The environmental dependence of brightest cluster galaxies: Implications for large-scale flows

In a much-noticed recent study Lauer & Postman found that the inertial frame defined by a sample of 119 nearby Abell clusters with cz. < 15,000 km s(-1) showed a highly significant motion with respect to the cosmic microwave background (CMB) frame. We construct a subsample of their sample which comprises 64 Abell/ACO clusters with X-ray luminosities from ROSAT and brightest cluster galaxy (BCG) photometry from Lauer & Postman. We find that both BCG metric luminosities and residuals from the L-m-alpha relation of Lauer & Postman are significantly correlated with the X-ray luminosity of the host 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 the structure parameter alpha. Taking this correlation into account, we obtain a new distance indicator for BCGs, the L-m-alpha-L-X relation. Applying the L-m-alpha-L-X relation to our sample, we find that the frame defined by these clusters has a bulk motion of 494 km s(-1) toward l = 285 degrees, b = 47 degrees with respect to the CMB frame but the 95% confidence range on the amplitude is 306-1419 km s(-1). When the covariance of the components of the bulk motion is properly taken into account, these results are inconsistent with this frame being at rest in the CMB frame at only the 98.6% confidence level but are consistent with the 300-400 km s(-1) amplitude flows found by other studies on scales cz less than or similar to 6000 km s(-1). In order to obtain an estimate of the bulk flow on scales beyond local perturbations such as the ''Great Attractor,'' we have also examined the subsample of 57 clusters with X-ray data and cz(LG) > 6000 km s(-1). For this subsample, the random errors in the bulk motion are large. We find that the bulk motion is consistent both with no motion in the CMB frame and with the motion found by Lauer & Postman. However, our analysis of all 107 Lauer & Postman BCGs with cz(LG) > 6000 km s(-1) indicates that, even with no X-ray correction, the motion of these clusters with respect to the CMB frame is not significantly different from zero. Furthermore, the correction to the bulk motion of the subsample with X-ray data goes in the sense of reducing the amplitude (by 663 km s(-1)) and significance (from 98.8% to 83.8%) of its motion in the CMB frame, as well as reducing the internal inconsistency between its motion and that of the remainder of the Lauer & Postman sample with no X-ray data. Claims of large-scale, large-amplitude bulk flows should therefore be regarded with caution until X-ray data become available for more clusters or until cluster distances are confirmed by independent methods.