A whole sky study of quasars known population starting from the LQAC-2 compiled catalogue

Observatoire de Paris, SYRTE, CNRS/UMR 8630, 75014 Paris, France
e-mail: cesar.gattano@obspm.fr

Received: 12 December 2013
Accepted: 22 February 2014

Abstract

Context. Thanks to huge surveys, such as the Sloan Digital Sky Survey (SDSS), the last decade has shown a dramatic increase in the number of known quasars. In the second release of the general compiled catalogue Large Quasar Astrometric Catalogue (LQAC), 187 504 objects are recorded.

Aims. From this catalogue, we carry out statistical studies dealing with several topics: the astrometric accuracy of the quasars, their spatial location, the distribution of the distance to the closest neighbour, the identification of binary quasars, the completness of catalogues at a given magnitude and the estimation of the number of quasars expected to be detected by the astrometric space mission Gaia.

Methods. We analyse the astrometric improvements brought by the LQAC-2 in terms of equatorial coordinates off-sets. We plot the bi-dimensional spatial distribution of the LQAC-2 quasars according to their equatorial, galactic, and ecliptic coordinates, thus exploring the anisotropy of the distribution. We compare the observed distribution of closest neighbours with the theoretical values based on a Poisson distribution. Moreover, we perform a comparison between two catalogues, the SDSS and the 2dF inside a huge common field. By extrapolating to the whole sky we deduce the number of quasars that will be detected by Gaia.

Results. We show how the equatorial, ecliptic, and galactic distribution of recorded quasars is strongly affected by the galactic extinction as well as by the deficiency of detections in the southern hemisphere. In homogenous zones covered by the SDSS survey we identify a significant excess of closest neighbours at short angular distances, with respect to the theoretical estimation, which is caused by the presence of binary quasars. Moreover, we detail the incompletness of systematic survey catalogues at any magnitude threshold, when comparing two huge surveys such as the SDSS and 2dF. Following this study we deduce the number of quasars detected by Gaia under a given magnitude threshold. For V = 20, this number should be at least 1 million objects.