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Galaxy clusters are the largest andmost massive discrete structures in theUniverse. They represent the endpointof gravity’s influence on the growth andcollapse of the Universe’s large scalestructure. Mass in the Universe, astraced by galaxies, is distributed insheets surrounding large, nearly emptyspherical voids. At the intersection ofthese sheets, at places referred to as fil-aments, the density of the universe iseven larger. At the intersection of fila-ments, sitting much like spiders in a3-D cosmic web, are galaxy clusters.Clusters are extremely dense, with cen-tral densities ∼1000 times that of theirsurroundings. As the Universe ages,galaxy clusters are thought to becomelarger and larger, as mass drains alongthe filaments into the central clusters.Galaxy clusters are extremely importantlaboratories for the study of a number ofquestions in astronomy, and will be oneof the most important targets for obser-vations by both ground-based andspace observatories in the comingyears.

The Red-Sequence Cluster Survey isan ambitious project designed to identi-fy a large sample of galaxy clusters overa wide range of redshifts (distances).The resulting sample of galaxy clusterswill yield answers about the way inwhich structures formed and grew in theUniverse, and will facilitate a number ofother projects. The survey is the largestarea survey ever conducted on 4-mclass telescopes, and as such will yield

important new insights on hitherto poor-ly measured or unknown phenomena.Despite the large area of the survey (90square degrees of sky - roughly 500times the apparent area of the fullMoon) a very efficient observing strate-gy allowed this unprecedented area tobe covered in only 25 nights of observ-ing time. Two telescopes were used tocomplete the project (the Canada-France-Hawaii 3.6 m telescope for thenorthern hemisphere, and the CerroTololo Inter-American Observatory 4 mtelescope for the southern hemisphere).The survey began in mid-1999, and ob-servations were finished by late 2001.We are currently using the powerfulESO VLT telescopes for following upsome of the highest redshift clusters inthe sample.

The method

In conjunction with the extensive datafrom the RCS project, we have deviseda new algorithm for finding clusters intwo-filter survey data. This algorithm ex-ploits the fact that all clusters so far ob-served have a central population of oldred galaxies. While the properties of theoverall cluster galaxy population doevolve with redshift (i.e. the fraction ofblue or actively star-forming galaxies isgenerally higher at higher redshift), in allwell-formed significant clusters so farobserved there is a red population. Inessence, we define a cluster as anoverdensity in both position and colour.The distribution of galaxy clusters, evenin systems with a large fraction of bluegalaxies, represents a colour distribu-tion not generally found in the field (i.e.non-cluster) galaxy population. Addi-tionally, the filters used for the RCS sur-vey provide a particularly deviant (andhence readily identified) colour for clus-ters at modest to very high redshifts.Thus, simple colour cuts allow us to se-lect 2-D groupings of red galaxies,which are very likely to be real 3-D clus-ters of galaxies at high redshifts. Wehave tested this method extensively us-ing both real redshift survey data, andusing complex and thorough simula-tions, and find that it works extremelywell (a detailed description of themethod can be found in Gladders & Yee2000).

Clusters and Cosmology

The number density of galaxy clus-ters as a function of their mass and red-

shift, N(M,z), is strongly dependent onthe cosmological parameters Ωm andσ8. Ωm describes the matter density ofthe Universe, and σ8 describes the am-plitude of the early fluctuations in theUniverse, which seed the growth ofstructures on the physical scale ofgalaxy clusters. In an expanding low-density universe (small Ωm), structureslike galaxy clusters must form relativelyearly, when the universe was still com-pact and relatively dense. Only in thissetting does such a universe have suf-ficient mass density to cause largestructures to collapse under the influ-ence of gravity, and even then only if theinitial fluctuations which seed thegrowth of structure are relatively large(high σ8). Conversely, in a universe withmuch greater mass content (large Ωm)structure continues to form as the uni-verse expands and ages, even whenthe seed fluctuations are relatively mod-est (low σ8).

We have known for a long time thatthe local universe contains many galaxyclusters. These clusters could resultfrom either a low matter density, largefluctuation cosmology, or a high matterdensity, small fluctuation cosmology.However, as described above, the pasthistory of this local cluster population iswildly different in these two cosmolo-gies, and so studies of clusters at greatdistances (which, due to light travel-time effects also corresponds to the dis-tant past) offer a powerful constraint oncosmological models. The RCS seemsto contain many high-redshift, massiveclusters, and hence initial resultsstrongly favour a low Ωm and high σ8universe. However, this conclusion de-pends critically on correct mass esti-mates for these systems, and our ongo-ing spectroscopy with the VLT+FORS2,for which the first data have just beentaken, represents a critical step in con-firming this initial result. A summary ofthe first clusters with confirmed red-shifts is shown in Figure 1.

