotal quasar sample.
galaxy clusters. Inspiteoftheuncertaintiesduetothesmallamountofdata
Triplets of quasars in a relative small volume are ex- forsystemsofquasars,thepresenceofthesetwopeaksmay
tremely rare and until now, no cluster-scale triplets have be interpreted in terms of differences in the growth of self-
been published or investigated. A very compact quasar as- regulated black holes for different masses (Di Matteo et al.
sociation (∼ 50 Kpc separation) was recently reported by 2005). They proposed a fast evolutionary growth for the
Djorgovski et al.(2007)suggestingthatthiscouldbeacom- massiveblack holesformed intheearly Universe,which ex-
pact galaxy group in a merging process. We propose to use haustedthesurroundinggas duetoahigheraccretion rate.
triplets of quasars as “lighthouses” to find and study clus- Ontheotherhand,thelifetimeoftheactivephaseincreases
ters of galaxies that favour such rare events. With this aim for black holes of smaller masses implying an excess of low
in mind, we performed an optical and infrared photometric luminosity quasars in the local Universe.
study of the environments of triplets of quasars at differ- The total sample consists of 12 triplets of quasars: 3
ent redshifts on the scale of galaxy clusters. The images triplets with z < 0.2 and 9 within the range of 0.9 < z
allow us to observe galaxies in the early Universe and they < 2.5. As mentioned above, pairs of quasars - as a class -
are used to investigate the environments of quasar triplets havebeenextensivelystudiedandsowewillconcentrateour
We also study the group and cluster morphology, and the efforts hereonly on triplets of quasars, which are veryrare.
photometric properties of individual galaxies in the neigh- Ourfirstsubsampleoftripletsofquasars,thosenearby
bourhood of each triplet.Wehavealready found that low-z triplets with z < 0.2 is discussed in S¨ochting et al. (2007).
triplets of Seyfert 1 galaxies are associated with extremely Using the photometric and spectroscopic catalogs of the
richclustersofgalaxies (S¨ochting et al. 2007).Ifthequasar SDSS-DR4,wefoundastrongassociationofz<0.2triplets
triggering mechanism doesnotevolvestrongly overredshift with the richest central parts of superclusters. In all cases
wecanexpecttofindsomeoftherichestgalaxyclustersever the individual members of the triplets have been found on
discoveredatz∼1.Anegativeresultwill beclearevidence theperiphery of the main galaxy concentrations.
of the evolution of thequasar formation mechanism. For some of our second subsample, those triplets at
This paper is organized as follows: In section 2 we de- higherredshifts,weperformedmulticolorphotometryofthe
scribe the sample selection and section 3 shows the optical regions near to these triplets in order to locate associated
and infrared observations anddata reduction.Section 4de- galaxy groups and clusters.
scribes thephotometric galaxy catalog, and in section 5 we
provideasummaryofthemainresults.Weadoptthelatest
cosmologywithΩ =0.3,Ω =0.7andaHubbleconstant,
M λ 3 OBSERVATIONS AND DATA REDUCTION
H0=70 km s−1 Mpc−1.
In this paper, we concentrate on four high-redshift triplets
from the sample. We obtained multicolor photometry of
their neighbourhoods with the CTIO 4m telescope using
2 THE SAMPLE
two instruments: MOSAIC II for the optical and ISPI for
the near IR (Proposal Ids. 06A-0146 and 06B-0117). Ta-
The quasar sample is defined by quasars which have at
ble 1 shows the observed triplets, including the triplet and
least one emission line with a full width at half maxi-
mum larger than 1000 km s−1, luminosities brighter than quasaridentifications,coordinates,redshiftsandabsoluteV
M =−23,PSFimagnitude<19.1andhighlyreliablered- magnitudes, respectively.
i
shifts (Schneideret al. 2003). To analyze the regions in the
neighbourhoodsofgroupsofquasars,weidentifieddifferent
3.1 Optical imaging
activesystemsusingthefriend-of-friend,three-dimensional,
percolation algorithm (Huchra& Geller (1982)). This tech- Triplets 5 and 6 were observed in the R and z′ bands using
niquewasappliedtothesampleof54000quasarstakenfrom MOSAICIIduring5halfnightsfromFebruary2nd through
SDSS-DR4which satisfy the criteria defined above. Taking 6th, 2006. This optical instrument is an array of 8 2048 ×
intoaccountgalaxiesinclustersandtheirsurroundings,and 4096SITeCCDswithagainof2e-/ADUandascaleof0.27
the low spatial density of quasars (about 15 quasars per arcsec/pixel,givingatotalfieldofviewof36×36arcmin2.
