The environment of active galaxies in the SDSS-DR4

INAF - Osservatorio Astronomico di Capodimonte, via Moiariello, 16, 80131 Napoli, Italy e-mail: [gsorrent;radovich;rifatto]@na.astro.it

Received: 14 October 2005
Accepted: 14 January 2006

Abstract

Aims.We study the environment of active galaxies and compare it with that of star-forming and normal galaxies.

Methods.From the Fourth Data Release (DR4) of the Sloan Digital Sky Survey (SDSS) we extracted the galaxies in the redshift range and with (that is, ). Emission-line ratios and/or widths were used to separate active galactic nuclei (AGNs) from star-forming galaxies (SFGs); AGNs were classified as Seyfert-1 (Sy1) and Seyfert-2 (Sy2) 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 the morphological type of the host galaxies into account.

Results.We find no difference in the large-scale environment of Sy1 and Sy2 galaxies; however, a larger fraction of Sy2 (~) than Sy1 (~) is found in systems that are smaller than  kpc, mainly in low-density environments (pairs or triplets). For comparison, this fraction is ~ for star-forming galaxies and ~ for normal galaxies.

Conclusions.We find no evidence of 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).