Line emission in the brightest cluster galaxies of the NOAO fundamental Plane and Sloan digital Sky Surveys

We examine the optical emission-line properties of brightest cluster galaxies (BCGs) selected from two large, homogeneous data sets. The first is the X-ray selected National Optical Astronomy Observatory Fundamental Plane Survey (NFPS), and the second is the C4 catalogue of optically selected clusters built from the Sloan Digital Sky Survey Data Release 3 (SDSS DR3). Our goal is to better understand the optical line emission in BCGs with respect to properties of the galaxy and the host cluster. Throughout the analysis we compare the line emission of the BCGs to that of a control sample made of the other bright galaxies near the cluster centre. Overall, both the NFPS and SDSS show a modest fraction of BCGs with emission lines (similar to 15 per cent). No trend in the fraction of emitting BCGs as a function of galaxy mass or cluster velocity dispersion is found. However, we find that, for those BCGs found in cooling flow clusters, 71(-14)(+9) have optical emission. Furthermore, if we consider only BCGs within 50kpc of the X-ray centre of a cooling flow cluster, the emission-line fraction rises further to 100(-15)(+0) per cent. Excluding the cooling flow clusters, only similar to 10 per cent of BCGs are line emitting, comparable to the control sample of galaxies. We show that the physical origin of the emission-line activity varies: in some cases it has LINER-like line ratios, whereas in others it is a composite of star-formation and LINER-like activity. We conclude that the presence of emission lines in BCGs is directly related to the cooling of X-ray gas at the cluster centre.