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

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× 107 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 easily 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.

A New Hydrodynamic Spherical Accretion Exact Solution and Its Quasi-spherical Perturbations

“…Regarding a first quantitative comparison to numerical experiments, Sun & Yang (2021) model the hydrodynamical accretion of optically transparent hot gas onto a central black hole for low luminosity AGNs, assuming spherical symmetry and an adiabatic index of γ = 5/3, and obtain density profiles consistent with the expectations of the new model presented here, see their Figure 7. Along these lines, in Section 5 we present the first numerical simulations of the new spherical solution presented here, showing it to be stable.…”

“…Similarly, other physical processes at play in the actual observed cases include the nonadiabatic character of the gas flows. Fortunately, as shown by, e.g., Sun & Yang (2021), given the strong density dependences of both heating (from irradiation by a central X-ray emission) and cooling rates, this lack of adiabaticity will again be confined to the very central regions, typically at parsec scales much smaller than the radial ranges fitted in Figure 4 and Appendix A.…”