Cl 1205+44, a fossil group at z = 0.59 M. P. Ulmer Department Physics & Astronomy, Northwestern University, Evanston, IL 60208-2900 [email protected] 5 C. Adami, G. Covone 0 0 Laboratoire d’Astrophysique de Marseille, Traverse du Siphon, 13012 Marseille, France 2 [email protected], [email protected] y a M F. Durret Institut d’Astrophysiqe de Paris, CNRS, 98 bis Boulevard Arago, 75014 Pars, France 3 [email protected] 2 v 6 G. B. Lima Neto 8 Instituto de Astronomia, Geof´ısica e C. Atmosf./USP, R. do Mata˜o 1226, 05508-090 Sa˜o Paulo/SP, Brazil 4 [email protected] 1 0 5 0 K. Sabirli1 / Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213, USA h p [email protected] - o r t B. Holden2 s a Department of Physics, University of California, Davis, 1 Shields Avenue, Davis, CA 95616 : v [email protected] i X r R.G. Kron a Fermi National Accelerator Laboratory, MS 127, Box 500, Batavia, IL 60510 [email protected] A.K. Romer1 Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213, USA [email protected] ABSTRACT This is a report of Chandra, XMM-Newton, HST and ARC observations of an extended X- ray source at z = 0.59. The apparent member galaxies range from spiral to elliptical and are all relatively red (i′-K about 3). We interpret this object to be a fossil group based on the s difference between the brightness of the first and second brightest cluster members in the i′- band, and because the rest-frame bolometric X-ray luminosity is about 9.2×1043 h−2 erg s−1. 70 This makes Cl 1205+44 the highest redshift fossil group yet reported. The system also contains a central double-lobed radio galaxy which appears to be growing via the accretion of smaller galaxies. We discuss the formation and evolution of fossil groups in light of the high redshift of Cl 1205+44. 1 Subject headings: groups of galaxies: general — X-rays: individual (Cl 1205+44) 1. Introduction et al., 2003) independently to identify this object as a fossil group and to obtain HST and redshift InaROSATsurveyofextendedsources(Adami observations. The HST and redshift data nicely et al. 2000;Romer et al. 2001),we found a source complement the Chandra, XMM-Newton, i′- and showing extended X-ray emission but with only a K -band data we have obtained. s singlefaintoptical(R-band)counterpart. Ondeep Our study of Cl 1205+44 also adds one more opticalimages,the colorsofthe galaxiesinthe X- datapointtothetopicofpreheating. Recentstud- ray area were red enough that the program HY- ies of simulations of preheating can be found, for PERZderivedaphotometricredshiftgreaterthan example, in Borgani et al. (2004). The possibility 1. This led to Chandra and XMM-Newton obser- of preheating of the ICM in groups and clusters vations of the object we designate as Cl 1205+44. has taken on added significance, as it relates to We find that the system is a fossil group at the the Suyanev-Zel’dovich (SZ) effect, which can in- highest redshift yet published (0.59 versus typical fluence the interpretation of the high order power < values of ∼0.1). The system is particularly inter- spectrum of the Cosmic Microwave Background estingbecauseitallowsustoexploreevolutionary (CMB; Lin et al. 2004). tracksoffossilgroupsandtoconsiderscenariosfor Inthispaper,wereporttheresultsofourmulti- the evolution of galaxies in groups. wavelength analysis and discuss how Cl 1205+44 Fossilgroupshavebeendefined(e.g.Jonesetal. fitsintothelargerpictureoffossilgroupformation 2003) as being similar to groups (poor clusters) and evolution. in which the luminosity function of the member WewilluseH =70kms−1Mpc−1,Ω =0.7and galaxieshasbeenmodifiedbyaccretionofgalaxies 0 Λ Ω =0.3hereafter. Ataredshiftof0.5915,whichis onto the central galaxy while the rest of the sys- m themostlikelyredshiftofCl1205+44,theangular tem remains unevolvedfor approximately 4 Gyrs. scale for this cosmology is 6.64 kpc/′′. The galaxy accretion onto the D galaxy leads to a magnitude difference between the first and sec- 2. Observations and Analysis ond brightest galaxies (m ) of 2 or higher in the 12 rest frame R or V bands within one half a virial 2.1. Optical and IR observations radius,whereJoneset al.(2003)use the following to calculate the virial radius: rvir = 3.89×(T/10 We made observations in the i′ and Ks-bands keV)0.5×(1+z)−3/2 h−1 Mpc. at the ARC telescope2. For the i′-band, we used 50 the Spicam camera for a net total exposure time Moreover, Cl 1205+44 is interesting because of7,200secondsatameanairmassof1.15,andfor it harbors a double lobed radio source which is theK -bandweusedtheGRIMIIcameraforanet also a D (central dominant) galaxy. Such systems s totalexposuretimeof7,200seconds. Wetooksep- can be directly compared with the model of West arate 10 minute exposures to acquire the i′-band (1994). Also, the radio source simply makes the dataand10to30secondexposurestoaccumulate group more complex and its formation could be related to cooling flows1 on one hand, and energy theKs data. Thei′-banddatawerereducedusing theESO-MIDASpackageandtheK datawerere- injection on the other (e.g. Sun et al. 2003). s duced using DIMSUM, a NOAO/IRAF tool. The A D galaxy that is a radio source is a tracer zero points were computed using standard stars for finding fossil groups which led J. S. Mulchaey observed at the same time as the scientific data (2003, private communication; see also Mulchaey and at similar air masses. 