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Register a new userThe colour-magnitude relations of ClJ1226.9+3332, a massive cluster of galaxies at z=0.89
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(Abridged) The colour-magnitude relations of one of the most massive, high redshift clusters of galaxies known have been studied. Photometry has been measured in the V, R, I, z, F606W, F814W, J and K bands to a depth of K*+2.5 and spectroscopy confirms 27 K band selected cluster members. The V-K colours are equivalent to a rest-frame colour of ~2700A-J, and provide a very sensitive measure of star-formation activity. HST ACS imaging has been used to morphologically classify the galaxies. The cluster has a low early-type fraction compared to nearby clusters, with only 33% of the cluster members having types E or S0. The early-type member galaxies form a clear red-sequence in all colours. The scatter and slope of the relations show no evolution compared to the equivalent Coma cluster relations, suggesting the stellar populations are already very old. The normalisation of the relations has been compared to models based on synthetic stellar populations, and are most consistent with stellar populations forming at z>3. Some late-type galaxies were found to lie on the red-sequence, suggesting that they have very similar stellar populations to the early-types. These results present a picture of a cluster in which the early-type galaxies are all old, but in which there must be future morphological transformation of galaxies to match the early-type fraction of nearby clusters. In order to preserve the tight colour-magnitude relation of early-types seen in nearby clusters, the late-type galaxies must transform their colours, through the cessation of star-formation, before the morphological transformation occurs. Such evolution is observed in the late-types lying on the colour-magnitude relation.
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A detailed X-ray analysis of an XMM-Newton observation of the high-redshift (z=0.89) galaxy cluster ClJ1226.9+3332 is presented. The X-ray temperature is found to be 11.5{+1.1}{-0.9}keV, the highest X-ray temperature of any cluster at z>0.6. In contrast to MS1054-0321, the only other very hot cluster currently known at z>0.8, ClJ1226.9+3332 features a relaxed X-ray morphology, and its high overall gas temperature is not caused by one or several hot spots. The system thus constitutes a unique example of a high redshift, high temperature, relaxed cluster, for which the usual hydrostatic equilibrium assumption, and the X-ray mass is most reliable. A temperature profile is constructed (for the first time at this redshift) and is consistent with the cluster being isothermal out to 45% of the virial radius. Within the virial radius (corresponding to a measured overdensity of a factor of 200), a total mass of (1.4+/-0.5)*10^15 M_solar is derived, with a gas mass fraction of 12+/-5%. The bolometric X-ray luminosity is (5.3+/-0.2)*10^45 erg/s. The probabilities of finding a cluster of this mass within the volume of the discovery X-ray survey are 8*10^{-5} for Omega_M=1 and 0.64 for Omega_M=0.3, making Omega_M=1 highly unlikely. The entropy profile suggests that entropy evolution is being observed. The metal abundance (of Z=0.33{+0.14}{-0.10} Z_solar), gas mass fraction, and gas distribution are consistent with those of local clusters; thus the bulk of the metals were in place by z=0.89.
Deep XMM and Chandra observations of ClJ1226.9+3332 at z=0.89 have enabled the most detailed X-ray mass analysis of any such high-redshift galaxy cluster. The XMM temperature profile of the system shows no sign of central cooling, with a hot core and a radially declining profile. A temperature map shows asymmetry with a hot region that appears to be associated with a subclump of galaxies at the cluster redshift, but is not visible in the X-ray surface brightness. This is likely to be result of a merger event in the cluster, but does not appear to significantly affect the overall temperature profile. The XMM temperature profile, and combined Chandra and XMM emissivity profile allowed precise measurements of the global properties of ClJ1226.9+3332; we find kT=10.4+/-0.6keV, Z=0.16+/-0.05Zsol, and M=5.2^{+1.0}_{-0.8}x10^{14}Msol. We obtain profiles of the metallicity, entropy, cooling time and gas fraction, and find a high concentration parameter for the total density profile of the system. The global properties are compared with the local LT and MT relations, and we are able to make the first observational test of the predicted evolution of the YM relation. We find that departures from these scaling relations are most likely caused by an underestimate of the total mass by ~30% in the X-ray hydrostatic mass analysis due to the apparent recent or ongoing merger activity.
