Properties of galaxies in SDSS Quasar environments at z < 0 . 2

We analyse the environment of low redshift, z < 0.2, SDSS quasars using the spectral and photometric information of galaxies from the Sloan Digital Sky Survey Third Data Release (SDSS-DR3). We compare quasar neighbourhoods with field and high density environments through an analysis on samples of typical galaxies and groups. We compute the surrounding surface number density of galaxies finding that quasar environments systematically avoid high density regions. Their mean environments correspond to galaxy density enhancements similar to those of typical galaxies. We have also explored several galaxy properties in these environments, such as spectral types, specific star formation rates, concentration indexes, colours and active nuclei activity. We find a higher relative fraction of blue galaxies in quasar environments compared to groups and typical galaxy neighbourhoods. Consistent with this picture, the distribution of the concentration index of these galaxies also indicate a larger fraction of late-type objects. By analysing the available information of galaxy spectra we have also studied the distribution of the star formation rates of these neighbour galaxies finding that quasar environments are populated by objects with an enhanced star formation activity. An analysis of the relative flux ratios of [OIII]λ5700/Hβ and [NII]λ6583/Hα of emission-line galaxies shows no an excess of nuclei activity in quasar neighborhood with respect to the environment of a typical galaxy. We conclude that low redshift quasar neighbourhoods (rp < 1h Mpc, ∆V < 500kms) are populated by bluer and more intense star forming galaxies of disktype morphology than galaxies in groups and in the field. Although star formation activity is thought to be significantly triggered by interactions, we find that quasar fueling may not require the presence of a close companion galaxy (rp < 100h kpc, ∆V < 350kms). As a test of the unified AGN model, we have performed a similar analysis to the neighbours of a sample of active galaxies. The results indicate that these neighbourhoods are comparable to those of quasars giving further support to this unified scenario.