1present address: Astronomy Center, Department of The HST data were retrieved from the archive Physics and Astronomy, University of Sussex, Falmer, (proposal ID: 8131, PI: Mulchaey). They con- Brighton,BN19QH sist of 6×1200 second dithered R-band WFPC2 2present address: UCO/Lick Observatories, 1156 High (F702W filter) exposures. We reduced the data Street,SantaCruz,CA95065 using the drizzle IRAF package (Fruchter & Hook 1Coolingflowliteratureistooextensivetocomprehensively 2002). We generated catalogs of detected objects reference; a few references that directly relate radio lobes to X-rays in relatively poor systems with X-ray emission are: Carilli et al. (1994); Harris et al. (2000); McNamara 2Seehttp://www.apo.nmsu.edu fordetails. etal.(2000) 2 using SExtractor (version 2.3 Bertin & Arnouts mostlikely in ourlist not to be a cluster member. 1996) using a detection threshold of 1.5σ and an The uncertainty in L does not affect any con- R analysis threshold of 2σ. clusion we draw in the discussion section of this Weusedthefixedaperture3′′ magnitudesfrom paper. The F702W total luminosity is 6.0×1011 SExtractor for the HST magnitudes and we used LJ. Converting to the RJ band used by Jones the SExtractor auto-magnitude feature to derive et al. (2003),we obtain LR = 5.5×1011 LJ. The the magnitudes for the ARC data. Figure 1 K-correction to z = 0 gives a (rest frame) value gives the magnitude histogramsin the three mag- of LR = 7.8×1011 LJ. Finally, to compare with nitude bands available to us (in the ARC Spi- Jones et al., we use H0=50 and q0 = 0.5 which cam field of view for i′, in the ARC GRIMII leads to LR =8.9×1011 LJ. field of view for K and in the HST WFPC2 s 2.2. Redshift field of view for F702W). These histograms al- low us to estimate an upper limit to the com- SincetheX-raydatawerenotofsufficientsignal pleteness value of the magnitude in each bands: to noiseratiotoconstrainthe clusterredshift(see i′∼22.25, F702W∼26.25 and K ∼19.75. The re- s below), we needed optical spectra of the galax- spective solid angle coverage of the instruments ies belonging to the system. By chance, this sys- is: WFPC2 image: 2.2×2.2 arcmin2, i′ image: temwasobservedbyJ.S.Mulchaey(2003,private 6×4.3 arcmin2, K image: 2×2 arcmin2. s communication) in his ongoing fossil group sur- In order to assess the quality of our data, vey and he kindly provided us with the value of we plotted the magnitudes in the different bands theredshiftofthebrightestgalaxyoftheputative against each other as shown in Figure 2. We fossil group, z = 0.5915. These data plus the po- estimate from the results a relative 1σ uncer- sitional coincidence of the brightest group galaxy tainty between magnitudes (limiting ourselves to (called the D galaxy hereafter) with the centroid the brightest completeness limit) of 0.26 mag be- (see §2.3.4) of the X-ray emission firmly identi- tween F702W and i′ and of 0.36 mag between i′ fiestheextendedX-rayemissionwiththebrightest and Ks. This gives an upper limit to the magni- galaxy and the associated galaxies in Table 1. tude uncertainty since some of the scatter is due to the intrinsic color variations of the objects. 2.3. X-ray Observations As an external test, we compared our K s We were granted time to observe Cl 1205+44 magnitudes with the estimates for the two ob- with the XMM-Newton (June 2003,52,200s) and jects in the K field bright enough to be de- s Chandra (October 2003,31800 s) satellites. tected by 2MASS3 (including the D galaxy). We have reasonable agreement (for the two objects, 2.3.1. Chandra data K −K =−0.38 and +0.26) given that these s 2MASS objects are below the 2Mass completeness limit. The Chandra observation was made in “Very Faint”modewithatimeresolutionof3.24secand For use in our discussion below, we calculated a CCD temperature of −120◦C. The data were the total R-band luminosity. To derive this num- reduced using CIAO version 3.0.15 following the ber,weusedthetotalF702Wfluxenclosed4bythe Standard Data Processing, producing new level 1 X-ray contours in Figure 3. We estimate that ap- and 2 event files. proximately90%ofourderivedvalueofL comes R fromthe galaxiesenclosedby rectanglesin Figure We have further filtered the level 2 event file, 4. Therefore, the L value is probably accurate keeping only ASCA grades 0, 2, 3, 4 and 6, and R to within 20%. The 20% uncertainly is also con- restricted our data reduction and analysis to the sistentwithsubtractingtheflux of#6inFigure3 back-illuminated chip, ACIS-S3. We checked that (seealsoTable1)which,basedonitscolors,isthe no afterglow was present and applied the Good Time Intervals (GTI) supplied by the pipeline. 