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

Abstract

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 $L_{\mathrm{X}}>2.3\times 10^{44}$ 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, $\hat{I}\pm$, 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-$L_{\mathrm{X}}$ clusters. By contrast, we find that the brightest members of low-$L_{\mathrm{X}}$ 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-$L_{\mathrm{X}}$ clusters yield a value of $\hat{I}{\text{c◯}}_{\mathrm{M}}=0.28\pm 0.24$ if the cosmological constant $\hat{I}›=0$. For a flat Universe we find $\hat{I}{\text{c◯}}_{\mathrm{M}}=0.55_{-0.15}^{+0.14}$ with a 95 per cent confidence upper limit to the cosmological constant corresponding to $\hat{I}{\text{c◯}}_{\hat{I}}›<0.73$. These results are discussed in the context of other methods used to constrain the density of the Universe, such as Type Ia supernovae.

Accepted for publication in MNRAS. 15 pages, 13 Postscript figs included. Uses epsfig and mn.sty