The environment of active galaxies in the SDSS-DR4

Abstract

We study the environment of active galaxies and compare it with that of star forming and normal galaxies. We extracted from the Fourth Data Release (DR4) of the Sloan Digitaly Sky Survey (SDSS) the galaxies in the redshift range 0.05 \le z \le 0.095 and with M(r) \le -20.0 (that is M* + 1.45). Emission line ratios and/or widths are used to separate Active Galactic Nuclei (AGN) from Star-Forming Galaxies (SFGs); AGN are classified as Seyfert-1 and Seyfert-2 galaxies according to emission line widths. The environmental properties, as defined by a density parameter and the number of companions, are compared for the different types of galaxies, taking into account the morphological type of the host galaxies. We find no difference in the large-scale environment of Seyfert-1 and Seyfert-2 galaxies; however, a larger fraction of Seyfert-2 (~2%) than Seyfert-1 (~1%) is found in systems which are smaller than r_{max} \le 100 kpc, mainly in low-density environments (pairs or triplets); for comparison, this fraction is ~2% for star forming galaxies and ~1% for normal galaxies. We find no evidence for a relation between large-scale environment properties and activity. If activity and environment are related, this more likely occurs on small scales (e.g. galaxy interaction, merging).