Angular and spatial clustering of photometrically classified quasar
  candidates from SDSS NBCKDE
  The present paper analyses the quasar clustering using the two-point
correlation function (2pCF) and the largest existing sample of photometrically
selected quasars: the SDSS NBCKDE catalogue (from the SDSS DR6). A new
technique of random catalogue generation was developed for this purpose, that
allows to take into account the original homogeneity of the survey without
knowledge of its imaging mask. When averaged over photometrical redshifts
0.8<z_phot<2.2 the 2pCF of photometrically selected quasars is found to be
approximated well with the power law w(\theta)=(\theta/\theta_0)^{-\alpha} with
\theta_0=4".5+/-1".4, \alpha=0.94+/-0.06 over the range 1'<\theta<40'. It
agrees well with previous results by Myers et al. (2006,2007), obtained for
samples of NBCKDE quasars with similar mean z_phot, but averaged over broader
z_phot range. The parameters of the deprojected 2pCF averaged over the same
z_phot range and modelled with a power law \xi(r)=(r/r_0)^{-\gamma}, are
r_0=7.81^{+1.18}_{-1.16} Mpc/h, \gamma=1.94+/-0.06, which are in perfect
agreement with previous results from spectroscopic surveys. We confirm the
evidence for an increase of the clustering amplitude with z, and find no
evidence for luminosity dependence of the quasar clustering. The latter is
consistent with the models of the quasar formation, in which bright and faint
quasars are assumed to be similar sources, hosted by dark matter halos of
similar masses, but observed at different stages of their evolution. Comparison
of our results with studies of the X-ray selected AGNs with similar z shows
that the clustering amplitude of optically selected quasars is similar to that
of X-ray selected quasars, but lower than that of samples of all X-ray selected
AGNs. As the samples of all X-ray selected AGNs contain AGNs of both types, our
result serves as an evidence for different types of AGNs to reside in different
environments.
