Line Emission in Brightest Cluster Galaxies: The Nature of Recent Activity

Louise Edwards

The Brightest Cluster Galaxy (BCG) is a giant elliptical, typically located at the center of the gravitational potential of a rich cluster. Because the BCG is often found at the peak of the cluster's X-ray surface brightness, where the hot intracluster gas can potentially cool to form molecular clouds, an additional stellar or gaseous component arising via the cooling intracluster medium is plausible.

We investigate the frequency of occurrence and properties of optical line emission in nearby BCGs in an effort to pinpoint current or recent activity in these large systems. To investigate the role of the host cluster properties, we examine the BCGs and controls in two large homogeneous datasets: the NOAO Fundamental Plane Survey (NFP) and the Sloan Digital Sky Survey Data Release 3 (SDSS). Subsequently, using integral field spectroscopy (IFU) on a select subsample of BCGs, we explore the detailed properties of the optical emission.

Analysis of the survey results 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, for those BCGs located in cooling flow clusters, about 70% have optical emission. Furthermore, considering only BCGs within 50 kpc of the X-ray center of a cooling flow cluster, the emission-line fraction rises to nearly 100%. 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. The conclusion is that the presence of emission lines in BCGs is directly related to the cooling of X-ray gas at the cluster center.

The morphology, kinematics and diagnostic line ratios from optical IFU observations of the central regions of BCGs is presented. Occasionally, the emission mechanism is found to be dominated by hot stars (2 of 9 cases), however most often there are dominating signatures of a harder ionizing source, such as an AGN. The conclusion is that the emission mechanism is not universal among BCGs. Different ionization processes are more or less important for determining the emission line properties in the individual BCGs.