The Environmental Dependence of Brightest Cluster Galaxies: Implications for Large‐Scale Flows

(Shortened) We construct a subsample of the Lauer & Postman (LP) brightestcluster galaxy (BCG) sample which comprises 64 Abell/ACO clusters with X-rayluminosities from ROSAT. We find that both BCG metric luminosities andresiduals from the L-alpha relation of LP are significantly correlated with theX-ray luminosity of the cluster at the 99.6% confidence level, in the sensethat more X-ray luminous clusters have brighter BCGs. The strength of thiscorrelation increases with increasing X-ray luminosity and with increasingvalues of alpha. We obtain a new distance indicator for BCGs, the L-alpha-L_Xrelation. Applying this new relation to our sample, we find that the framedefined by these clusters has a bulk motion of 494 km/s in the CMB frame butthe 95% confidence range on the amplitude is 306 to 1419 km/s. These resultsare inconsistent with this frame being at rest in the CMB frame at the 98.6%confidence level. For the outer shell subsample (57 BCGs with cz > 6000 km/s),the bulk motion is consistent both with no motion in the CMB frame and with themotion found by LP. However, our analysis of the 107 Lauer and Postman BCGs inthis outer shell indicates that, even with no X-ray correction, the CMB-framebulk motion of these clusters is not statistically different from zero.Furthermore, for the subsample with X-ray data, the X-ray correction goes inthe sense of reducing both the amplitude (by 663 km/s) and significance (from98.8% to 83.8%) of its CMB-frame bulk motion, as well as reducing the internalinconsistency between its motion and that of the remainder of the Lauer andPostman sample with no X-ray data. Claims of large-scale, large-amplitude bulkflows should be regarded with caution until X-ray data become available formore clusters.