On The Fine Tuning and Physical Origin of Line-Locked Absorption Systems in Active Galaxies

TL;DR

This paper investigates line locking in quasar absorption systems, revealing its rarity, stability, and implications for quasar emission models, while challenging existing theories on the formation of narrow absorption lines.

Contribution

It provides a detailed analysis of the physical conditions for line locking in quasars and explores its implications for quasar outflows and models of the UV emission.

Findings

Line locking occurs in a small, finely-tuned phase space.

Line locking stability is affected by quasar luminosity variations.

High occurrence of line locking suggests underestimation of extreme-UV emission in models.

Abstract

Line locking (LL) of absorption line systems is a clear signature of the dynamical importance of radiation pressure force in driving astrophysical flows, with recent findings suggesting that it may be common in quasars exhibiting multiple intrinsic narrow absorption-line (NAL) systems. In this work we probe the phase space conducive to LL and follow the detailed kinematics of those systems that may lock at the velocity separation of the CIV $\lambda\lambda 1548.19,1550.77$ doublet. We find that a small volume of the phase-phase admits LL, suggesting a high-degree of fine-tuning between the physical properties of locked systems. The stability of LL against quasar luminosity variations is quantified with implications for the long-term variability amplitude of quasars and the velocity-separation statistic between multiple NAL systems. The high occurrence of LL by the CIV doublet implies that the hidden extreme-UV emission from quasars is unlikely to be significantly under-estimated by current models. Further, the ratio of the LL velocity to the outflow velocity may serve as a powerful constraint on the composition of the accelerating medium. We conclude that LL poses significant challenges to current theories for the formation of non-intervening NAL systems, and speculate that it may be a manifestation of expanding circumstellar shells around asymptotic giant branch (AGB) stars in the quasar-host bulge.