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 datasets. 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 (~15%). 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^{+9}_{-14}% 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^{+0}_{-15}%. Excluding the cooling flow clusters, only ~10% 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.Comment: Accepted for publication in MNRAS. 13 pages mn2e style with 7 figures and 2 table

CFHT Legacy Ultraviolet Extension (CLUE): Witnessing Galaxy Transformations up to 7 Mpc from Rich Cluster Cores

Using the optical data from the Wide component of the CFHT Legacy Survey, and new ultraviolet data from GALEX, we study the colours and specific star formation rates (SSFR) of ~100 galaxy clusters at 0.16<z<0.36, over areas extending out to radii of r~7Mpc. We use a multicolour, statistical background subtraction method to study the galaxy population at this radius; thus our results pertain to those galaxies which constitute an excess over the average field density. We find that the average SSFR, and its distribution, of the star-forming galaxies (with SFR>0.7 M_sun/yr at z~0.2 and SFR>1.2 M_sun/yr at z~0.3) have no measurable dependence on the cluster-centric radius, and are consistent with the field values. However, the fraction of galaxies with SFR above these thresholds, and the fraction of optically blue galaxies, are lower for the overdense galaxy population in the cluster outskirts compared with the average field value, at all stellar masses M*>10^{9.8} M_sun and at all radii out to at least 7Mpc. Most interestingly, the fraction of blue galaxies that are forming stars at a rate below our UV detection limit is much higher in all radial bins around our cluster sample, compared with the general field value. This is most noticeable for massive galaxies M*>10^{10.7} M_sun; while almost all blue field galaxies of this mass have detectable star formation, this is true for less than 20% of the blue cluster galaxies, even at 7Mpc from the cluster centre. Our results support a scenario where galaxies are pre-processed in locally overdense regions, in a way that reduces their SFR below our UV detection limit, but not to zero.Comment: MNRAS accepte