Incidence of Mg II absorption systems toward flat-spectrum radio quasars

Abstract

The conventional wisdom that the rate of incidence of Mgii absorption systems, dN/dz (excluding “associated systems” having a velocity βc relative to the active galactic nucleus (AGN) of less than ∼5000 km s⁻¹.), is totally independent of the backgroundAGNs has been challenged by a recent finding that dN/dz for strongMgii absorption systems toward distant blazars is 2.2±⁰•⁸₀.₆ times the value known for normal optically selected quasars (QSOs). This has led to the suggestion that a significant fraction of even the absorption systems with β as high as ∼0.1 may have been ejected by the relativistic jets in the blazars, which are expected to be pointed close to our direction. Here, we investigate this scenario using a large sample of 115 flat-spectrum radio-loud quasars (FSRQs) that also possess powerful jets, but are only weakly polarized. We show, for the first time, that dN/dz toward FSRQs is, on the whole, quite similar to that known for QSOs and that the comparative excess of strong MgII absorption systems seen toward blazars is mainly confined to β < 0.15. The excess relative to FSRQs probably results from a likely closer alignment of blazar jets with our direction; hence, any gas clouds accelerated by them are more likely to be on the line of sight to the active quasar nucleus.