Clumpy AGN outflows due to thermal instability

TL;DR

This paper presents the first numerical simulations of clumpy AGN outflows caused by thermal instability, revealing key physical conditions and effects that enable clump formation in parsec-scale winds.

Contribution

It introduces novel 1-D and 2-D simulations demonstrating thermal instability-induced clumpiness in AGN outflows, addressing previous modeling limitations.

Findings

Clumpiness arises when the pressure ionization parameter decreases along a streamline.

Insufficient heating, stretching effects, and flow speed influence TI development.

Fast flows can pass through the TI zone too quickly for clumps to form.

Abstract

One of the main mechanisms that could drive mass outflows on parsec scales in AGN is thermal driving. The same X-rays that ionize and heat the plasma are also expected to make it thermally unstable. Indeed, it has been proposed that the observed clumpiness in AGN winds is caused by thermal instability (TI). While many studies employing time-dependent numerical simulations of AGN outflows have included the necessary physics for TI, none have so far managed to produce clumpiness. Here we present the first such clumpy wind simulations in 1-D and 2-D, obtained by simulating parsec scale outflows irradiated by an AGN. By combining an analysis of our extensive parameter survey with physical arguments, we show that the lack of clumps in previous numerical models can be attributed to the following three effects: (i) insufficient radiative heating or other physical processes that prevent the outflowing gas from entering the TI zone; (ii) the stabilizing effect of stretching (due to rapid radial acceleration) in cases where the gas enters the TI zone; and (iii) a flow speed effect: in circumstances where stretching is inefficient, the flow can still be so fast that it passes through the TI zone too quickly for perturbations to grow. Besides these considerations, we also find that a necessary condition to trigger TI in an outflow is for the pressure ionization parameter to decrease along a streamline once gas enters a TI zone.