We present a weak-lensing analysis of the galaxy cluster CL J1226+3332 at z=0.89 using Hubble Space Telescope Advanced Camera for Surveys images. The cluster is the hottest (>10 keV), most X-ray luminous system at z>0.6 known to date. The relaxed X-ray morphology, as well as its high temperature, is unusual at such a high redshift. Our mass reconstruction shows that on a large scale the dark matter distribution is consistent with a relaxed system with no significant substructures. However, on a small scale the cluster core is resolved into two mass clumps highly correlated with the cluster galaxy distribution. The dominant mass clump lies close to the brightest cluster galaxy whereas the other less massive clump is located ~40 (~310 kpc) to the southwest. Although this secondary mass clump does not show an excess in the X-ray surface brightness, the gas temperature of the region is much higher (12~18 keV) than those of the rest. We propose a scenario in which the less massive system has already passed through the main cluster and the X-ray gas has been stripped during this passage. The elongation of the X-ray peak toward the southwestern mass clump is also supportive of this possibility. We measure significant tangential shears out to the field boundary (~1.5 Mpc), which are well described by an Navarro-Frenk-White profile with a concentration parameter of c200=2.7+-0.3 and a scale length of rs=78+-19 (~600 kpc) with chi^2/d.o.f=1.11. Within the spherical volume r200=1.6 Mpc, the total mass of the cluster becomes M(r<r200)=(1.4+-0.2) x 10^15 solar mass. Our weak-lensing analysis confirms that CL1226+3332 is indeed the most massive cluster known to date at z>0.6.
We present BVI photometry of 190 galaxies in the central region of the Fornax Cluster observed with the Michigan Curtis Schmidt Telescope. The cluster members exhibit a strong surface brightness-magnitude relation in our data: both giant and dwarf ellipticals decrease in surface brightness as luminosity decreases. However the surface brightness-magnitude relation is no longer a reliable method for determining cluster membership at surface brightnesses fainter than 22 mag/sq.arcsec. The newly discovered ultra-compact dwarf galaxies (UCDs) lie well off the normal surface brightness-magnitude relation. We present the colour-magnitude relation for a sample of 113 cluster galaxies as a function of morphological type. The UCDs also lie off the locus of this relation. Their mean V-I colours are redder than dwarf galaxies of similar luminosity, but similar to those of globular clusters associated with NGC 1399. The location of the UCDs on both surface brightness and colour-magnitude plots supports the hypothesis that they are the remnants of tidally stripped nucleated dwarf elliptical galaxies.
We investigate the development of the red sequence (RS) of cluster galaxies by using a semi-analytic model of galaxy formation. Results show good agreement between the general trend of the simulated RS and the observed relation in early-type galaxies. However, the most luminous galaxies ($M_V lesssim -20$) depart from the linear fit to observed data, displaying almost constant colours. We analyze the dependence with redshift of the fraction of stellar mass contributed to each galaxy by different processes (i.e., quiescent star formation, disc instability and mergers), finding that the evolution of the bright end, since $zapprox 2$, is mainly driven by minor and major dry mergers. Since the most luminous galaxies have a narrow spread in ages ($1.0times 10^{10}$ yr $<t<1.2times 10^{10}$ yr), their metallicities are the main factor that affects their colours. Galaxies in the bright end reach an upper limit in metallicity as a result of the competition of the mass of stars and metals provided by the star formation within the galaxies and by the accretion of merging satellites. Star formation activity in massive galaxies (M_star gtrsim 10^{10} M_{odot}$) contribute with stellar components of high metallicity, but this fraction of stellar mass is negligible. Mergers contribute with a larger fraction of stellar mass ($approx 10-20$ per cent), but the metallicity of the accreted satellites is lower by $approx 0.2$ dex than the mean metallicity of galaxies they merge with. The effect of dry mergers is to increase the mass of galaxies in the bright end, without significantly altering their metallicities, and hence,their colours, giving rise to the break in the RS. These results are found for clusters with different virial masses, supporting the idea of the universality of the CMR in agreement with observational results.
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