Rare Blue-core BCGs as Optical Signatures of ICM Cooling

Abstract

Blue cores in brightest cluster galaxies (BCGs) trace recent central activity associated with cooling cluster atmospheres. We search for blue cores at $z<0.6$ in five eROSITA all-sky surveys using X-ray data, optical imaging, and optical spectroscopy. We identify 379 blue-core BCGs among 11352 systems (3.3%), although contamination increases toward higher redshift. Of these, 98 (0.86%) are X-ray bright. Their defining signature is a central $g-r$ color at least 0.2 mag bluer than the reference population within $a=20$ kpc before returning to the normal BCG envelope. Ha emission equivalent width correlates strongly with the blue continuum excess, consistent with young stellar components comprising ~0.1-1% of pre-existing stellar mass. Line ratios indicate widespread LINER-like excitation, with a larger star-forming contribution in blue Ha emitters but substantial non-stellar excitation. This activity is closely linked to the thermodynamics of the host-cluster gas. At $0.1R_{500}$, cooling times decrease from BCGs without significant Ha emission to red and then blue Ha emitters. Blue BCGs preferentially occur in clusters with $L_{\rm X,300}>10^{44}$ erg s$^{-1}$; red Ha emitters also favor higher X-ray luminosities. The blue-core fraction rises with optical richness. For systems with $t_{\rm cool}(0.1R_{500})<20$ Gyr, doubling richness increases the odds of hosting a blue-core BCG by $2.45\pm0.39$. This indicates that cluster scale carries information beyond cooling time, plausibly through the amount of gas available for cooling and condensation. The scarcity of blue BCGs without significant Ha emission disfavors isolated short bursts followed by long Ha-quiet fading phases. It can instead be explained by sustained or closely recurrent star formation over timescales much longer than 100 Myr, or by Ha persisting through additional excitation mechanisms as the young stellar component fades.