3http://www.ipac.caltech.edu/2mass/ Then, we have checked for flares using the light- 4Weexcludedthestar,whichisthebrightestobjecttoward the top of the figure but inside the contours; it is south- 5http://asc.harvard.edu/ciao/ southwestoftheDgalaxy 3 curve in the [10–12 keV] band; no flare was de- 2.3.3. X-ray Spectral fits tected and the total exposure time was 29,711 s. Spectra were analyzed with XSPEC 11.3. We We have used the CTI-corrected ACIS back- have simultaneously fit all four spectra from the groundevent files (“blank-sky”), produced by the XMM-Newton MOS1, MOS2, PN and Chandra ACIS calibration team6, available from the cal- ACIS-S3 cameras. The spectra were rebinned so ibration data base (CALDB). The background that we could use the standard χ2 minimization events were filtered, keeping the same grades as when fitting the spectra. We have applied the the source events, and then were reprojected to mekal plasma spectral model (Kaastra & Mewe matchtheskycoordinatesoftheCl1205+44ACIS 1993; Liedahl et al. 1995). observation. The photoelectric absorption (mainly due to neutralhydrogeninourgalaxy)wascomputedus- 2.3.2. XMM-Newton data ing the cross-sectionsgivenby Balucinska-Church Cl 1205+44 was observed in standard Full & McCammon (1992), available in xspec. Given Frame mode using the “thin” filter with the two the low count-rate, the hydrogen column density EPIC MOS1 and MOS2 and the PN detectors. N could not be well constrained by the spectral H Thebasicdataprocessing(the “pipeline”removal fit, therefore we fixed it to the galactic value at of bad pixels, electronic noise and correction for the cluster position. The interpolation of the HI chargetransferlosses)wasdonewithpackageSAS map of Dickey & Lockman (1990), using the task V5.3, thus creating calibrated event files for each nh from ftools, yields N = 1.27×1020cm−2. H detector. The results are shown in Figure 5. The metallic- For the MOS1 and MOS2 cameras, following ity is poorly constrained; as shown in Table 3, at the standard procedure, we have discarded the a1σ confidencelevelwefoundZ <0.6Z and,at J events with FLAG 6= 0 and PATTERN > 12; 90% confidence level, Z < 1.1Z . For complete- J for the PN, we have restricted the analysis to the ness in Figure 6 we show how NH and metallicity events with PATTERN ≤ 4 and Flag =0. correlate with kT in the spectral fits. The light-curves in the [10–12 keV] band that We restricted the spectral analysis within the we have produced showed that there were se- energyrangewheretheclusteremissionwasabove vere flaring events during the observation. Fil- the background and the detectors were well cali- tering outthe periods with flaressubstantially re- brated. FortheMOS1and2cameras,thisinterval duced the exposure times: 21,223 s, 20,861 s, and was[0.3–8.0keV],forthePN[0.5–8.0keV]andfor 16,478 s for the MOS1, MOS2 and PN, respec- ACIS-S3[0.4–7.0keV].Thespectrawereextracted tively (from an initial 52.2 ks). insideacircleof1.4arcminradius,centeredonthe With the cleaned event files, we have created cluster. The strong northeast X-ray point source theredistributionmatrixfile(RMF)andancillary (probably an AGN) is outside this radius. response file (ARF) with the SAS tasks rmfgen 2.3.4. X-ray luminosity and arfgen for each camera and for each region that we have analyzed. For the reasons stated above (§2.2), we assume The backgroundwas taken into account by ex- that the cluster redshift is z = 0.5915 and fixed tracting spectra from the blank sky templates de- this value from hereon. Table 3 summarizes the scribed by Lumb et al. (2002), and reprojected best spectral fits, either fixing the metallicity or to the same coordinates and roll angle of the the hydrogen column density or both. Clearly, Cl1205+44XMM-Newton observation. Thesame themetallicityisnotwellconstrainedandweonly filtering procedure was applied to the background haveupperlimitsforN . Fixingthemetallicityto H eventfiles. We giveabreakdownofthe numberof the typical value found in clusters (e.g. Fukazawa counts used per detector in the fitting in Table 2. et al. 2000, 0.3Z ) and N to the galactic value, J H the mean temperature is kT = 3.0+0.3 keV. This 6http://cxc.harvard.edu/cal/Acis/WWWacis cal.html system therefore has an X-ray tem−p0e.3rature typ- ical of a poor cluster (such as Abell 194, which has a temperature of 2.6±0.15 keV [Nikogossyan 4 etal. 1999],andthepoorclusterRXJ0848+4456, of total flux = 56.4 ± 1.7 mJy at 20 cm (Con- which has a temperature of 3.2±0.3 keV [Holden don et al. 2002). These facts are relevant to the et al., 2001]) and warmer than that of a typical model proposed by West (1994). He proposed fossil group (e.g. Jones et al. 2003). an anisotropic merger model for the origin of the Withredshiftz=0.5915,thecorrespondingun- formation of D galaxies, and the model predicts absorbedluminosityandfluxinthe[2.0–10.0keV] that the D-galaxy will be associated with a pow- are (3.3±0.3)h−2×1043erg s−1 and (1.5±0.2)× erful radio source, which is the case here. Fur- 70 10−14erg s−1 cm−2. The bolometric luminosity is thermore,the model predicts that the radio lobes (9.2±0.4)h−2 ×1043erg s−1. The cluster prop- will be aligned with the major axis of the X-ray 70 erties are summarized in Table 4. We define the emission in the cluster. Therefore, we attempted position of the cluster to be that of the D galaxy, to determine the major axis of the X-ray emis- as there is a peak in the contours (see Figure 3) sion, which is rather ill defined. In order to make at this location. a determination of the major axis, we heavily smoothed7 thedatatoproduceFigure11(seealso For its measured bolometric luminosity, Cl Figure 10). Then, the major axis of the X-ray 1205+44 is hotter than the best fit to the local emissionof Cl1205+44can be defined by the line L –T relationbasedontwo fossilgroups(Jones X X joining points “A” and “B” in Figure 11. In this etal.