square degree), we finally defined as candidates for triplets In the R band,we made eight exposures of 600s for Triplet
ofquasarsthosesystemswithpercolationlongitudessmaller 5andelevenexposuresof600s for Triplet 6.Moreover, four
than 2 h−1 Mpc and 2000 km s−1. These limits are about exposures of 600s were taken in z′ band for both triplets.
twice the typical region for rich galaxy clusters. We found Table 2 shows the triplet Ids together with a summary of
Triplets of Quasars at high redshift I: Photometric data 3
the optical observations, in R and z’, in columns 2 and 3, 0.302 arcsec for R-band and ∆α = −0.110±0.236 arcsec
respectively. and ∆δ = −0.074±0.302 arcsec for z′-band. The scatter
in the coordinates is found to be typically smaller than 0.3
arcsec and it is consistent with the astrometric accuracy of
3.1.1 Photometry theUSNO-A2.0catalog. Wealso compared thepositions of
thestarswithSDSSsourcesandwefoundsystematicoffsets
The images were reduced using the SMSN pipeline
of 0.218 ± 0.096 and 0.434 ± 0.107 in right ascension and
(Rest et al. 2005; Garg et al. 2007; Miknaitis et al. 2007)
declination, respectively - similar to those found by Fadda
originally created for the SuperMACHO and ESSENCE
et al. (2004).
projects. The SMSN pipeline reduces the raw images using
the IRAF 1 mosaic routines XTALK and MSCCMATCH,
aswellasitsownroutinesinC,perlandpython.Aspartof
3.2 Infrared imaging
thisreduction,theimagesarecrosstalkcorrected,astromet-
rically calibrated, and split into the 16 separate amplifiers. Thetripletsofquasarswerealsoobservedintheinfraredus-
The images are then bias subtracted and flat fielded. The ingISPIintworunsduring2006:5halfnightsfromApril2nd
z-band images were also defringed. to6th and3completenights-October7th to9th.ISPI(In-
We flux calibrated the R and z′ images using our frared SidePort Imager) isa1-2.4 µm imagerwith an 2048
observations of several Landolt (1992) and Smith et al. ×2048HgCdTearraywithanscaleof0.3arcsec/pix,giving
(2002) standard stars, respectively. In our images, a field of view of 10.5 x 10.5 arcmin2 (van derBliek et al.
point-source magnitudes were obtained using SExtractor 2004).Thegainis4.25e-/ADUandthedarkcurrentisvery
(Bertin & Arnouts 1996) with an aperture diameter of 6′′, low - about 0.1 e-/sec for long exposures. However, this ar-
whichprovedtobeadequatebymonitoringthegrowthcurve ray has a number of bad pixels which are located along a
of all the measured stars. For the final calibration, we in- few rows and columns in theupperright corner.
clude the zero point and extinction term. The colour term AllfourtripletswereobservedintheKs bandbutonly
wasnegligiblecomparedtomagnitudeerrorsanditwasnot Triplets6and7wereobservedintheJband.Weusedacir-
included (as in Gawiser et al. (2006)). cular dithering pattern of 15 different positions to observe
For R magnitudes, the zero point is in agreement with thetriplet fields. For each image in thesequence, theexpo-
the published value for the instrument. In order to better sure time was 130s × 1 coadds and 20s × 5 coadds for J
analyze the precision of our calibration, we also compared and Ks,respectively.Table 2 includesa summary of theIR
themagnitudesofthestarsinourfieldsobtainedwithSEx- observations in columns 2 to 5.
tractor (MAG BEST) with the PSF magnitudes computed by
SDSS. Before performing the comparison we transformed
′ 3.2.1 Photometry
SDSSr magnitudestooursystemusingFaddaetal.(2004)
transformations.Figure2showsintheupperpanelthiscom-
Forthereduction,weusedstandardtoolsinIRAFandsome
parison of R magnitudes showing the good agreement with tasks from the CTIO Infrared reduction package (cirred)
those of the SDSS. The mean difference is ∆ R = 0.162 ±
written by R. Blum for specific ISPI processing. The basic
0.139.
