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

TL;DR

This study compares Mg II absorption systems in flat-spectrum radio quasars and blazars, finding that the excess absorption in blazars is likely due to jet alignment rather than ejection of gas by jets.

Contribution

It provides the first detailed comparison of Mg II absorption incidence between flat-spectrum radio quasars and blazars, challenging previous assumptions about jet ejection effects.

Findings

Mg II absorption incidence in FSRQs is similar to that in QSOs.

Excess Mg II systems in blazars are mainly at beta< 0.15.

Jet alignment likely causes the observed excess in blazars.

Abstract

The conventional wisdom that the rate of incidence of Mg II absorption systems, dN/dz (excluding `associated systems' having velocity beta*c relative to the AGN of less than ~5000 km/s) is totally independent of the background AGN, has been challenged by a recent finding that dN/dz for strong Mg II absorption systems towards distant blazars is 2.2 \pm_{0.6}^{0.8} 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 beta 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) which too possess powerful jets, but are only weakly polarized. We show, for the first time, that dN/dz towards FSRQs is, on the whole, quite similar to that known for QSOs and the comparative excess of strong \mgii absorption systems seen towards blazars is mainly confined to beta< 0.15. The excess relative to FSRQs can probably result from a likely closer alignment of blazar jets with our direction and hence any gas clouds accelerated by them are more likely to be on the line of sight to the active quasar nucleus.