Cluster Galaxy Evolution

Clusters of galaxies also provide uswith natural laboratories to study galaxyevolution, since we find a number ofgalaxies in a relatively small region ofthe universe that can be traced to highredshift, or equivalently to an epochwhen the universe was much younger.

Our studies are focussed on measur-ing properties such as the galaxy lumi-nosity function (LF), blue fraction, and

The Red-Sequence Cluster SurveyFELIPE BARRIENTOS1, MICHAEL GLADDERS2, HOWARD YEE3, ERICA ELLINGSON4,PATRICK HALL1,5, LEOPOLDO INFANTE1

1P. Universidad Católica de Chile; 2Observatories of the Carnegie Institution of Washington; 3University of Toronto;4University of Colorado; 5Princeton University

Figure 1: Spectroscopic confirmation of thefirst clusters found in the RCS. The meas-ured spectroscopic redshift for the clustersin this sample agrees very well with the esti-mates obtained from the photometric dataalone. We are currently working with the VLTand FORS2 to populate this diagram in theregion at z > 0.9.

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the CMR (colour-magnitude relation,i.e. red sequence), for clusters over arange of richnesses and redshift (indi-vidually for rich clusters, in redshift binsfor poor clusters). These data will allowinvestigations of the evolution of clustergalaxies, and will constrain their forma-tion time and process through a detailedanalysis of the evolution of the slope,scatter and colour of the CMR. As thesample will be volume limited for richclusters over a large portion of the red-shift range, we will be able to trace clus-ter galaxy evolution without strong se-lection biases.

RCS0439.6-2905: A ClusterGalaxy Evolution Case Study

As an example of such studies wepresent in Figure 2 an image ofRCS0439.6-2905, one of the most mas-sive distant clusters found in the RCS.Recent spectroscopy confirms thatRCS0439.6-2905 is at z = 0.97 makingit more distant than all but a handful ofknown galaxy clusters. The I, J and K-band colour composite in Figure 2 (I-band taken at Magellan and J and K-bands at the VLT) shows numerousgalaxies with similar colours that pre-sumably are cluster members.

Figure 3 shows the colour-magnituderelation for the galaxies in the field ofthis cluster. The quality of the imagestaken with the VLT was very good, witha seeing of ∼ 0.4 arcsec, and this allowsus to perform a morphological study byapplying galaxy image fitting tech-

niques. The galaxies selected as ellipti-cal or lenticular galaxies (E/S0s) on thebasis of their 2-D light profiles areshown as red circles. Clearly thesegalaxies define a tight colour sequencein this cluster. The red sequence as itwould appear at the distant redshift forthe E/S0 galaxies in the Coma cluster, anearby cluster of galaxies and thecanonical example of a rich cluster, isshown as the blue broken line. Thegalaxies in RCS0439.6-2905 appear in-trinsically slightlybluer than those inthe Coma cluster,consistent with thesegalaxies being likethose seen in Coma,but at a much youn-ger age.

The excellent im-age quality of theVLT not only allowedus to segregate theE/S0 galaxies in thiscluster at z = 0.97 butalso to determine thesize of the galaxies.The size of the galax-ies, accounting forthe seeing profile, isgiven by the effectiveradius, the radiusthat encloses half ofthe light of thegalaxy, and is showin Figure 4 as a func-tion of their absolutemagnitude. The red

symbols are the galaxies in the clusterat z = 0.97 and the blue crosses aretheir equivalent in local clusters ofgalaxies. The red solid line and the bro-ken blue line are the best fit for thegalaxies at z = 0.97 and for those in lo-cal clusters, respectively, keeping thesame slope for both sets of galaxies.The offset between the lines is approxi-mately 1.2 magnitudes. This brightnessshift can be interpreted as the galaxiesat z = 0.97 following a similar size-mag-nitude relation than those in local clus-ters but being 1.2 magnitudes brighterthan their local counterparts. Much likethe colour evolution, this is consistentwith models of early formation of thestellar population in these galaxies, withsubsequent fading and reddening asthey age.

Strong-Lensing Clusters

The large area and depth covered bythe RCS provides large samples ofgalaxy clusters spanning a wide rangeof properties. A particularly interestingsubsample is the strong-lensing clus-ters, which due to gravitational effectsact as lenses magnifying and distortingthe images of distant objects back-ground to the cluster. We have so farfound 8 new strong lensing systems inthe RCS, some of them with the pres-ence of multiple giant arcs (Gladders etal. 2003). The incidence of such a largenumber of strong lensing clusters in thesurveyed area is discrepant with currenttheoretical predictions (standard expec-tations are 0-1 such clusters in a surveythe size of the RCS).

