First tentative detection of anisotropy in the QSO distribution around nearby edge-on spiral galaxies

Instituto de Astrofísica de Canarias, C/.Vía Láctea, s/n, 38200 La Laguna (S/C de Tenerife), Spain e-mail: martinlc@iac.es

Received: 16 March 2006
Accepted: 13 September 2006

Abstract

Aims.To check whether the polar angle distribution of QSOs around nearby spiral galaxies is isotropic or not.

Methods.A statistical analysis of the polar angle distribution of large samples of QSOs from the SDSS survey and Monte Carlo simulations to calculate their significance are carried out.

Results.There is a clear excess of QSOs near the minor axis with respect to the major axis of nearby edge-on spiral galaxies, significant at a level 3.5 up to angular distances of ~ (or ∼1.7 Mpc) from the centre of each galaxy. The significance is increased to 3.9 with the  QSOs, and it reaches 4.8 if we include galaxies whose circles of radius 3 degrees are covered by the SDSS in more than 98% (instead of 100%) of the area.

Conclusions.Gravitational lensing in the halo of nearby galaxies or extinction seem insufficient to explain the observed anisotropic distribution of QSOs. The anisotropic distribution agrees qualitatively with the predictions of Arp's models, which claim that QSOs are ejected by galaxies along the rotation axis, although Arp's prediction give a distance of the QSOs ∼3 times smaller than that found here. In any case, a chance fluctuation, although highly improbable, might be a possibility rather than a true anisotropy, and the present results should be corroborated by other groups and samples, so we prefer to consider it as just a first tentative detection.