(2003);seeourFigure7),butagreeswiththe case the radio lobe axis is aligned within 10 de- localrelationofNovickietal.(2002),andthevalue grees of the X-ray axis, which is also consistent fallswithin1σoftheL –T no-evolutionrelation X X withthemodelofWest. However,theopticalaxis derived for the hzi=0.34 sample of Novickiet al. of the D galaxy is offset by about 30 degrees with (2002). For the purpose of later discussion, we respectto the radio lobe axis andthe West model have plotted the values of kT versus L for Cl X also predicts alignment with the galaxy distribu- 1205+44 on figures taken from Jones et al. and tion. If there is a galaxy distribution that defines Novicki et al. in our Figures 7 and 8. a direction, it is the almost north-south line of galaxies running from galaxies #7 to #8 in Fig- 2.3.5. X-ray surface brightness fits ure 4. Overall, then, the data are not consistent We carriedout a standardβ surface brightness with the model of West. fittoboththeXMM-Newton dataandtheXMM- Newton plus Chandra data combined. We used 3. Discussion the 0.5–8 keV bands in both cases. Then with I (b) ∝ [1 + [b/r ]2]−3β+1/2 (e.g. Sarazin 1986), 3.1. Nature of the System x c where I is the surface brightness as function of b The most compelling reason for calling Cl projected radius, b and r is the core radius. For c 1205+44 a fossil group is that the value of m the more robust XMM-Newton data alone case, 12 withinonehalfthe projectedvirialradius8 isvery we found, using a maximum likelihood method: close to that of the Jones et al.(2003)criterionof β =0.45±0.02,andr =21′′±3′′,1σ errors. The c being >2 forz ∼0 (for R- orV-bandrestframe), fitappearsbettertotheeye(seeFigures9aandb) i.e. m (i′) = 1.93. In addition, that the value for the XMM-Newton plus Chandra case, but we 12 foundthe resultsso sensitiveto the normalization 7Note that the heavy smoothing causes the center of the between the two data sets and the binning of the second highest contour level in the X-ray emission to be data, that we only quote this fit as a 2σ lower differentfromthatinFigure3,buttheexactpositionofthe bound to the core radius = 15′′; the value of β X-raypeak is not important. For, it isextremely unlikely thattheDgalaxywouldfallsoclosetothecenterofthisX- was again 0.45 for this minimum χ2 fit. rayemissionandnotbeassociatedwiththeX-rayemission. Thehighestcontourlevelhastwopeaks,oneofwhichfalls 2.4. Radio Source ontheDgalaxy;seetheblackandwhiteversionofFigure 10orthecolorimageofFigure11. Thecentralpeakwithinthesecondhighestcon- 8ForconsistencywithJonesetal. rvir =70′′ weusedtheir tour level in Figure 3 is located on the D galaxy. cosmology,H0 =50,q0=0.5;thenthescaleis7.57kpc/′′, TheDgalaxyisalsoadouble-lobedFIRST(White andfromtheirformulaforthevirialradius,reproducedin −1 etal.1998)radiosourceF1205+44(seeFigure10) sthpeonidnstrtoodu7c0t′′io.n, rvirial/2 = 0.53 h50 Mpc, which corre- 5 of m is slightly smaller can be attributed to Cl sured by J. S. Mulchaey (2003, private commu- 12 1205+44being younger (by up to ∼4 Gyrs) than nication). The conclusion that these galaxies are z = 0 fossil groups; in the Jones et al. scenario, allgroupmembers is basedonthe followingfacts: the central galaxy grows in brightness with time theiri′ magnitudesareallsimilarexceptfortheD as it accretes more galaxies. In older systems the galaxy;theircolorsareallsimilarexceptforgalaxy central galaxy will have had more time to accrete #6; their apparent sizes are all similar, and their galaxies, and hence be brighter compared to the average color (see also § 3.6) corresponds to the remaining ones. peak in the histogram of the i′–K distribution s The X-ray emitting AGN we have listed in Ta- shown in Figure 12. We will assume therefore, ble 1 is brighterthan the D galaxyin the i′-band, that except for #6, all these galaxies are cluster so if it were a cluster member then Cl 1205+44 members. Basedontheirdisk-likemorphology,we would certainly not be a fossil group. However, classified3outofthe6groupmembersaslatetype wehave(withoutredshifts)twoargumentsagainst (spiral)galaxiesandtheotherthree(includingthe this AGN being a group member: (1) its color is D galaxy) as early type (elliptical) galaxies. We significantly bluer than the other (probable) clus- will refer to the galaxy population and colors in ter members in Table 1; (2) the AGN becomes our discussion of the scenarios of this fossil group the dominant galaxyand then is >∼2 (X-ray)core formation in §3.6 and §3.8 below. radiiawayfromthecentroidoftheextendedX-ray 3.3. Possible Cooling Evolution