Brightest cluster galaxies as standard candles

Abstract

We investigate the use of brightest cluster galaxies (BCGs) as standard candles for measuring galaxy peculiar velocities on large scales. We have obtained precise large-format CCD surface photometry and redshifts for an all-sky, volume-limited (z <= 0.05) sample of 119 BCG. We reinvestigate the Hoessel (1980) relationship between the metric luminosity, L_m_, within the central 10 h^-1^ kpc of the BCGs and the logarithmic slope of the surface brightness profile, α. The L_m_-α relationship reduces the cosmic scatter in L_m_ from 0.327 mag to 0.244 mag, yielding a typical distance accuracy of 17% per BCG. Residuals about the L_m_-α relationship are independent of BCG luminosity, BCG B - R_c_ color, BCG location within the host cluster, and richness of the host cluster. The metric luminosity is independent of cluster richness even before correcting for its dependence on α, which provides further evidence for the unique nature of the BCG luminosity function. Indeed, the BCG luminosity function, both before and after application of the α-correction, is consistent with a single Gaussian distribution. Half the BCGs in the sample show some evidence of small color gradients as a function of radius within their central 50 h^- 1^ kpc regions but with almost equal numbers becoming redder as becoming bluer. However, with the central 10 h^-1^ kpc the colors are remarkably constant-the mean B - R_c_ color is 1.51 with a dispersion of only 0.06 mag. The narrow photometric and color distributions of the BCGs, the lack of "second-parameter" effects, as well as the unique rich cluster environment of BCGs, argue that BCGs are the most homogeneous distance indicators presently available for large-scale structure research.