2007MNRAS.377..179B

We study the distant red galaxy (DRG; J - K_s > 2.3) neighbour population of quasi-stellar objects (QSOs) selected from the Sloan Digital Sky Survey (SDSS) in the redshift range 1 ≲ z ≲ 2. We perform a similar analysis for optically obscured active galactic nuclei (AGNs; i.e. with a limiting magnitude I > 24) detected in the mid-infrared (24 µm) with the Spitzer Space Telescope and a mean redshift z ∼ 2.2 in the Flamingos Extragalactic Survey (FLAMEX). Both QSOs and obscured AGN target samples cover 4.7 deg² in the same region of the sky. We find a significant difference in the environment of these two target samples. Neighbouring galaxies close to QSOs tend to be bluer than galaxies in optically obscured source environments. We also present results on the cross-correlation function of DRGs around QSOs and optically faint mid-infrared sources. The corresponding correlation length obtained for the QSO sample targets is r₀= 5.4±1.6 Mpc h^–1 and a slope of γ = 1.94±0.10. For the optically obscured galaxy sample, we find r₀= 8.9±1.4 Mpc h^–1 and a slope of γ = 2.27±0.20. These results indicate that optically faint obscured sources are located in denser environment of evolved red galaxies compared to QSOs.