Tidally disrupted dusty clumps as the origin of broad emission lines in active galactic nuclei

TL;DR

This paper proposes that tidally disrupted dusty clumps near supermassive black holes can explain the origin of broad emission lines in active galactic nuclei, matching observed spectral profiles.

Contribution

It introduces a novel model where disrupted dusty clumps form the broad-line region, supported by dynamical calculations and spectral profile comparisons.

Findings

Profiles of emission lines match observations of Palomar-Green quasars.

Asymmetry and shifts in line profiles depend on [O III] luminosity.

Tidally disrupted clumps can serve as the broad-line region source.

Abstract

Type 1 active galactic nuclei display broad emission lines, regarded as arising from photoionized gas moving in the gravitational potential of a supermassive black hole. The origin of this broad-line region gas is unresolved so far, however. Another component is the dusty torus beyond the broad-line region, likely an assembly of discrete clumps that can hide the region from some viewing angles and make them observationally appear as Type 2 objects. Here we report that these clumps moving within the dust sublimation radius, like the molecular cloud G2 discovered in the Galactic center, will be tidally disrupted by the hole, resulting in some gas becoming bound at smaller radii while other gas is ejected and returns to the torus. The clumps fulfill necessary conditions to be photoionized. Specific dynamical components of tidally disrupted clumps include spiral-in gas as inflow, circularized gas, and ejecta as outflow. We calculate various profiles of emission lines from these clouds, and find they generally agree with H$\beta$ profiles of Palomar-Green quasars. We find that asymmetry, shape and shift of the profiles strongly depend on [O III], luminosity, which we interpret as a proxy of dusty torus angles. Tidally disrupted clumps from the torus may represent the source of the broad-line region gas.