The environments of quasars have been studied in the past on widely different scales ranging all the way from host galaxies to Mpc scales. Very early studies have shown that typical quasar environments have galaxy densities comparable to galaxy groups or poor clusters. At Mpc scales, comparing the environments of quasars to those of galaxies has given conflicting results. Early studies on Mpc scales suggest that quasars are more strongly clustered than galaxies, while later studies based on surveys such as the Two Degree Field (2dF) and the Sloan Digital Sky Survey (SDSS) have found the environmental galaxy densities of quasars and galaxies to be comparable to each other
The quasar sample was extracted from the fifth release of the SDSS Quasar Catalog (Schneider et al. 2010), which uses data from the seventh SDSS data release (Abazajian et al. 2009). The catalog consists of quasars fainter than i ∼ 15.0, with an absolute magnitude of Mi <-22.0 and a reliably measured redshift. Additionally the quasars either have at least one emission line with FWHM>1000 km/ sec, or show complex/interesting absorption lines. The full catalog con- tains ∼106.000 spectroscopically confirmed quasars. In this work we only used objects in the Stripe 82 (Annis et al. 2011) region.
Low redshift quasars in the Stripe82: The local environments
Karhunen K, Kotilainen J.K.,, Falomo, R. , Bettoni, 2014, MNRAS 441, 1802
We used quasars that were imaged in the SDSS Stripe82 that is up to 2 mag deeper than standard Sloan images. For 52 quasars we undertake a study of the color of the host galaxies in u, g, r, i and z bands. This sample is part of a larger dataset of ∼400 quasars at z < 0.5 for which both the host galaxies and their galaxy environments were studied (Falomo et al. 2014; Karhunen et al. 2014).
The mean colors of the QSO host galaxy (g−i=0.82±0.26; r−i=0.26±0.16 and u−g=1.32±0.25) are very similar to the values of a sample of inactive galaxies matched in terms of redshift and galaxy luminosity with the quasar sample.
Low redshift quasars in the Stripe82: The host galaxy colors and close environment.
Bettoni, D., Falomo, R. , Kotilainen J.K., Karhunen K, and Uslenghi, M. 2015, MNRAS ,in press (arXiv:1509.07605)