"Red" but Not "Dead": Actively Star-forming Brightest Cluster Galaxies at Low Redshifts

Brightest cluster galaxies (BCGs) are believed to have assembled most of their stars early in time and therefore should be passively evolving at low redshifts and appear "red-and-dead." However, there have been reports that a minority of low-redshift BCGs still have ongoing star formation rates (SFRs) of a few to even similar to 100 M(circle dot)yr(-1). Such BCGs are found in "cool-core" ("CC") clusters, and their star formation is thought to be fueled by "cooling flow." To further investigate the implications of low-redshift, star-forming BCGs, we perform a systematic search using the 22 mu m data ("W4" band) from the Wide-field Infrared Survey Explorer (WISE) on the GMBCG catalog, which contains 55,424 BCGs at 0.1 less than or similar to z less than or similar to 0.55 identified in the Sloan Digital Sky Survey (SDSS). Our sample consists of 389 BCGs that are bright in W4 ("W4BCGs"), most being brighter than 5 mJy. While some (less than or similar to 20%) might host active galactic nuclei, most W4BCGs should owe their strong mid-IR emissions to dust-enshrouded star formation. Their median total IR luminosity (L-IR) is 5 10(11) L-circle dot (SFR similar to 50 M-circle dot yr(-1)) and 27% of the whole sample has L-IR > 10(12) L-circle dot (SFR > 100 M-circle dot yr(-1)). Using 10 W4BCGs that have Chandra X-ray data, we show that 7 of them are possibly in CC clusters. However, in most cases (five out of seven) the mass deposition rate cannot account for the observed SFR. This casts doubt on the idea that cooling flows are the cause of the star formation in non-quiescent BCGs.