A fraction of brightest cluster galaxies ( BCGs ) Êshows Êbright emission in the UV Êand the blue part of the optical spectrum , which has been interpreted as evidence of recent star formation.Ê Most of these results are based on the analysis of broadband photometric data .
ÊHere , 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 ( CCCP ) sample .
We identify plausible star formation histories of the galaxies by fitting Simple Stellar Populations ( SSPs ) 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 ( \sim 200 \mathrm { Myr } ) stellar populations in 4 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 % to 3 % , but rising to 7 % 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 \mathrm { kpc } of their host cluster ’ s X-ray peak , and the diffuse , X-ray gas surrounding the BCGs exhibit a ratio of the radiative cooling-to-free-fall time ( t _ { c } / t _ { ff } ) of \leq 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 AGN 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 \sim 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 timescale for star formation to restart of the order of \sim 200 Myrs .