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

Abstract

(Shortened) We construct a subsample of the Lauer & Postman (LP) brightest cluster galaxy (BCG) sample which comprises 64 Abell/ACO clusters with X-ray luminosities from ROSAT. We find that both BCG metric luminosities and residuals from the L-alpha relation of LP are significantly correlated with the X-ray luminosity of the cluster at the 99.6% confidence level, in the sense that more X-ray luminous clusters have brighter BCGs. The strength of this correlation increases with increasing X-ray luminosity and with increasing values of alpha. We obtain a new distance indicator for BCGs, the L-alpha-L_X relation. Applying this new relation to our sample, we find that the frame defined by these clusters has a bulk motion of 494 km/s in the CMB frame but the 95% confidence range on the amplitude is 306 to 1419 km/s. These results are 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 the motion found by LP. However, our analysis of the 107 Lauer and Postman BCGs in this outer shell indicates that, even with no X-ray correction, the CMB-frame bulk motion of these clusters is not statistically different from zero. Furthermore, for the subsample with X-ray data, the X-ray correction goes in the sense of reducing both the amplitude (by 663 km/s) and significance (from 98.8% to 83.8%) of its CMB-frame bulk motion, as well as reducing the internal inconsistency between its motion and that of the remainder of the Lauer and Postman sample with no X-ray data. Claims of large-scale, large-amplitude bulk flows should be regarded with caution until X-ray data become available for more clusters.

20 pages, LaTeX, ApJ in press. Also available via http://astrowww.phys.uvic.ca/rsocs/hudson/pubs/pubs.html

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