Optical Line Emission in Brightest Cluster Galaxies at 0 < z < 0.6: Evidence for a Lack of Strong Cool Cores 3.5 Gyr Ago?

TL;DR

This study investigates the evolution of optical line emission in brightest cluster galaxies from redshift 0 to 0.6, revealing a minimum at z~0.3 and suggesting changes in cool core activity over cosmic time.

Contribution

It provides the first continuous analysis of optical emission-line nebulae evolution in BCGs over this redshift range, linking optical emission to cool core evolution.

Findings

Minimum in optical line emission at z~0.3

Upturn in strongly-emitting systems beyond z>0.3

Overall agreement with X-ray cool core fraction evolution

Abstract

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 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 brightest cluster galaxies over the range 0 < z < 0.6 is determined. A minimum in the fraction of BCGs with optical line emission is found at z \sim 0.3, suggesting that complex, filamentary emission in systems such as Perseus A are 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 cool cores 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 cool cores and, thus, provide insights into the balance of heating and cooling processes at early cosmic times.