The nature of obscuration in AGN: II. insights from clustering properties

Based on large optical and mid-infrared (IR) surveys, we investigate the relation between nuclear activity in local Seyfert 2 galaxies and galaxy interactions using a statistical neighbour counting technique. At the same level of host galaxy star formation (SF), we find that active galactic nuclei (AGNs) with stronger [OIII] emission lines do not show an excess of near neighbours, while AGNs with stronger mid-IR emission do have more near neighbours within a projected distance of 100 kpc. The excess neighbour count increases with decreasing projected radius. These results suggest a phase of torus formation during galaxy interactions.Comment: 5 pages, 2 figures, accepted to MNRA

Stellar mass versus stellar velocity dispersion: which is better for linking galaxies to their dark matter halos?

It was recently suggested that, compared to its stellar mass (M*), the central stellar velocity dispersion (sigma*) of a galaxy might be a better indicator for its host dark matter halo mass. Here we test this hypothesis by estimating the dark matter halo mass for central alaxies in groups as function of M* and sigma*. For this we have estimated the redshift-space cross-correlation function (CCF) between the central galaxies at given M* and sigma* and a reference galaxy sample, from which we determine both the projected CCF, w_p(r_p), and the velocity dispersion profile (VDP) of satellites around the centrals. A halo mass is then obtained from the average velocity dispersion within the virial radius. At fixed M*, we find very weak or no correlation between halo mass and sigma*. In contrast, strong mass dependence is clearly seen even when sigma* is limited to a narrow range. Our results thus firmly demonstrate that the stellar mass of central galaxies is still a good (if not the best) indicator for dark matter halo mass, better than the stellar velocity dispersion. The dependence of galaxy clustering on sigma* fixed M*, as recently discovered by Wake et al. (2012), may be attributed to satellite galaxies, for which the tidal stripping occurring within halos has stronger effect on stellar mass than on central stellar velocity dispersion.Comment: 4 pages, 4 figures, accepted for publication in ApJ Letters, minor revisions in the tex