Discovery of a very X-ray luminous galaxy cluster at z=0.89 in the Wide Angle ROSAT Pointed Survey (WARPS)

H Ebeling, Laurence Jones, Bruce Fairley, E Perlman, C Scharf, D Horner

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We report the discovery of the galaxy cluster Cl J1226.9+3332 in the Wide Angle ROSAT Pointed Survey (WARPS). At and z = 0.888 and L-x = 1.1 x 10(45) h(50)(-2) ergs s(-1) (0.5-2.0 keV), Cl J1226.9+3332 is the most distant X-ray luminous cluster currently known. The mere existence of this system represents a huge problem for world models Omega (o) = 1 world models At the modest (off-axis) resolution of the ROSAT Position Sensitive Proportional Counter observation in which the system was detected, Cl J1226.9+3332 appears relaxed; an off-axis High Resolution Imager observation confirms this impression and rules out significant contamination from point sources. However, in moderately deep optical images (R and I band), the cluster exhibits signs of substructure in its apparent galaxy distribution. A first crude estimate of the velocity dispersion of the cluster galaxies based on six redshifts yields a high value of 1650 km s(-1), indicative of a very massive cluster and/or the presence of substructure along the line of sight. While a more accurate assessment of the dynamical state of this system requires much better data at both optical and X-ray wavelengths, the high mass of the cluster has already been unambiguously confirmed by a very strong detection of the Sunyaev-Zeldovich effect in its direction. Using Cl J1226.9+3332 and Cl J0152.7-1357 (z = 0.835), the second most distant X-ray luminous cluster currently known and also a WARPS discovery, we obtain a first estimate of the cluster X-ray luminosity function at and 0.8 <z <1.4 and L-x > 5 x 10(44) h(50)(-2) ergs s(-1) (0.5- 2.0 keV). Using the best currently available data, we find the comoving space density of very distant, massive clusters to be in excellent agreement with the value measured locally (z <0.3) and conclude that negative evolution is not required at these luminosities out to z 1. Our findings are in conflict with earlier claims of highly significant (>3 sigma) negative evolution already at 0.3 <z <based on the cluster samples of the Einstein Extended Medium-Sensitivity Survey (EMSS) and the Center for Astrophysics 160 degrees survey. Our results agree, however, with the lack of significant evolution of very X-ray luminous clusters out to z similar to 0.4 reported by the MAssive Cluster Survey team. Our findings are also consistent with the abundance of very X-ray luminous clusters at z similar to 0.8 inferred from the EMSS cluster sample, provided Cl J0152.7-1357 (which was missed by the EMSS) is added in.
Original languageEnglish
Pages (from-to)L23-L27
JournalThe Astrophysical Journal
Issue number1
Publication statusPublished - 10 Feb 2001


  • cosmology : observations
  • galaxies : clusters : general galaxies : clusters : individual (Cl J1226.9+3332, Cl J0152.7-1357)
  • X-rays : general


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