emission. Tobeafossilgroup,thelowerlimitJonesetal. PerhapsthefactthatCl1205+44ishotterthan (2003) place to LXbol is 1 × 1042 h−502 ergs s−1, thetwoz=0fossilgroupswithmeasuredtemper- which Cl 1205+44 easily meets. Although Jones atures discussed by Jones et al. (2003) is due to et al. do not set an upper limit to the X-ray cooling between z ∼ 0.6 and z ∼ 0. We now con- temperature or luminosity, the LXbol (corrected sider this possibility. We used the cooling time to H0 = 75 for their Figure 3, and our Figure 7) equation from Sarazin (1986) for our calculations pointfallsintheregionwheretheJonesetal. fos- (see §3.5 fordetails). To derive a lowerbound, we sil group sample lies. Cl 1205+44 is also distin- used our β =0.45, r =15′′ model fit and we de- c guished from “normal clusters” in that the LXbol rivedanaveragecoolingtime of6.5 Gyrswithin1 value is about a factor of two lower at kT ∼ 3 core radius and ∼ 11 Gyrs within 2r . The time c keV than the cluster sample compiled by Lumb betweenz=0.59andz=0.2(the highestredshift et al. (2004) for clusters at z ∼ 0.4−0.6. In con- inthe sampleonwhichJonesetal. basetheirdis- trast, the LXbol– LR point lies approximatelyhalf cussionof fossilgroupformationand evolution)is way between the Jones et al. lines for “normal” only 3 Gyrs. Even the longer time of 4 Gys (the X-ray bright groups and fossil groups. On the averageageoffossilgroupsintheJonesetal. sce- other hand, the X-ray luminosity and kT values nario) is less than the 6.5 Gyrs we derived for the for Cl 1205+44 are similar to the values Holden average cooling time within the core. We derive et al. (2001) assigned to an extended X-ray emit- a 2σ lower bound of ∼ 4 Gyrs at the very core tingregiontheysimplyclassifiedasa“cluster.” In which could lead to cooling in the center of the theend,however,them12valueofnearly2(within cluster. It is unlikely, therefore, that the ICM of one half the projected rvir) and LXbol >1042 ergs high redshift fossil groups is hotter than that of s−1 meet the primary Jones et al. criteria, which low redshift ones due to cooling between z ∼ 0.6 leads us to conclude Cl 1205+44 is fossil group, and z ∼ 0.0. We defer a discussion of the energy and we will assume it is a fossil group in what input to the ICM until §3.8.1. follows. The issue of cooling, gas infall (or the suppres- sion of gas infall), resulting heating due to gas in- 3.2. Fossil Group Members fall, etc. is a complicated one and has been dis- We must make an assumption about galaxy cussed extensively in the literature, see for exam- membership, since redshifts for the specific galax- ple, some recent works (Clarke et al. 2004; Kaas- ies are not available to us, and except for the D tra et al. 2004; Peterson et al. 2004, and refer- galaxynoneofthosemarkedinFigure4wasmea- ences therein). The purpose of the above discus- 6 sion was not to argue for or against cooling per over 1 Gyr. se, but rather to determine if it is possible for Furthermore, there is the lack of a correlation the gas to have cooled enough between z ∼ 0.6 betweenthe radioandthe X-rayemission. There- and 0.1 to explain the temperature difference be- fore we have no evidence that the radio source tween Cl 1205+44 and nearby fossil groups. We is responsible for extra energy input to the intra haveshownthatifweassumethesimplestcircum- clustermedium(ICM)ofCl1205+44. Regardless, stances, i.e. a collisionless gas without a tangled Sunetal.(2003)suggestthatcDgalaxiescanpro- magnetic field, the gas could just barely cool over videheatingtotheICMviagalacticwindsover10 this z ∼ 0.6-0.1 time interval. Then, since heat Gyr. input is likely in any event, we conclude that the nearby fossil groups do not have lower tempera- 3.5. Gas Mass and Total Mass tures than Cl 1205+44because ofcooling. It may Inordertocomparewithpreviouswork,itisin- just be that hot fossil groups such as Cl 1205+44 terestingtocalculatethegasmassinvariousways. arerareperunit volumebut arethe ones thatare Belowwegivethe valuesbasedonthe assumption easiest to detect at high z. of a core radius of 21′′. The values are approxi- 3.4. The Radio andX-ray Source Relation mately 40% lower if the other best lower limit of 15′′ is used. Thereforethese results are simply for The issue is whether or not there is evidence qualitative comparison with previous work. for the radiosource interacting with the ICM and We used the relationships between X-ray sur- producing X-rays via the inverse Compton effect. face brightness and mass as described in Sarazin Theradiolobes,aswediscussedin§2.4,seemtobe (1986). We assumed the electron density is 1.1 aligned with the emission that defines the major times the proton density and a mean molecular axis in the X-ray emission, but there are no radio weight of 0.6 for the gas. Within 100 kpc for the lobes oneither the FIRST (White etal.1998)im- XMM model, we find a gas mass ∼8×1011 M . J age or the NVSS (Condon et al. 2002)image that Within one core radius the gas mass is 1.9×1012 coincide with the X-ray emission features called M . If we assume no temperature gradient, we J “A” and “B” in §2.4. Therefore, this alignment derive a dynamical mass (e.g., Sarazin, 1986) of is probably accidental. Regardless of whether we 1 × 1013 M . If we use the Sun et al. (2003) J assumethealignmentisaccidentalornot,thecon- value of M/L of 5 and our value of L , we find R tribution of inverse Compton flux from the radio the total galaxy mass of Cl 1205+44is 7.5×1012 lobes to the 1-10keVX-rayemissionappearsneg- M whichisover10timeshigherthantheirvalue J ligible as there is no (detectable) X-ray enhance- of about 5×1011 M for the group (not classi- J ment associated with the radio lobes. fied as a fossil group, however) surrounding NGC BasedontheratiooftheDgalaxytoNGC1550 1550. Within 100kpc, their value of the gas mass (atthe center ofanX-raybrightgroup;Sun etal. (M )ofabout3×1011M isabout2timeslower gas J (2003)20cmfluxes(56.4±1.7mJy/16.6±1.6mJy, than our value for Cl 1205+44. We also calcu- fromtheVLANVSS;Condonetal.,2002)andon lated M from Allen et al. (2001) (see also Sun 2500 the measured redshifts, we find that the radio lu- etal.2003)whoderivedaformulaforM using 2500 minosity of the D galaxy in Cl 1205+44 is about a set of X-ray luminous relaxed clusters. Then, 1.4×104 timesthatofNGC1550. Thetotalradio M = 2.7 × 1013 M × (T/1.37)1.5/E(z) = 2500 J flux(∼2.5×10−14ergscm−2 s−1 assumingaflux 3×1013 M . E(z) = H/H = (Ω (1+z)4 + J 0 r,0 spectral index of −0.7 and that the flux extends Ω (1 + z)3 + Ω + (1 − Ω )(1 + z)2)1/2; for m,0 Λ,0 0 from0.1GHz to10GHz)iscomparableto the to- Ω = 0 as assumed here, this can be simplified r,0 tal X-ray flux (and luminosity) of the gas. But if to (1+z)(1+zΩ +Ω (1+z)2−Ω )(1/2). m,0 Λ,0 Λ,0 we assume the radio source is only “radio active” (e.g. Lara et al. 2004, and references therein) for 3.6. Galaxy Colors 108 years,plusthatitprobablycaninjectnomore To compare with other galaxies at this red- than 10% of its maximum amount of energy out- shift we have used the Sloan Digital Sky Survey put into the ICM, this reduces the total energy inputto1%ofthe totalenergyoutputoftheICM 7 (SDSS)9 and the 2MASS catalog. For galaxies ters that formed earlier in these simulations tend bright enough to have spectra measured with the to have fewer galaxies and less continuous infall SDSS,wefindthatthecolorsofthosegalaxiesatz (to produce the BO effect) than the systems that between0.5and0.6aretypicalofthoseinTable1 formed later. These simulations predict, then, assumed to be members of CL 1205+44. that the galaxy to total cluster mass ratio should The six galaxies for which we have colors that be lowerforthe earliestformedclusterscompared weassociatewithCl1205+44areallrelativelyred, to later ones. With a sample of one fossil group even the late-type galaxies. In contrast, Butcher withredgalaxies,however,itisprematuretomake & Oemler (1984) found a high fraction of blue a comparison between the data and the simula- galaxiesatthisredshiftcomparedtolowredshifts. tions at this level. The Butcher-Oemler effect can be explained by 3.7. The D Galaxy assuming that at 0.6 compared to 0, there is a higherfractionofgalaxiesthathavejustfalleninto The D galaxy of our system is similar to the the cluster and have not had their gas stripped z = 0.25–0.5 simulated galaxies in West (1994). yet (e.g. Kauffmann 1995). Hence, the newer (at Our value of the i′ magnitude of 19.27 converts leastspiral)clustermemberstendtobe bluer,the into a rest frame absolute R magnitude of −24.1 J higher the redshift. For Cl 1205+44, the fossil (luminosity distance = 3.4×103 Mpc, where we group could have formed at z of about 2, could used 0.7 for the K correction for an elliptical at have aged about 4 Gyrs, and could have had no z = 0.59). This value of −24.1 is about 0.5 mag- galaxy infall since birth. The only galaxy popula- nitudes brighter than the brightest cluster galaxy tion evolution that has taken place has been the magnitudes compiledby Collins et al.(2003). Re- merging of galaxiesinto the centralD galaxyplus cent galaxy mergers may be responsible for this ram pressure stripping of the gas from the galax- relatively high brightness. ies. Then,therearenorecentinfallspiralsandthe Note that ongoing minor mergers still appear spirals have the same colors as the elliptical (un- to be visible on the HST image (see Figure 13). dertheassumptionthatallbutgalaxy#6andthe These ongoing mergers at a relatively early stage AGNinTable1areclustermembers;for,asnoted of this fossil group history do not favor the fossil in §3.1 it would be peculiar to have the brightest group formation scenario of Mulchaey & Zablud- clustergalaxysofarremovedfromtheclustercen- off(1999),whoproposedaformationofthese sys- ter). Inthis hypothesis,the spiralshavehadtheir tems with an unusual initial luminosity function. gas removed via ram pressure stripping, and they An alternative proposal is the evolutionary sce- havebeenclustermemberssinceits formationap- nario of fossil groups proposed by Borne et al. proximately 4 Gyrs ago. The key issue is what (2000) and Jones et al. (2003). This links com- suppressesgalaxyinfallforfossilgroupscompared pact groups of galaxies and giant elliptical galax- to typical groups and clusters. iesviaanultra-luminousinfraredgalaxy(ULIRG) Fossil