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B. Holden, S. A. Stanford (IGPP/LLNL - UC Davis), P. Rosati (ESO), P. Tozzi (Osservatorio Astronomico di Trieste), S. Borgani (INFN, Sezione di Trieste), P. Eisenhardt (Jet Propulsion Laboratory)
Using the Chandra X-ray Observatory, we have observed some of the highest redshift clusters in the ROSAT Deep Cluster Survey. We use these X-ray observations to study the intracluster medium in two clusters at a redshift of z ~ 1.27. Using these data, we measure fluxes, temperatures and compare the distribution of the X-ray emitting gas with the underlying galaxy population.
First, we examined the spatial distribution of these two clusters. One has quite regular X-ray isophotes, so we fit an isothermal \beta model to constrain the mass of X-ray emitting gas. We find, with a value of \beta = 0.61 and a compact core radius of rc = 65 kpc (Ho = 65 km s-1 Mpc-1, \Omegam = 0.3, \Omega\Lambda = 0.7) a gas mass of 2.3\times 1013 M\odot(<1 Mpc). The second cluster has a much more irregular morphology, with no central concentration.
For both clusters, we used the spatial distribution of X-ray events to define apertures for measuring the fluxes and temperatures of the clusters. The irregular cluster is quite faint, ~ 120 events inside a 30 arcsecond aperture. For that cluster, we find a temperature of ~ 1.6 keV but with large uncertainties. The more regular cluster has a temperature of ~ 7.0 keV, but a luminosity of only 1.3\times 1044 erg s-1 (0.7--10.0 keV, rest frame) based on 450 events within a 36 arcsecond aperture.
Both clusters lie slightly to the right of the low redshift luminosity-temperature relation, in contradiction with the expectation from \Omegam = 1 intracluster medium evolution models. However, for both clusters, the intracluster medium is well correlated with the galaxy population which is apparently dominated by older stellar populations, a tantalizing clue for the formation of the intracluster medium.