datareduction consisted increatingthemasterflatfieldim-
′
In the z-band the comparison with SDSS was made age from a median of lamps on – lamps off dome flats and
withoutanytransformationbecausethefiltersareidentical.
theconstructionofthebadpixelmask.Thefinalmasterflat
The bottom panel of Figure 2 shows this comparison. The
wascorrectedbyillumination effectsandnormalized bythe
′
mean difference is ∆ z = 0.14 ± 0.08 mag. This offset is average intensity. Median sky images were created for each
consistentwiththeestimateduncertaintyoftheoverallzero-
ditheredsequence.Thescientificimageswereflatfielded,bad
point,mainlyduetotheverysmallnumberofstandardstars
pixelcorrected,andskysubtracted.Theastrometrywasper-
′
usedinthisband.Ourfinalz magnitudeswerecorrectedby formedwiththepackageWCSTools2(Mink2006).Thenfor
this offset. eachsequencetheimageswereweightedcombinedwithindi-
Weappliedtheoffsetsinbothbandstocorrectourfinal vidualweight mapimagesusingSWarpwritten byE.Bertin
magnitudes. Note also that all magnitudes are in the Vega 3.
system.
To calibrate the magnitudes, we observed some stan-
dard stars from Persson et al. (1998). We also used those
starsinourfieldsbelongingtothe2MASSpointsourcecat-
3.1.2 Astrometric precision
alog (Cutri et al. 2003)toimproveprecision. Asfortheop-
Inordertoassesstheaccuracyoftheastrometriccalibration, tical magnitudes, the stellar instrumental magnitudes were
′′
wecompared theposition of thestars in theobservedfields obtained with an aperture diameter setting of 6 . For the
with those of the USNO-A2.0 Catalog (Monet et al. 1998). finalcalibration, again we did not includethecolor term.
We used only unsaturated stars from our images. Figure 3 To verify the photometric precision we compared our
shows only the results of the comparison for the z′-band. magnitudes of detected objects (those with 5 σ above the
The mean offset in the positions between both catalogs is sky background) with the corresponding, uncontaminated,
negligible: ∆α=−0.057±0.281 arcsec and ∆δ=−0.097± point sourcesfrom the2MASScatalogue. Figure 4presents
comparisons of the magnitude differences for the stars in
1 ImageReductionandAnalysisFacility(IRAF),asoftwaresys-
temdistributedbytheNationalOpticalAstronomyObservatories 2 Availableatftp://cfa-ftp.harvard.edu/pub/gsc/WCSTools/
(NOAO) 3 http://terapix.iap.fr/soft/swarp
4 Alonso et al.
common with 2MASS in J and K bands, respectively; the the image seeing. In Figure 8 we show the half-light radius
s
agreement is excellent. In the J band, for objects brighter vs K magnitude. Again, as in Figure 7 point sources with
s
than 16 mag, ∆J = −0.007±0.153 mag. In the K band, CLASS STAR > 0.95 are plotted with star symbols. As men-
s
thecomparisonslooksimilar;∆K =−0.066±0.109magfor tioned in Sub-SectionInfrared imaging, thereare some bad
s
objects brighter than 14.5 mag. Finally, in both bands, we pixels located along a few rows and columns and yet they
can see from Figure 4 a progressive increase in the scatter were not rejected during the reduction process. These ob-
at fainter magnitudes due to larger errors in the 2MASS jects have r1/2 < 1 in the Figure and they were discarded
catalogue at these magnitudes. in the final catalog. Also, the stellar locus is clearly visible
at r1/2 ∼ 2 and Ks < 18. Therefore, we define as galaxies
those objects with J−K > 1 and K < 18 mag.
s s
3.2.2 Astrometric precision
We created the final photometric catalog discarding
spurious objects and stars.
