Dynamical and thermal properties of the parsec-scale gases spherically accreted onto low luminous active galactic nuclei

TL;DR

This paper analytically investigates the dynamical and thermal behavior of parsec-scale, optically-thin gases accreting onto low luminous active galactic nuclei, highlighting how radiation and galaxy potential influence stability and accretion rates.

Contribution

It provides a new analytical framework accounting for radiative heating, cooling, and stellar potential, revealing differences from the classical Bondi accretion model.

Findings

Bondi model underestimates accretion rates

Thermal stability boundary depends on Compton temperature

Higher Compton temperature promotes thermal instability

Abstract

We analytically study the dynamical and thermal properties of the optically-thin gases at the parsec-scale when they are spherically accreted onto low luminous active galactic nuclei (LLAGNs). The falling gases are irradiated by the central X-ray radiation with the Compton temperature of 5--15$\times10^7$ K. The radiative heating/cooling and the bulge stellar potential in galaxies are taken into account. We analyze the effect of accretion rate, luminosity, gas temperature, and Compton temperature on steady solutions of dynamical and thermal properties. The steady solutions are obviously different from Bondi solution. Compared to our models, the Bondi model underestimates the accretion rate. We give the boundary between thermal stability and instability. The boundary is significantly affected by Compton temperature. When Compton temperature is higher, the falling gases tend to become thermally unstable. When thermal instability takes place in the irradiated gases, the gases become two phases (i.e. hot gases and cool gases) and the hot gases may become outflows. This effect may reduce the accretion rates.