OPTICAL LINE EMISSION IN BRIGHTEST CLUSTER GALAXIES AT 0 < z < 0.6: EVIDENCE FOR A LACK OF STRONG COOL CORES 3.5 Gyr AGO?

In recent years the number of known galaxy clusters beyond z ∼> 0.2 has increased drastically with the release of multiple catalogs containing >30,000 optically detected galaxy clusters over the range 0 < z < 0.6. Combining these catalogs with the availability of optical spectroscopy of the brightest cluster galaxy (BCG) from the Sloan Digital Sky Survey allows for the evolution of optical emission-line nebulae in cluster cores to be quantified. For the first time, the continuous evolution of optical line emission in BCGs over the range 0 < z < 0.6 is determined. A minimum in the fraction of BCGs with optical line emission is found at z ∼ 0.3, suggesting that complex, filamentary emission in systems such as Perseus A is a recent phenomenon. Evidence for an upturn in the number of strongly emitting systems is reported beyond z > 0.3, hinting at an earlier epoch of strong cooling. We compare the evolution of emission-line nebulae to the X-ray-derived cool core (CC) fraction from the literature over the same redshift range and find overall agreement, with the exception that an upturn in the strong CC fraction is not observed at z > 0.3. The overall agreement between the evolution of CCs and optical line emission at low redshift suggests that emission-line surveys of galaxy clusters may provide an efficient method of indirectly probing the evolution of CCs and thus provide insights into the balance of heating and cooling processes at early cosmic times.