Star formation in quasar and active galaxy environments

Abstract

We use the 2dF public 100 K data release of galaxies and samples of quasars and active galaxies taken from the Veron-Cetty and Veron catalogue to study the nature of galaxies in the surroundings of active objects withredshifts in the range 0.1 3.5, in the vicinity of quasars and active galaxies compared to that in the neighbourhood of typical galaxies, bright early types and groups. This effect extends up to projected distance rp ∼ 1 h - 1 Mpc for active galaxies and r p ∼ 3 h - 1 Mpc for quasars. We also find a tendency for companion galaxies of quasars to be brighter than the neighbours of active galaxies within r p ∼ 3 h - 1 Mpc. We estimate average star-formation rates for objects at different distances from quasars, active galaxies, galaxies and groups. We find a significantly higher star-formation activity within ∼2.0 h - 1 Mpc from quasars with respect to typical galaxies, which reinforces the idea that star formation is enhanced in the neighbourhood of quasars. Our tests with the group environment provide evidence against quasars being associated with groups. Also, our analysis of the neighbours of bright early types shows that although these galaxies are typical hosts of quasars, their companion galaxies are significantly different in terms of the star-formation activity.