We present equivalent widths of the [OII] and Ha nebular emission lines for 77 brightest cluster galaxies (BCGs) selected from the 160 Square Degree ROSAT X-ray survey. We find no [OII] or Ha emission stronger than -15 angstroms or -5 angstroms, respectively, in any BCG. The corresponding emission line luminosities lie below 6E40 erg/s, which is a factor of 30 below that of NGC1275 in the Perseus cluster. A comparison to the detection frequency of nebular emission in BCGs lying at redshifts above z = 0.35 drawn from the Brightest Cluster Survey (Crawford et al. 1999) indicates that we should have detected roughly one dozen emission-line galaxies, assuming the two surveys are selecting similar clusters in the X-ray luminosity range 10E42 erg/s to 10E45 erg/s. The absence of luminous nebular emission (ie., Perseus-like systems) in our sample is consistent with an increase in the number density of {\it strong} cooling flow (cooling core) clusters between z=0.5 and today. The decline in their numbers at higher redshift could be due to cluster mergers and AGN heating.
@article{arxiv.1101.4966,
title = {Evidence for Rapid Redshift Evolution of Strong Cluster Cooling Flows},
author = {R. Samuele and B. R. McNamara and A. Vikhlinin and C. R. Mullis},
journal= {arXiv preprint arXiv:1101.4966},
year = {2015}
}