In order to verify the precision of our astrometry, we also
checked our measured positions for 2MASS objects in our
fieldsagainst the2MASScatalogue. Figure7showsthedif-
4.1 Results. Number counts
ferencesinbothcoordinates,rightascensionanddeclination,
betweenthetwosetsofdatainKsband.Theaveragediffer- Thefinalcatalog inall bandswas obtained with total mag-
encesare∆α=0.096±0.396 arcsecand∆δ=0.006±0.347 nitudes, and no need to apply an aperture correction. Our
arcsec for J band for objects brighter than 16 mag; and fields are located at | b |> 40◦, so the Galactic extinction
∆α = 0.038±0.313 arcsec and ∆δ = 0.018±0.371 arcsec effect is generally small. Only the R magnitudes were cor-
for Ks band for objects brighter than 14.5 mag, an overall rectedforgalacticextinctionusingtheE(B−V)valuespro-
astrometric precision sufficient for ourpurposes. vided in the NASA / IPAC Extragalactic Database (NED)
4,whicharebasedontheE(B−V)valuesfromtheextinc-
tion maps of Schlegel et al. (1998).
4 SOURCE DETECTION AND CATALOGUE Thenumbercountsasafunctionofmagnitudeallowus
toanalyzethehomogeneityanddepthofthecatalogandto
Ourphotometric catalog in thefour bandswas constructed compare it with results obtained from the literature.
using SExtractor. An object was considered detected if the Table 3 shows the main result, the number of galaxies
flux had an excess of 1.5 times the local background noise persquaredegree permagnitudein thefour bands:R,z′,J
level overat least 5 connected pixels. andK andtheirrespectiveerrors.TheFigures9,10,11and
s
The catalog includes magnitudes in apertures of 2, 3 12 show the galaxy number counts, in the four bands. We
and4arcsec,totalmagnitudes,star-galaxyseparation,ellip- compared our results with galaxy number counts from the
ticity, etc. The star-galaxy classification represented by the literature,mainly:MacDonald etal.(2004) andMetcalfe et
stellarity index,CLASS STARallows ustodecontaminate the al.(2001)intheRband;Kashikawaetal.(2004)andCapak
galaxy catalog. Figure6showsthisparameterasafunction et al. (2004) in the z′ band; Olsen et al. (2006), Saracco
′
of R magnitudes. The results are similar for z band. From et al. (1999) and Iovino et al. (2005) for the J band; and
theFigurewecanseeclearlytwoobjectsequences:starsde- finallyBestetal.(2003),Olsenetal.(2006),Best(2000)and
finedwith CLASS STAR ∼1 andgalaxies with CLASS STAR ∼ Bornancini et al. (2004) for K band.From theseFigures it
s
0. At fainter magnitudes, Bertin & Arnouts (1996) showed isclearthatwereachmagnitudesofR=24.5mag,z′=22.5
that the stellarity index is strongly seeing dependent. Tak- mag,J=20.5magandK =19.0mag.Withintheselimiting
s
ing this into account, we defined as galaxies those objects magnitudes,wefindgoodagreementwiththegalaxycounts
with CLASS STAR< 0.9 and CLASS STAR < 0.75 for Rand z′ coming from otherauthors in all bands.
bands, respectively.
TheIRphotometriccatalogwasconstructedusingSEx-
tractorindouble-imagemodewith theK imageasarefer-
s
5 SUMMARY
ence on account of its betterquality.