The first studied RCS lensing cluster,RCS0224.5-0002, is shown in Figure 5.Spectroscopy of this cluster was ac-quired using the 8.2 m KueyenTelescope and FORS-2 in Director’s

Figure 2: IJK colour composite image of the field centred on RCS0439.6-2905. North is upand East to the left. This image shows approximately the central 1.1 × 1.1 Mpc.

Figure 3: IR colour-magnitude diagram in the field of RCS0439.6-2905. All the objects are included and shown as filled circles. Themorphologically selected E/S0 galaxies are shown as open cir-cles. These galaxies define a tight sequence, similar to that foundin local clusters. The solid line shows the best fit to the sequenceof E/S0 galaxies in the cluster. The broken line corresponds to thecolour-magnitude relation for the E/S0 galaxies in Coma clusterredshifted to z = 0.97.

Discretionary Time, soon after the clus-ter was discovered. This initial spec-troscopy demonstrated that the clusterwas at z = 0.773, and showed that oneof the arcs (the arc labelled ’C’, visiblein Figure 5) was extremely distant, at aredshift of 4.8786 (Gladders et al.2002). The FORS spectrum of this arcat Ly-α is shown in Figure 6, along withthe images of the arc in the R and I-bands. The Ly-α emission (shown byboth the spectrum and the R-bandlight), with an equivalent width of sever-al hundred Angstroms, is spatially ex-tended compared to the UV continuumjust to the red of Ly-α (shown by the I-band light). At the time of discovery,RCS0224.5-0002 was the most distantcluster known with such spectacularstrong lensing, and the high redshift arcwas one of only two known giant arcs atsuch great distances, the other being anarc at z = 4.92 formed around thez = 0.33 cluster CL 1358+62 (Franx etal. 1997). Notably, the distant arc inRCS0224.5-0002 appears rather differ-ent in detail; it is spatially smooth with anextended Ly-α halo surrounding a com-pact star-forming core, and shows novelocity structure, whereas the in CL1358+62 is knotty in appearance andshows velocity structure of ∼ 300 km/s.

Since the discovery of RCS0224.5-0002 we have found several other spec-tacular multiple-arc strong lens clusters.Of the total of 8 strong-lens systems, 2more are comparable to RCS0224.5-0002. This high proportion of multiplearc systems argues that there must ex-ist a class of “super-lenses” which, forsome yet undetermined reason, act asparticularly powerful lenses. Notably,the strong lens clusters in the RCS areall at z > 0.64, and it thus seems likelythat the source of this lensing boost isassociated with early times when clus-ters are still forming, and that whateveris responsible for the lensing boostevolves away as clusters age (Gladderset al. 2003).

Additional Projects: Clusteringand Evolution of Small GalaxyGroups

The wide area, depth and homo-geneity of the RCS data allow us to pur-sue other relevant problems in galaxyevolution, such as the study of the for-mation and evolution of structure in theUniverse, ranging from clusters,groups, pairs of galaxies to individualgalaxies.

As is well known, the distribution ofgalaxies in the universe is believed tobe different from the distribution of darkmatter; the distribution of galaxies is abiased tracer of the matter. To under-stand the clustering pattern of galaxiesthrough a good interpretation of obser-vational data and to compare them tocurrent predictions of cosmologicalmodels, this bias has to be understood.

In simulations, pairs, triples, smallgroups, groups and clusters of galaxies

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Figure 4: Size-magnitude diagram for theE/S0 galaxies (filled circles) in the field ofRCS0439.6-2905. The size has been ob-tained from the 2-D galaxy light profile fittingalgorithm. For comparison the E/S0 galaxiesin local clusters are show as crosses, in-cluding the best linear fit to these galaxies(broken line). The solid line corresponds tothe fit for the galaxies in RCS0439.6-2905,constrained to have the same slope as thatfor the local E/S0 galaxies. There is an off-set for the fit at z = 0.97 from the local rela-tion that amounts to ∆MB(AB) = –1.20 ± 0.09.

Figure 5: This 4040 image is a colour composite of zI+R+BV images of RCS0224.5-0002.Various features – two candidate radial arcs, and excess blue light in the cluster centre – arehighlighted in grey-scale inserts. Arc C is at z = 4.8786. (Adapted from Gladders et al. 2002)

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are fully characterized by their corre-sponding halo dark matter mass. If so,how can observations be used to meas-ure the “bias” ? Theoretically, there is a

strong relation between the dark matterhalo mass and the number of memberswith similar luminosities in clusters,cluster number richness, m. In the ob-

servational plane, the measured quanti-ties are N(z,m) and the two-point corre-lation function, ξ(r,m).

