Constraining FeLoBAL outflows from absorption line variability

TL;DR

This study investigates the physical properties of FeLoBAL quasar outflows through multi-epoch absorption line variability, revealing that the outflows are located within tens of parsecs from the central black hole and exhibit transverse motions.

Contribution

First variability-based constraints on FeLoBAL outflow distances, indicating their location within tens of parsecs and supporting transverse motion as the cause of variability.

Findings

Absorption line variability detected in 3 out of 12 quasars.

Outflow distances constrained to less than 69, 7, and 60 parsecs.

Transverse motions of gas are favored as the variability mechanism.

Abstract

FeLoBALs are a rare class of quasar outflows with low-ionization broad absorption lines (BALs), large column densities, and potentially large kinetic energies that might be important for `feedback' to galaxy evolution. In order to probe the physical properties of these outflows, we conducted a multiple-epoch, absorption line variability study of 12 FeLoBAL quasars spanning a redshift range between 0.7 and 1.9 over rest frame time-scales of approximately 10 d to 7.6 yr. We detect absorption line variability with greater than 8 sigma confidence in 3 out of the 12 sources in our sample over time-scales of 0.6 to 7.6 yr. Variable wavelength intervals are associated with ground and excited state Fe II multiplets, the Mg II 2796, 2803 doublet, Mg I 2852, and excited state Ni II multiplets. The observed variability along with evidence of saturation in the absorption lines favors transverse motions of gas across the line of sight (LOS) as the preferred scenario, and allows us to constrain the outflow distance from the supermassive black hole (SMBH) to be less than 69, 7, and 60 pc for our three variable sources. In combination with other studies, these results suggest that the outflowing gas in FeLoBAL quasars resides on a range of scales and includes matter within tens of parsecs of the central source.