We present IR and UV photometry for a sample of brightest cluster galaxies (BCGs). The BCGs are from a heterogeneous but uniformly characterized sample, the Archive of Chandra Cluster Entropy Profile Tables (ACCEPT), of X-ray galaxy clusters from the
Chandra X-ray telescope archive with published gas temperature, density, and entropy profiles. We use archival GALEX, Spitzer, and 2MASS observations to assemble spectral energy distributions (SEDs) and colors for BCGs. We find that while the SEDs of some BCGs follow the expectation of red, dust-free old stellar populations, many exhibit signatures of recent star formation in the form of excess UV or mid-IR emission, or both. We establish a mean near-UV to 2MASS K color of 6.59 pm 0.34 for quiescent BCGs. We use this mean color to quantify the UV excess associated with star formation in the active BCGs. We use fits to a template of an evolved stellar population and library of starburst models and mid-IR star formation relations to estimate the obscured star formation rates. Many of the BCGs in X-ray clusters with low central gas entropy exhibit enhanced UV (38%) and mid-IR emission (43%), above that expected from an old stellar population. These excesses are consistent with on-going star formation activity in the BCG, star formation that appears to be enabled by the presence of high density, X-ray emitting gas in the the core of the cluster of galaxies. This hot, X-ray emitting gas may provide the enhanced ambient pressure and some of the fuel to trigger the star formation. This result is consistent with previous works that showed that BCGs in clusters with low central gas entropy host H{alpha} emission-line nebulae and radio sources, while clusters with high central gas entropy exhibit none of these features. UV and mid-IR measurements combined provide a complete picture of unobscured and obscured star formation occurring in these systems.
We present a detailed investigation of the X-ray luminosity (Lx)-gas temperature (Tvir) relation of the complete X-ray flux-limited sample of the 64 brightest galaxy clusters in the sky (HIFLUGCS). We study the influence of two astrophysical processe
s, active galactic nuclei (AGN) heating and intracluster medium (ICM) cooling, on the Lx-Tvir relation, simultaneously for the first time. We determine best-fit relations for different subsamples using the cool-core strength and the presence of central radio activity as selection criteria. We find the strong cool-core clusters (SCCs) with short cooling times (< 1Gyr)to display the steepest relation (Lx ~ Tvir^{3.33}) and the non-cool-core clusters (NCCs) with long cooling times (> 7.7Gyr) to display the shallowest (Lx ~ Tvir^{2.42}). This has the simple implication that on the high-mass scale (Tvir > 2.5keV) the steepening of the Lx-Tvir relation is mainly due to the cooling of the intracluster medium gas. We propose that ICM cooling and AGN heating are both important in shaping the Lx-Tvir relation but on different length-scales. While our study indicates that ICM cooling dominates on cluster scales (Tvir > 2.5keV), we speculate that AGN heating dominates the scaling relation in poor clusters and groups (Tvir < 2.5keV). The intrinsic scatter about the Lx-Tvir relation in X-ray luminosity for the whole sample is 45.4% and varies from a minimum of 34.8% for weak cool-core clusters to a maximum of 59.4% for clusters with no central radio source. We find that after excising the cooling region, the scatter in the Lx-Tvir relation drops from 45.4% to 39.1%, implying that the cooling region contributes ~ 27% to the overall scatter. Lastly, we find the true SCC fraction to be 25% lower than the observed one and the true normalizations of the Lx-Tvir relations to be lower by 12%, 7%, and 17% for SCC, WCC, and NCC clusters, respectively. [abridged]
