The K-band Hubble diagram for brightest cluster galaxies in X-ray clusters

This paper concerns the K band Hubble diagram for the brightest cluster galaxies (BCGs) in a sample of X-ray clusters covering the redshift range $0.05<z<0.8$. We show that BCGs in clusters of high X-ray luminosity are excellent standard candles: the intrinsic dispersion in the raw K band absolute magnitudes of BCGs in clusters with $L_{\rm X} > 2.3 \times 10^{44}$ erg s$^{-1}$ (in the 0.3 - 3.5 keV band) is no more than 0.22 mag, and is not significantly reduced by correcting for the BCG structure parameter, $\alpha$, or for X-ray luminosity. This is the smallest scatter in the absolute magnitudes of any single class of galaxy and demonstrates the homogeneity of BCGs in high-$L_{\rm X}$ clusters. By contrast, we find that the brightest members of low-$L_{\rm X}$ systems display a wider dispersion ($\sim 0.5$ mag) in absolute magnitude than commonly seen in previous studies, which arises from the inclusion, in X-ray flux-limited samples, of poor clusters and groups which are usually omitted from low redshift studies of BCGs in optically rich clusters....[abstract shortened].. The BCGs in our high-$L_{\rm X}$ clusters yield a value of $\Omega_{\rm M}=0.28\pm0.24$ if the cosmological constant $\Lambda=0$. For a flat Universe we find $\Omega_{\rm M}=0.55^{+0.14}_{-0.15}$ with a 95 per cent confidence upper limit to the cosmological constant corresponding to $\Omega_{\Lambda}<0.73$. These results are discussed in the context of other methods used to constrain the density of the Universe, such as Type Ia supernovae.Comment: Accepted for publication in MNRAS. 15 pages, 13 Postscript figs included. Uses epsfig and mn.st

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IN MEMORIAM: ALFRED BLOSE, and DUMA WILBERFORCE GWAL

The Properties of Brightest Cluster Galaxies in X-Ray Selected Clusters

We present the K-band Hubble diagram for 162 brightest cluster galaxies (BCGs) in X-ray selected clusters, 0.01<z<0.83. The sample incorporates that of Burke, Collins, & Mann (2000) and includes additional infrared data from the 2MASS extended source catalogue. We show that below z=0.1 the BCGs show no correlation with their environment, however, above z=0.1 BCGs in more X-ray luminous clusters are more uniform in their photometric properties. This suggests that there may be two populations of BCGs which have different evolutionary histories.Comment: 2 pages, to appear in the proceedings of the Sesto 2001 conference on tracing cosmic evolution with galaxy cluster

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