No clustering studies of small galaxygroups with m > 3 have been carriedout, basically because of small surveyarea, bad number statistics and lack ofdeep homogeneous data. To findgroups at different redshifts, deep widefield imaging is needed. In this project,SDSS and RCS provide the low andhigh-z groups, to z < 0.7 respectively. Anumber of groups have been detectedon the RCS data. In order to measurethe spatial clustering properties, inver-sion of 2-dimensional data is required.Currently, redshifts of group membersselected from SDSS and RCS are beingmeasured.

ReferencesGladders, M. D., & Yee, H. K. C. 2000, AJ,

120, 2148 Gladders, M. D., Yee, H. K. C., & Ellingson

E. 2002, AJ, 123, 1 Gladders, M. D., Hoekstra, H., Yee, H. K. C.,

Hall, P. H., & Barrientos, L. F. 2003, ApJ,in press

Franx, M., Illingworth, G. D., Kelson, D. D.,van Dokkum, P. G., & Tran, K.-V. 1997,ApJ, 486, L75

Figure 6: Arc C in R band (left), I band (middle), and in Ly emission (right). The spectral im-age has not been sky subtracted. The position of the slit as reconstructed after the FORS ob-servations, is also indicated in the broadband images.(Adapted from Gladders et al. 2002)

Long Period Variables in the Giant EllipticalGalaxy NGC 5128: the Mira P–L Relation at 4 MpcM. REJKUBA1,2, D. MINNITI2, D. R. SILVA1, T. R. BEDDING3

1ESO, Garching, Germany; 2Department of Astronomy, P. Universidad Católica, Chile; 3School of Physics,University of Sydney, Australia

In a stellar population older than afew hundred Myr, the near-infrared lightis dominated by red giants. Amongthese, the stars lying on the red giantbranch (RGB) are the brightest amongthe metal poor stars older than 1– 2 Gyr.In intermediate-age populations (~ 1– 5Gyr old) numerous bright asymptotic gi-ant branch (AGB) stars are locatedabove the tip of the RGB. However, alsoamong old populations like Galacticglobular clusters with [Fe/H] ≥ –1.0 andin the Galactic bulge, bright stars havebeen detected above the tip of the RGBimplying the presence of bright AGBstars in metal-rich and old populations.All of the bright giants above the RGBtip in globular clusters seem to be longperiod variables (LPVs; Frogel & Elias1988). Old populations of lower metal-licity are known not to have AGB starsbrighter than the RGB tip.

Long period variables are thermallypulsing asymptotic giant branch (TP-AGB) stars with main sequence mass-es between 1 and 6 M. They have vari-ability with periods of 80 days or longer,and often the longest period variablesshow variable or multiple periods. Twomain classes of LPVs are Mira variables(Miras) and semi-regular variables

(SRs). SRs usually have smaller ampli-tudes as well as shorter and more ir-regular or even multiple periods. Theyare sometimes divided into subclasses(SRa, SRb) depending on the regularityand multiplicity of their periods andshape of their light curves. The separa-tion between Miras and SRs is not al-ways very clear. The classical definitionrequires that Miras have V-band ampli-tudes larger than 2.5 mag and regularperiods in the range of 80–1000 days.Mean K-band amplitudes of Miras aretypically 0.6 mag. SRs show more ir-regular variability, as their name indi-cates, and have smaller amplitudes.

So far, LPVs have been studied in theMilky Way, Magellanic Clouds and a fewother Local Group galaxies. However,the Local Group lacks the importantclass of giant elliptical galaxies. At thedistance of about 4 Mpc (Harris et al.1999), NGC 5128 (Centaurus A) is theclosest giant elliptical galaxy, the clos-est active galactic nucleus (AGN), oneof the largest and closest radio sourcesand a classical example of a recentmerger. It is the dominant galaxy of thenearby Centaurus Group of galaxies.Rejkuba et al. (2001) presented optical-near-IR colour-magnitude diagrams

(CMDs) of two fields in the halo of thisgiant elliptical galaxy (Figure 1). TheseCMDs show broad giant branches indi-cating a large spread in metallicity. TheRGB tip is detected at K ~ 21.3.

A large number of sources are ob-served brighter than the tip of the RGB.These can belong to one of the threecategories: (i) intermediate-age AGBstars with abundances similar to thosefound in Magellanic Clouds, (ii) old andmetal-rich AGB stars similar to thosefound in the Galactic Bulge and metal-rich globular clusters, or (iii) blends oftwo or more old first ascent giant branchstars. While Rejkuba et al. (2001) haveshown with simulations that only a smallpart of these sources can be ascribed toblends, a definite proof that these brightred giants belong to the AGB populationin NGC 5128 is through the detection ofvariability of these sources. Further-more, the near-IR properties of long pe-riod AGB variable stars can be used toinvestigate the presence or absence ofan intermediate-age component in thestellar populations of this giant ellipticalgalaxy. This has important conse-quences for the formation and evolutionof giant elliptical galaxies.

Using the multi-epoch K-band pho-