Figure 7 shows the IR colour-magnitude diagram and We selected a sample of triplets of quasars as those triple
how the J −Ks colour alone provides a reasonable means systemswithpercolationlongitudessmallerthan2h−1 Mpc
ofperformingthestar-galaxyseparation.Thehorizontalse- and 2000 km s−1. The goal is to use them as “lighthouses”
quence at J −Ks < 1 corresponds to the stellar sequence to find richest clusters. For this purpose we conducted a
of galactic stars with types later than G5 and earlier than photometric study of the neighbourhoods of 4 triplets of
K5 (Finlator et al. 2000). In the Figure we also plot point quasars in the redshift range of 0.9 to 1.7. Images in R,
sourcesdetectedwithCLASS STAR>0.95(starsymbols).As z′, J and K bands were obtained with MOSAIC II and
s
canbeseenmostoftheseobjectsarelocatedwithinthestel- ISPI at CTIO. Magnitudes and coordinates were found to
lar locus box. But those point sources with redder colours beinexcellentagreementwiththeUSNO-A2.0and2MASS
(J −Ks >1 mag and Ks > 18) are too faint and the star- catalogs. We analyzed the homogeneity and depth of the
galaxy separation becomes unreliable. In spite of this, we photometric catalog by comparing the number counts with
prefer to include them in the final catalog (also using star resultscoming from theliterature. Theexcellent agreement
symbols). shows that our magnitudes are reliable, down to limiting
To reinforce our star-galaxy separation we also use the
half-light radius parameter, r1/2. This parameter measures
the radius that encloses 50% of the object’s total flux. It 4 http://nedwww.ipac.caltech.edu/ - the NASA-IPAC Extra-
is clearly independent of magnitude and depends only of galacticDatabase
Triplets of Quasars at high redshift I: Photometric data 5
magnitudesofR=24.5,z′ =22.5,J=20.5andK =19.0. Haines,C.P.,Clowes, R.G.,Campusano, L.E.,&Adam-
s
Theseresultsallow ustostudylarge- scalestructureandin son, A.J. 2001, MNRAS, 323, 688
aforthcomingpaperwewilldealwiththestatisticalanalysis Haines, C. P., Campusano, L. E., & Clowes, R. G. 2004,
of this data. A&A,421, 157
Hall, P.B., & Green, R.F. 1998, ApJ, 507, 558
Hopkins, P. F., Hernquist, L., Cox, T. J., Di Matteo, T.,
Martini,P.,Robertson,B.,&Springel,V.2005,ApJ,630,
6 ACKNOWLEDGMENTS
705
Thisresearchwaspartially supportedbygrantsfromCON- Huchra,J. P., & Geller, M. J. 1982, ApJ, 257, 423
ICET,AgenciaC´ordobaCienciaandtheSecretar´ıadeCien- Iovino, A., et al. 2005, A&A,442, 423
ciayT´ecnicadelaUniversidadNacional deC´ordoba.GVC Kashikawa, N., et al. 2004, PASJ, 56, 1011
wouldliketothanktheCTIOforhospitalityduringherob- Landolt, A. U.1992, AJ, 104, 340
servational runs. IKS would like to acknowledge the kind MacDonald, E. C., et al. 2004, MNRAS, 352, 1255
hospitality of the IATE group during her stay at Cordoba. Metcalfe, N., Shanks, T., Campos, A., McCracken, H. J.,
WewouldalsoliketothankRogerClowesforsharingwithus & Fong, R.2001, MNRAS, 323, 795
hisfringeframesforMOSAICdata.Thispublicationmakes Miknaitis, G., et al. 2007, ApJ, 666, 674
use of data products from the Two Micron All Sky Survey, Mink, D. 2006, ASP Conf. Ser. 351: Astronomical Data
which is a joint project of the University of Massachusetts Analysis Software and SystemsXV,351, 204
andtheInfraredProcessingandAnalysisCenter/California Monet, D., Canzian, B., Harris, H., Reid, N., Rhodes, A.,
InstituteofTechnology,fundedbytheNationalAeronautics & Sell, S. 1998, VizieR Online DataCatalog, 1243, 0
and Space Administration and the National Science Foun- Olsen, L. F., et al. 2006, A&A, 456, 881
dation. Also, the authors make use of the NASA/IPACex- Persson, S. E., Murphy,D. C., Krzeminski, W., Roth, M.,
tragalactic database (NED) which is operated by the Jet & Rieke,M. J. 1998, AJ, 116, 2475
PropulsionLaboratory,Caltech,undercontractwiththeNa- Rest, A., et al. 2005, ApJ, 634, 1103
tional Aeronautics and Space Administration. Saracco, P., D’Odorico, S., Moorwood, A., Buzzoni, A.,
Cuby,J.-G., & Lidman, C. 1999, A&A, 349, 751
Schlegel, D. J., Finkbeiner, D. P., & Davis, M. 1998, ApJ,
500, 525
REFERENCES
Schneider,D. P., et al. 2003, AJ, 126, 2579
Bertin E., ArnoutsS.,1996, A & A Supp.117, 393. Smith,R.J.,Boyle,B.J.&Maddox,S.J.,1995,MNRAS,
Best, P. N.2000, MNRAS, 317, 720 277, 270.
