The regulation of star formation in cool-core clusters: imprints on the stellar populations of brightest cluster galaxies

A fraction of brightest cluster galaxies (BCGs) show bright emission in the ultraviolet and the blue part of the optical spectrum, which has been interpreted as evidence of recent star formation. Most of these results are based on the analysis of broad-band photometric data. Here, we study the optical spectra of a sample of 19 BCGs hosted by X-ray luminous galaxy clusters at 0.15 <z < 0.3, a subset from the Canadian Cluster Comparison Project sample. We identify plausible star formation histories of the galaxies by fitting simple stellar populations as well as composite populations, consisting of a young stellar component superimposed on an intermediate/old stellar component, to accurately constrain their star formation histories. We detect prominent young (∼200 Myr) stellar populations in four of the 19 galaxies. Of the four, the BCG in Abell 1835 shows remarkable A-type stellar features indicating a relatively large population of young stars, which is extremely unusual even amongst star-forming BCGs. We constrain the mass contribution of these young components to the total stellar mass to be typically between 1 and 3 per cent, but rising to 7 per cent in Abell 1835. We find that the four of the BCGs with strong evidence for recent star formation (and only these four galaxies) are found within a projected distance of 5 kpc of their host cluster's X-ray peak, and the diffuse, X-ray gas surrounding the BCGs exhibits a ratio of the radiative cooling-to-free-fall time (t_c/t_ff) of ≤10. These are also some of the clusters with the lowest central entropy. Our results are consistent with the predictions of the precipitation-driven star formation and active galactic nucleus feedback model, in which the radiatively cooling diffuse gas is subject to local thermal instabilities once the instability parameter t_c/t_ff falls below ∼10, leading to the condensation and precipitation of cold gas. The number of galaxies in our sample where the host cluster satisfies all the criteria for recent and ongoing star formation is small, but their stellar populations suggest a time-scale for star formation to restart of the order of ∼200 Myr.