groups have probably formed in initially phase. The D galaxy of our system (which is also above average over dense regions (so that the a radio source) has an i′-K color very similar to s group collapsed early), which, however, were not thatofULIRGs(fromNED10)inthe 2MASSsur- sufficiently rich in matter and galaxies to sustain vey. This D galaxy could be an ULIRG that is growth of the group beyond some point in time. justturningon,exceptthattheK -bandfluxfalls s The propertyofhavingrelativelya lowtotalmat- well below other ULIRGs (e.g. Yun et al. 2004). ter value and a negligible blue galaxy fraction is Therefore if the D galaxy were formed in the pro- directly related to arriving at an over density suf- cess suggested by West (1994), then the ULIRG ficient for collapse earlier than more massive sys- phase has probably already passed. tems. For example, simulations by Gao et al. Continuing with the idea of D galaxy evolu- (2004) have shown that the galaxy infall rate and tion in galaxy groups, we refer to Collins et al. number of galaxies in a cluster is related to the (2003). They show that in poor groups the vari- age of the universe when the cluster formed; clus- ancebetweencentralgalaxypropertiesfromgroup 9http://www.sdss.org/dr2/ 10http://nedwww.ipac.caltech.edu/ 8 to group is quite large (factors of 10 or more) in (2002). Babul et al. made predictions of initial terms of total optical output. Thus, we expect energy input which they comparedwith the data. there would be a variation in the optical prop- The two fossilgroupsfoundby Jones etal.(2003) erties of the D galaxies in fossil groups such as with strong enough signal to measure a tempera- Cl 1205+44, unless there is a direct correlation ture were consistent with an entropy floor of 100 between the kT value and the D galaxy stellar keV cm2. In contrast, Cl 1205+44 lies a factor content. Only a high redshift (>∼ 0.6) survey of of two or more above the 100 keV cm2 line and fossilgroupswillbe ableto shed lightonthe vari- instead is nearly on the line occupied by normal anceofinitialconditionsandasurveyingeneralis richclusters and normalgroups (initial energy in- needed to shed light on the relationship between put 427 keVcm2; see Figure 7). This implies that kT and the D galaxy. eitherCl1205+44isreallynotafossilgroup,con- trary to our classification, or that it is unlike the 3.8. Scenarios nearby fossil groups,and that we were able to de- tectCl1205+44preciselybecauseitismuchmore To summarize a formation scenario for Cl luminous than low z fossil groups. This could be 1205+44: There was an initial potential well of due tothe resultofarelativelyhigh(comparedto dark matter. Energy injection occurred via some theaveragefossilgroup)entropyfloorinputinthe process or processes such as perhaps supernovae, ICM, or indicate that Cl 1205+44 simply is more radio galaxies, or ULIRGs. This process heated massive that other fossil groups. the gas as galaxiesfell into the potential well at a Opposed to the model of Babul et al. (2002), redshift of about 2. The galaxies passed through wefindamuchlowervalueforthe pseudoentropy the ICM several times and the gas and dust were swept out of the spirals. This ram pressure strip- = Tne−2/3 keV cm2 of 85 keV cm2. We derived pingsuppressedcontinuousstarformationandleft this value at r = 0.1r200 = 0.1rvirial from our β thespiralstobecomeasredastheellipticals. The fit(rc =21′′),whereweestimateanaveragekT= systemformedinarelativevoidofgalaxiessothat 3 keVand an electron density at the core to be ∼ there was no continuous infall of galaxies to keep 8.1×10−3cm−3,i.e. 6.5×10−3 cm−3 at0.1rvirial. the systemfedwith blue spiralfield galaxies. The This implies some refinement to the Babul et al. centralgalaxygrewtobecomearelativelylargeD model such as continuous rather than impulsive galaxy by merging with other galaxies to form a heating (pre-heating)atformation. Note that our m value >∼ 2. The entire evolutionary sequence entropyfora3keV(thetemperatureoftheICMof 12 took about 4 Gyrs. Cl1205+44)temperatureis lowerthan wasfound for3keVvirializedobjects(including clustersand TheabovefitswiththeJonesetal.(2000,2003) groups) at z <∼0.2, by Ponman et al. (2003). scenario, which assumes that the time it takes for the merging of galaxies to form a bright cen- The fact that the entropywe find is lowerthan tral galaxy is approximately 4 Gyrs and no con- thatfoundformorenearbyclustersandgroupsat tinuous field galaxy infall has taken place. The the sametemperature is consistentwiththe “neg- fact that the D galaxy in Cl 1205+44 appears to ative” entropy evolution suggested by Maughan have nearly completed the merging process and is et al. (2004). Maughan et al. proposed a form approximately 4 Gyrs younger than z ≃ 0 fossil E(z)−4/3 based on self-similar scaling and on the groups, implies that (at least some) fossil groups evolution of the critical density of the Universe at z = 0 are much older than 4 Gyrs. with z. The scaled-by-temperature (i.e. divided by kT) entropies for similar temperature objects 3.8.1. Consequences found by Ponman et al. (2003) at