Evidence for recent star formation in BCGs: a correspondence between blue cores and UV excess

TL;DR

This study finds a direct link between blue cores and UV excess in Brightest Cluster Galaxies, indicating recent star formation likely fueled by intracluster medium cooling, challenging previous assumptions about AGN heating effects.

Contribution

First demonstration of a one-to-one correspondence between blue cores and UV enhancement in BCGs, linking recent star formation to intracluster medium cooling.

Findings

Blue UV colors correlate with blue optical cores in BCGs.

Recent star formation in BCGs is less than 200 Myrs old.

Star formation contributes less than 1% to galaxy mass.

Abstract

We present a joint analysis of near-ultraviolet (NUV) data from the GALEX mission and (optical) colour profiles for a sample of 7 Brightest Cluster Galaxies (BCGs) in the Canadian Cluster Comparison Project. We find that every BCG which has a blue rest-frame UV colour also shows a blue-core in its optical colour profile. Conversely, BCGs that lack blue cores and show monotonic colour gradients typical of old elliptical galaxies, are red in the UV. We interpret this as evidence that the NUV enhancement in the blue BCGs is driven by recent star formation and not from old evolved stellar populations such as horizontal branch stars. The recent star formation in the blue BCGs typically has an age less than 200 Myrs and contributes mass fractions of less than a percent. Although the sample studied here is small, we demonstrate, for the first time, a one-to-one correspondence between blue cores in elliptical galaxies (in particular BCGs) and a NUV-enhancement observed using GALEX. The combination of this one-to-one correspondence and the consistently young age of recent star formation, coupled with additional correlations with the host cluster's X-ray properties, strongly suggests that the star formation is fueled by gas cooling out of the intracluster medium. In turn, this implies that any AGN heating of the intracluster medium in massive clusters only acts to reduce the magnitude of the cooling flow and that once this flow starts, it is nearly always active. (abridged)