Incidence of Mg ii absorbers towards blazars and the GRB/QSO puzzle

摘要

In order to investigate the origin of the excess of strong Mg ii systems towards GRB afterglows as compared to QSO sightlines, we have measured the incidence of Mg ii absorbers towards a third class of objects: the blazars. This class includes the BL Lac object population for which a tentative excess of Mg ii systems had already been reported. We observed with FORS1 at the ESO-VLT 42 blazars with an emission redshift 0.8 z 1.9, to which we added the three high z northern objects belonging to the 1 Jy BL Lac sample. We detect 32 Mg ii absorbers in the redshift range 0.35-1.45, leading to an excess in the incidence of Mg ii absorbers compared to that measured towards QSOs by a factor ～2, detected at 3σ. The amplitude of the effect is similar to that found along GRB sightlines. Our analysis provides a new piece of evidence that the observed incidence of Mg ii absorbers might depend on the type of background source. In front of blazars, the excess is apparent for both "strong (w(2796) > 1.0) and weaker (0.3 (2796) < 1.0) Mg ii systems. The dependence on velocity separation with respect to the background blazars indicates, at the ～1.5σ level, a potential excess for β v/c ～ 0.1. We show that biases involving dust extinction or gravitational amplification are not likely to notably affect the incidence of Mg ii systems towards blazars. Finally we discuss the physical conditions required for these absorbers to be gas entrained by the powerful blazar jets. More realistic numerical modelling of jet-ambient gas interaction is required to reach any firm conclusions as well as repeat observations at high spectral resolution of strong Mg ii absorbers towards blazars in both high and low states.