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

Abstract

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 (SFR) of a few to even $\sim$100 $M_\odot/yr$. 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 $0.1\lesssim z\lesssim 0.55$ 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 ($\lesssim 20\%$) might host AGN, most W4BCGs should owe their strong mid-IR emissions to dust-enshrouded star formation. Their median total IR luminosity ($L_{IR}$) is $5\times10^{11} L_{\odot}$ (SFR $\sim$50 $M_{\odot}/yr)$, and 27\% of the whole sample has $L_{IR}>10^{12} L_{\odot}$ (SFR $>$100 $M_{\odot}/yr$). Using ten W4BCGs that have Chandra X-ray data, we show that seven 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 to the idea that cooling flows are the cause of the star formation in non-quiescent BCGs.