Best, P. N., Lehnert, M. D., Miley, G. K., R¨ottgering, Smith, J. A., et al. 2002, AJ, 123, 2121
H.J. A.2003, MNRAS,343, 1 S¨ochting, I. K., Clowes, R. G., & Campusano, L. E. 2002,
Boris,N.V.,Sodr´e,L.,Jr.,Cypriano,E.S.,Santos,W.A., MNRAS,331, 569
deOliveira, C. M., & West, M. 2007, ApJ, 666, 747 S¨ochting, I. K., Clowes, R. G., & Campusano, L. E. 2004,
Bornancini,C.G.,Mart´ınez,H.J.,Lambas,D.G.,deVries, MNRAS,347, 1241
W.,vanBreugel, W., DeBreuck,C., &Minniti, D.2004, S¨ochting,I.K.,Coldwell, G.V.,Alonso, M. V.,Smith,M.
AJ, 127, 679 & Lambas, D. G. 2007, in preparation
Capak, P., et al. 2004, AJ, 127, 180 Stoughton,C., et al. 2002, AJ, 123, 485
Coldwell, G. V., Mart´ınez, H. J., & Lambas, D. G. 2002, Tanaka, I., Yamada, T., Arag´on-Salamanca, A., Kodama,
MNRAS, 336, 207 T., Miyaji, T., Ohta, K., & Arimoto, N. 2000, ApJ, 528,
Coldwell, G.V.,&Lambas,D.G.2003, MNRAS,344,156 123
Coldwell, G.V.,&Lambas,D.G.2006, MNRAS,371,786 Tanaka, I., Yamada, T., Turner, E. L., & Suto, Y. 2001,
Colless, M., et al. 2001, MNRAS, 328, 1039 ApJ, 547, 521
Croton, D. J., et al. 2006, MNRAS, 365, 11 van der Bliek, N. S., et al. 2004, SPIE–The International
Cutri,R.M.,et al.2003, TheIRSA2MASSAll-SkyPoint Society for Optical Engineering, 5492, 1582
Source Catalog, NASA/IPAC Infrared Science Archive. V´eron-Cetty,M.-P., & V´eron,P. 2003, A&A,412, 399
http://irsa.ipac.caltech.edu/applications/Gator/ Zhdanov,V. I., & Surdej, J. 2001, A&A, 372, 1
DiMatteo, T.,Springel,V.,&Hernquist,L.2005, Nature,
433, 604
Djorgovski, S.G.1999, TheHy-RedshiftUniverse:Galaxy
Formation and Evolution at High Redshift, 193, 397
Djorgovski, S. G., Stern, D., Mahabal, A. A., & Brunner,
R.2003, ApJ, 596, 67
Djorgovski, S. G., Courbin, F., Meylan, G., Sluse, D.,
Thompson, D., Mahabal, A., & Glikman, E. 2007, ApJ,
662, L1
Fadda, D., Jannuzi, B. T., Ford, A., & Storrie-Lombardi,
L. J. 2004, AJ, 128, 1
Finlator, K., et al. 2000, AJ, 120, 2615
Garg, A., et al. 2007, AJ, 133, 403
Gawiser, E., et al. 2006, ApJS, 162, 1
6 Alonso et al.
Table 1.TheObservedSampleofTripletsofQuasars
Triplet RA(J2000) DEC(J2000) z MV
QuasarId.
TRIPLET4
2QZJ015646-2828 1:56:46.6 -28:28:0 0.919 -24.5
2QZJ015647-2831 1:56:47.9 -28:31:43 0.919 -24.4
2QZJ015656-2823 1:56:56.0 -28:23:35 0.925 -24.3
TRIPLET5
SDSSJ10023+0155 10:02:19.5 01:55:37 1.509 -24.8
SDSSJ10025+0150 10:02:34.3 01:50:10 1.506 -25.7
SDSSJ10026+0159 10:02:36.7 01:59:48 1.516 -24.7
TRIPLET6
2QZJ120914+0035 12:09:14.9 00:35:51 1.316 -25.8
2QZJ120919+0029 12:09:19.6 00:29:26 1.319 -24.3
2QZJ120922+0026 12:09:22.4 00:26:46 1.322 -24.2
TRIPLET7
2QZJ235405-2845 23:54:05.3 -28:45:06 1.673 -25.2
2QZJ235430-2848 23:54:30.2 -28:48:41 1.670 -24.6
2QZJ235414-2848 23:54:14.2 -28:48:10 1.679 -25.8
Table 2.ObservingLogs
TripletId. R z′ J Ks
TRIPLET4 – – – 7×15×5×20s
TRIPLET5 8×600s 4×600s – 8×15×5×20s
TRIPLET6 11×600s 4×600s 15×15×130s 8×15×5×20s
TRIPLET7 – – 12×15×130s 7×15×5×20s
Table 3.NumberCountsinNDeg−1 0.5Mag−1
Magnitude R z′ J Ks
14.75 ... ... ... 126±63
15.25 ... ... ... 347±104
15.75 ... ... ... 663±144
16.25 ... ... 410±113 1359±207
16.75 ... ... 726±151 2117±258
17.25 ... 151±21 979±175 4013±356
17.75 ... 281±29 2180±262 5466±415
18.25 ... 446±36 2054±254 10110±565
18.75 ... 814±49 4013±356 13680±657
19.25 357±32 1489±66 6098±438 15580±701
19.75 804±48 2269±81 7520±487 15580±701
20.25 1416±64 3827±105 11630±606 13020±641
20.75 2112±78 5946±131 12950±639 5814±428
21.25 2945±92 8487±157 13110±643 ...
