Evolution in the iron abundance of the ICM

We present a Chandra analysis of the X-ray spectra of 56 clusters of galaxies at $z>0.3$, which cover a temperature range of $3> kT > 15$ keV. Our analysis is aimed at measuring the iron abundance in the ICM out to the highest redshift probed to date. We find that the emission-weighted iron abundance measured within $(0.15-0.3) R_{vir}$ in clusters below 5 keV is, on average, a factor of $\sim2$ higher than in hotter clusters, following $Z(T)\simeq 0.88 T^{-0.47} Z_\odot$, which confirms the trend seen in local samples. We made use of combined spectral analysis performed over five redshift bins at $0.3> z > 1.3$ to estimate the average emission weighted iron abundance. We find a constant average iron abundance $Z_{Fe}\simeq 0.25 Z_\odot$ as a function of redshift, but only for clusters at $z>0.5$. The emission-weighted iron abundance is significantly higher ($Z_{Fe}\simeq0.4 Z_\odot$) in the redshift range $z\simeq0.3-0.5$, approaching the value measured locally in the inner $0.15 R_{vir}$ radii for a mix of cool-core and non cool-core clusters in the redshift range $0.1<z<0.3$. The decrease in $Z_{Fe}$ with $z$ can be parametrized by a power law of the form $\sim(1+z)^{-1.25}$. The observed evolution implies that the average iron content of the ICM at the present epoch is a factor of $\sim2$ larger than at $z\simeq 1.2$. We confirm that the ICM is already significantly enriched ($Z_{Fe}\simeq0.25 Z_\odot$) at a look-back time of 9 Gyr. Our data provide significant constraints on the time scales and physical processes that drive the chemical enrichment of the ICM.Comment: 4 pages, 4 figures, to appear in the Proceedings of "The Extreme Universe in the Suzaku Era", Dicember 2006, Kyoto (Japan

A FIRST DETERMINATION OF THE SURFACE DENSITY OF GALAXY CLUSTERS AT VERY LOW X--RAY FLUXES

We present the first results of a serendipitous search for clusters of galaxies in deep ROSAT-PSPC pointed observations at high galactic latitude. The survey is being carried out using a Wavelet based Detection Algorithm which is not biased against extended, low surface brightness sources. A new flux--diameter limited sample of 10 cluster candidates has been created from $\rm\sim 3 \, deg^2$ surveyed area. Preliminary CCD observations have revealed that a large fraction of these candidates correspond to a visible enhancement in the galaxy surface density, and several others have been identified from other surveys. We believe these sources to be either low--moderate redshift groups or intermediate to high redshift clusters. We show X-ray and optical images of some of the clusters identified to date. We present, for the first time, the derived number density of the galaxy clusters to a flux limit of $\rm 1\cdot 10^{-14} erg cm^{-2} s^{-1}$ (0.5--2.0 keV). This extends the $\log N$--$\log S$ of previous cluster surveys by more than one decade in flux. Results are compared to theoretical predictions for cluster number counts.Comment: uuencoded compressed Postscript, 7 pages including 4 figures. Accepted for publication in Ap. J. Letters

The micro-Jy Radio Source Population: the VLA-CDFS View

We analyse the 267 radio sources from our deep (flux limit of 42 microJy at the field center at 1.4 GHz) Chandra Deep Field South 1.4 and 5 GHz VLA survey. The radio population is studied by using a wealth of multi-wavelength information, including morphology and spectral types, in the radio, optical, and X-ray bands. The availability of redshifts for ~ 70% of our sources allows us to derive reliable luminosity estimates for the majority of the objects. Contrary to some previous results, we find that star-forming galaxies make up only a minority (~ 1/3) of sub-mJy sources, the bulk of which are faint radio galaxies, mostly of the Fanaroff-Riley I type.Comment: 6 pages, 3 figures, to appear in the proceedings of "At the Edge of the Universe", Sintra, Portugal, Oct. 9 - 13, 200

The Evolutionary Status of Clusters of Galaxies at z ~ 1

Combined HST, X-ray, and ground-based optical studies show that clusters of galaxies are largely "in place" by $z \sim 1$, an epoch when the Universe was less than half its present age. High resolution images show that elliptical, S0, and spiral galaxies are present in clusters at redshifts up to $z \sim 1.3$. Analysis of the CMDs suggest that the cluster ellipticals formed their stars several Gyr earlier, near redshift 3. The morphology--density relation is well established at $z\sim1$, with star-forming spirals and irregulars residing mostly in the outer parts of the clusters and E/S0s concentrated in dense clumps. The intracluster medium has already reached the metallicity of present-day clusters. The distributions of the hot gas and early-type galaxies are similar in $z\sim1$ clusters, indicating both have largely virialized in the deepest potentials wells. In spite of the many similarities between $z\sim1$ and present-day clusters, there are significant differences. The morphologies revealed by the hot gas, and particularly the early-type galaxies, are elongated rather than spherical. We appear to be observing the clusters at an epoch when the sub-clusters and groups are still assembling into a single regular cluster. Support for this picture comes from CL0152 where the gas appears to be lagging behind the luminous and dark mass in two merging sub-components. Moreover, the luminosity difference between the first and second brightest cluster galaxies at $z\sim1$ is smaller than in 93% of present-day Abell clusters, which suggests that considerable luminosity evolution through merging has occurred since that epoch. Evolution is also seen in the bolometric X-ray luminosity function.Comment: 18 pages, 12 figures, to appear in Penetrating Bars through Masks of Cosmic Dust: the Hubble Tuing Fork Strikes a New Note, eds. D.L. Block, K.C. Freeman, I. Puerari & R. Groess. Figures degraded to meet astroph size limit; a version with higher resolution figures may be downloaded from: http://acs.pha.jhu.edu/~jpb/z1clusters/ford_clusters.pd