the same ra- dius (0.1r ) have values of about 75 cm2, which vir Energy to heat the ICM above and beyond ifscaledfromz=0toz=0.5915byE(z)−4/3 cor- gravitational infall results in an entropy (defined respondto 49cm2,whichis withinafactoroftwo here as the pseudo entropy kTne−2/3; where ne is of the value for Cl 1205+44 = 28 cm2 at 0.1r . thenumberofelectronspercm−3andkTisinkeV) vir These results then appear to be more consistent floor. The initial energy injection and its conse- withcontinuousratherthanimpulsiveinitialheat- quences were discussed in detail by Babul et al. ing to the ICM. 9 If there were some initial energy injection at Our main conclusion is that Cl 1205+44 is a z = 2 (whatever its cause), then this injection fossil group. A formation scenario is that the couldaffectthehighorderportionoftheCMBvia group formed in a peak density region of rela- the SZ effect fromclusters and groupsof galaxies. tively low total mass at z ∼ 2. The formation The possibility of this energy input and its effect regions did not have enough matter to sustain on SZ measurements has been discussed by Lin continuous infall of galaxies and gas, and such a et al. (2004) in the context of deriving cosmolog- scenario is consistent with the simulation carried ical parameters based on the power spectrum of out by Gao et al. (2004). Beyond the formation the CMB. Their impulsive heating model requires of a central dominant galaxy via accretion, fossil energy injection at z = 2. They fit their models groupsappearold,i.e.,fossil-likepreciselybecause tothe L -T normalclusterandgroupdatathat they have had little or no evolution in terms of X X are shown here in Figure 7, and from this we see galaxy infall since their inception. And, some fos- that Cl 1205+44 is consistent with the Lin et al. silgroupscouldbeconsiderablyolderthan4Gyrs. model(seealsoBorganietal.2004). Thefactthat No evolution of the galaxy population (at least in preheatingoccurredinthe Linetal. modelnearz the case of Cl 1205+44)is implied because of the ∼2 is consistentwith our suggestionthat atleast dearthofbluespirals,thekT-L valuepointon Xbol somefossilgroups,suchasCl1205+44,formedat theno-evolutiontracksforrichclustersofgalaxies z ∼2. However, if continuous heating models can (Novicki et al. 2002),and our cooling calculations also be shown to fit the data, then the amount of of the IGM are consistent with (but cannot ex- initial impulsive heating suggested by Lin et al. clude) no significant cooling of CL 1205+44 in 3 willnotbeaslargeasthey suggested,andthe im- Gyrs. pact on the high order CMB observations will be Some preheating of the gas in groups and clus- less. ters via supernovae or an active galaxy phase is likely to have occurred even in fossil groups (or 4. Summary and Conclusions normalgroups)asevidencedbytherelativelyhigh temperature of CL 1205+44, but it also appears Wehavefound,viaaROSATsurveyofarchival likely that there is continuous rather than impul- data, a group of galaxies with a relatively red sive heating at group formation, which results in population. HYPERZ calculations based on the “negative” evolution of the pseudo entropy (e.g. i′- and K -band data suggested the system might s Maughan et al. 2004). However, as the kT ver- > have z ∼ 1. Subsequent redshift work found that sus L of Cl 1205+44, fits within 1σ the no- the redshiftis = 0.5915,andChandra plus XMM- Xbol evolution with z of Novicki et al. (2002), the neg- Newton observationsconfirmedtheexistenceofX- ative evolution for entropy and the no-evolution ray emitting gas in the group. We classify this models for kT versus L (and galaxy popula- group as a fossil group based on its m in i′ Xbol 12 tion) may need to be fine tuned to be made con- and on the fact that its X-ray luminosity exceeds sistent with each other. 1×1042h−2ergss−1. Thismakesitbyfarthehigh- 50 Also,theamountofpreheatinghasimplications estredshiftfossilgroupyetreported. Thetemper- for the interpretation of high order measurements aturefortheICMisabnormallyhighcomparedto of the CMB. Our data are consistent with “neg- two z ∼ 0 fossil groups with a similar L , and Xbol ative” evolution of the pseudo entropy which im- the L –L point lies approximately half way Xbol R plies,ononehand,alesserimpactoftheSZ effect between the best fit lines for normal and fossil from high z clusters than suggested by Lin et al. groups Jones et al. (2003); Cl 1205+44has a kT– (2004). On the other hand, that the kT versus L value comparabletoanotherz ∼0.6system Xbol L value for our data and many other clusters that was classified as a cluster (RX J0848+4456; Xbol is consistent with no evolution, suggests that per- Holdenetal.,2001). But,althoughtherearesome haps a significantfaction of ICM heating did take similarities with normal groups and clusters, we placenearz =2, andhencedoes havea significant have classified it as a fossil group based on the primary criteria of m >∼ 2 in R- or V-band rest effect on the high order terms of the CMB. 12 frame and an X-ray luminosity ≥1×1042h−2 The discovery and X-ray and optical measure- 50 ments of still more z ≥ 0.6 clusters and fossil 10