Astronomy and Astrophysics – Astronomy
Scientific paper
Apr 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apj...424l..79c&link_type=abstract
Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X), vol. 424, no. 2, p. L79-L82
Astronomy and Astrophysics
Astronomy
67
Bremsstrahlung, Dark Matter, Galactic Clusters, Galactic Evolution, Hydrostatics, Sky Surveys (Astronomy), X Rays, Potential Fields, Red Shift, Rosat Mission, Stellar Luminosity
Scientific paper
We present and analyze deep Roentgen Satellite (ROSAT) observations of two fields containing the most distant (z greater than 0.7) optically selected clusters of galaxies currently known. We reliably detect X-ray emission from two clusters (including one at z approximately equals 0.9) out of five with available redshifts, but we do not detect any emission from a further five candidates without spectroscopic data. Although our distant clusters are expected to be among the richest found optically, their X-ray luminosities (Lx approximately equals 1044 h50-2 ergs/s) are much lower than those of present-day rich clusters. We argue that the clusters we have detected are the only X-ray-luminous examples in the fields surveyed. By considering the likely volume sampled we find evidence for a decline in the comoving number density of clusters to z approximately equals 1. On the basis of our current small sample, our results are inconsistent with standard hierarchical clustering models in which the gas evolves in a self-similar fashion, indicating that radiative and hydrodynamic processes may be required to account for the low observed luminosities. For example, a cold dark matter model in which the entropy of the intracluster gas is assumed constant predicts a low abundance of luminous clusters at z approximately equals 0.8 as inferred from our observations.
Castander Francisco J.
Dressler Alan
Ellis Richard S.
Frenk Carlos S.
Gunn James E.
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