The Impact of Galaxy Environment on the X-ray AGN Fraction

Daryl Haggard


Galaxy formation models must reproduce the AGN fraction as a function of host color and environment. To date, AGN surveys based on optical spectroscopic classification are inconclusive, e.g. some find no environmental dependence while others find a trend similar to starformation rate when selecting by luminosity, and optical emission line classification can falter for AGN at z > 0.4. X-ray selection, with no such limitation, yields greater completeness for AGN than even deep optical spectroscopy, and is much less biased against intrinsic absorption. Here we use a unique sample from the SDSS and the Chandra Multiwavelength Project (ChaMP) to determine the X-ray AGN fraction (log Lx > 42) as a function of local galaxy environment (Bayesian 10th nearest neighbor density). For each X-ray-detected galaxy in the Haggard et al. (2010) ChaMP/SDSS sample we define a control sample of SDSS galaxies with statistically equivalent stellar mass and redshift in order to quantify the relative over- or under-density of AGN environments and to quantify the relationships between star formation, accretion, and environment. Our sample includes ~1600 X-ray detections and over 10^5 SDSS galaxies with Chandra X-ray luminosity limits and thus represents the widest (~20 sq.deg), most uniform study of AGN activity as a function of galaxy environment to-date.