Thermal and dynamical properties of gas accreting onto a supermassive black hole in an AGN

TL;DR

This study investigates the thermal and dynamical stability of gas accretion onto supermassive black holes in AGNs, revealing the transition from smooth to two-phase flows and the formation of cold filaments, with implications for AGN feedback.

Contribution

It provides high-resolution grid-based simulations of gas accretion in AGNs, demonstrating the development of cold filaments and their evolution, extending previous SPH work with improved spatial resolution.

Findings

Gas becomes thermally and convectively unstable at certain X-ray luminosities.

Cold filaments form initially as long structures and later break into smaller clouds.

Cold phase accretion can dominate or coexist with hot phase depending on luminosity.

Abstract

(Abridged) We study stability of gas accretion in Active Galactic Nuclei. Our grid based simulations cover a radial range from 0.1 to 200 pc. Here, as in previous studies by our group, we include gas radiative cooling as well as heating by a sub-Eddington X-ray source near the central supermassive black hole of 10^8 M_{\odot}. Our theoretical estimates and simulations show that for the X-ray luminosity L_X \sim 0.008 L_{Edd}, the gas is thermally and convectivelly unstable within the computational domain. In the simulations, we observe that very tiny fluctuations in an initially smooth, spherically symmetric, accretion flow, grow first linearly and then non-linearly. Consequently, an initially one-phase flow relatively quickly transitions into a two-phase/cold-hot accretion flow. For L_X = 0.015 L_{Edd} or higher, the cold clouds continue to accrete but in some regions of the hot phase, the gas starts to move outward. For L_X < 0.015 L_{Edd}, the cold phase contribution to the total mass accretion rate only moderately dominates over the hot phase contribution. This result might have some consequences for cosmological simulations of the so-called AGN feedback problem. Our simulations confirm the previous results of Barai et al. (2012) who used smoothed particle hydrodynamic simulations to tackle the same problem. However here, because we use a grid based code to solve equations in 1-D and 2-D, we are able to follow the gas dynamics at much higher spacial resolution and for longer time in comparison to the 3-D SPH simulations. One of new features revealed by our simulations is that the cold condensations in the accretion flow initially form long filaments, but at the later times, those filaments may break into smaller clouds advected outwards within the hot outflow. These simulations may serve as an attractive model for the so-called Narrow Line Region in AGN.