Brightest Cluster Galaxies at the Present Epoch

Abstract

We have obtained photometry and spectroscopy of 433 z 0.08 brightest cluster galaxies (BCGs) in a full-sky survey of Abell clusters to construct a BCG sample suitable for probing deviations from the local Hubble flow. The BCG Hubble diagram over 0 < z < 0.08 is consistent to within 2% of the Hubble relation specified by a Ωm = 0.3, Λ= 0.7 cosmology. This sample allows us to explore the structural and photometric properties of BCGs at the present epoch, their location in their hosting galaxy clusters, and the effects of the cluster environment on their structure and evolution. We revisit the Lm—α relation for BCGs, which uses α, the log-slope of the BCG photometric curve of growth, to predict the metric luminosity in an aperture with 14.3 kpc radius, Lm, for use as a distance indicator. Residuals in the relation are 0.27 mag rms. We measure central stellar velocity dispersions, σ, of the BCGs, finding the Faber—Jackson relation to flatten as the metric aperture grows to include an increasing fraction of the total BCG luminosity. A three-parameter “metric plane” relation using α and σ together gives the best prediction of Lm, with 0.21 mag residuals. The distribution of projected spatial offsets, rx of BCGs from the X-ray-defined cluster center is a steep γ = − 2.33 power law over 1 < r x < 103 kpc. The median offset is ∼10 kpc, but ∼15% of the BCGs have rx > 100 kpc. The absolute cluster-dispersion normalized BCG peculiar velocity |ΔV1|/σc follows an exponential distribution with scale length 0.39± 0.03. Both Lm and α increase with σc. The α parameter is further moderated by both the spatial and velocity offset from the cluster center, with larger α correlated with the proximity of the BCG to the cluster mean velocity or potential center. At the same time, position in the cluster has little effect on Lm. Likewise, residuals from the metric plane show no correlation with either the spatial or velocity offset from the cluster center. The luminosity difference between the BCG and second-ranked galaxy, M2, increases as the peculiar velocity of the BCG within the cluster decreases. Further, when M2 is a close luminosity “rival” of the BCG, the galaxy that is closest to either the velocity or X-ray center of the cluster is most likely to have the larger α. We conclude that the inner portions of the BCGs are formed outside the cluster, but interactions in the heart of the galaxy cluster grow and extend the envelopes of the BCGs.