The second most distant cluster of galaxies in the extended medium sensitivity survey

Megan Donahue, G. Mark Voit, Caleb A. Scharf, Isabella M. Gioia, Christopher R. Mullis, John Hughes, John T. Stocke

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We report on our ASCA, Keck, and ROSAT observations of MS 1137.5 + 6625, the second most distant cluster of galaxies in the Einstein Extended Medium Sensitivity Survey (EMSS), at redshift 0.78. We now have a full set of X-ray temperatures, optical velocity dispersions, and X-ray images for a complete, high-redshift sample of clusters of galaxies drawn from the EMSS. Our ASCA observations of MS 1137.5 + 6625 yield a temperature of 5.7+2.1-1.1 keV and a metallicity of 0.43+0.40-0.37 solar, with 90% confidence limits. Keck II spectroscopy of 22 clster members reveals a velocity dispersion of 884+185-124 km s-1. This cluster is the most distant in the sample with a detected iron line. We also derive a mean abundance at z = 0.8 by simultaneously fitting X-ray data for the two z = 0.8 clusters, and obtain an abundance of ZFe = 0.33 ±0.260.23. Our ROSAT observations show that MS 1137.5 + 6625 is regular and highly centrally concentrated. Fitting of a β model to the X-ray surface brightness yields a core radius of only 71 h-1 kpc (q0 = 0.1) with β= 0.70 ± 0.450.15. The gas mass interior to 0.5 h-1 Mpc is thus 1.2 ± 0.20.3 x 1013 h-5/2 M (q0 = 0.1). If the cluster's gas is nearly isothermal and in hydrostatic equilibrium with the cluster potential, the total mass of the cluster within this same region is 2.1 ± 1.50.8 × 1014 h-1 M, giving a gas fraction of 0.06 ± 0.04 h-3/2. This cluster is the highest redshift EMSS cluster showing evidence for a possible cooling flow (∼ 20-400 M⊙ yr-1). The velocity dispersion, temperature, gas fraction, and iron abundance of MS 1137.5 + 6625 are all statistically the same as those properties in lower redshift clusters of similar luminosity. With this cluster's temperature now in hand, we derive a high-redshift temperature function for EMSS clusters at 0.5 < z < 0.9 and compare it with temperature functions at lower redshifts, showing that the evolution of the temperature function is relatively modest. Supplementing our high-redshift sample with other data from the literature, we demonstrate that neither the cluster luminosity-temperature relation, nor cluster metallicities, nor the cluster gas fraction has detectably evolved with redshift. The very modest degree of evolution in the luminosity-temperature relation inferred from these data is inconsistent with the absence of evolution in the X-ray luminosity functions derived from ROSAT cluster surveys if a critical density structure formation model is assumed.

Original languageEnglish (US)
Pages (from-to)525-534
Number of pages10
JournalAstrophysical Journal
Issue number2 PART 1
StatePublished - Dec 20 1999

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • Cosmology
  • Dark matter
  • Intergalactic medium
  • Observations
  • X-rays : Galaxies

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