We present equivalent widths of the [O II]λ3727 and Hα nebular emission lines for 77 brightest cluster galaxies (BCGs) selected from the 160 Square Degree ROSAT X-ray survey. We find no [O II]λ3727 or Hα emission stronger than –15 Å or –5 Å, respectively, in any BCG. The corresponding emission-line luminosities lie below L ~ 6 × 1040 erg s–1, which is a factor of 30 below that of NGC 1275 in the Perseus Cluster. A comparison to the detection frequency of nebular emission in BCGs at z , which is a factor of 30 below that of NGC 1275 in the Perseus Cluster. A comparison to the detection frequency of nebular emission in BCGs at z 42 erg s–1 to 5 × 1044 erg s–1. The absence of luminous nebular emission (i.e., Perseus-like systems) in our sample is consistent with an increase in the number density of 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 heating by active galactic nuclei.