21.25 2945±92 8487±157 9826±557 ...
21.75 4599±116 14490±205 6667±459 ...
22.25 6754±140 18820±233 ... ...
22.75 10340±173 17250±224 ... ...
23.25 16160±217 10180±172 ... ...
23.75 26070±275 ... ... ...
24.25 37260±329 ... ... ...
24.75 40550±343 ... ... ...
25.25 11050±179 ... ... ...
Triplets of Quasars at high redshift I: Photometric data 7
0.15
Triplets of QSOs
Systems of QSOs
0.1 Total sample of QSOs
0.05
0
0 1 2 3 4
z
Figure1.Relativeredshiftdistributionofthetotalquasarsam- Figure 3. Astrometric differences in coordinates between the
ple(dashedline),andsystemsofquasarsrepresentedbypairsand matchedMOSAICIIandUSNOA2.0starsinthez′-band.
triplets(solidline).Theredshiftsofthe12tripletsinoursample
aremarkedatthetopofthefigure.
Figure4.MagnitudecomparisonsforIRbandsbetweenstarsin
commonwith2MASS.Upperandbottompanelsshowmagnitude
Figure 2. Magnitude comparisons for optical bands between differencesvsourmagnitudes forJandKsbands,respectively.
starsinourobservedfieldsandthosestarsincommonwithSDSS.
Upper panel shows the R-bandmagnitude differences beforeap-
plyingFaddaetal(2004)transformationtoSDSSr′magnitudes.
Bottom panelshowsthez′-bandmagnitudedifferences.
8 Alonso et al.
5
4
3
2
1
0
-1
10 12 14 16 18 20 22
Figure 5.Differencesinrightascensionanddeclinationofstars
incommonwiththe2MASScatalogintheKs band. Figure 7. IR Color–magnitude diagram to discriminate among
starsandgalaxies.Starsymbolscorrespondtothoseobjectswith
CLASS STAR>0.95.
1
8
0.8
10
0.6 12
14
0.4
16
0.2
18
20
0
16 18 20 22 24 26
R
22
0 1 2 3 4 5 6 7
Figure6.Thestar-galaxyseparationversustotalmagnitudesfor
Rband.
Figure 8. Half-light radius vs Ks magnitude. As in previous
Figure,starsymbolscorrespondtothoseobjectswithCLASS STAR
>0.95.Theverticallinemarksourdetectabilitylimit.
Triplets of Quasars at high redshift I: Photometric data 9
1000
100
16 17 18 19 20 21 22
Figure 9. Number counts N(R) in the R band. We also show Figure 11. Number counts N(J) in the J band. We also show
number counts coming from other authors: MacDonald et al. number counts coming from other authors: Olsen et al. (2006),
(2004)andMetcalfeetal.(2001). Saraccoetal.(1999)andIovinoetal.(2005).
1000
100
15 16 17 18 19 20
Figure 10. Number counts N(z′) in the z′ band. We also show Figure12.NumbercountsN(Ks)intheKsband.Wealsoshow
number counts coming from other authors: Kashikawa et al. number counts coming from other authors: Best et al. (2003),
(2004)andCapaketal.(2004). Olsenetal.(2006), Best(2000)andBornancinietal.(2004).