On the polytropic Bondi accretion in two-component galaxy models with a central massive BH

TL;DR

This paper extends the classical Bondi accretion theory to a polytropic, two-component galaxy model with a central black hole, providing analytical and numerical solutions relevant for simulations and observations.

Contribution

It introduces a new class of galaxy models with analytical stellar profiles and explores polytropic Bondi accretion solutions, including thermodynamics, in these models.

Findings

Analytical solutions for isothermal and adiabatic cases.

Numerical exploration of non-adiabatic accretion.

Expressions for heat exchange with the environment.

Abstract

In many investigations involving accretion on a central point mass, ranging from observational studies to cosmological simulations, including semi-analytical modelling, the classical Bondi accretion theory is the standard tool widely adopted. Previous works generalised the theory to include the effects of the gravitational field of the galaxy hosting a central black hole, and of electron scattering in the optically thin limit. Here we apply this extended Bondi problem, in the general polytropic case, to a class of new two-component galaxy models recently presented. In these models, a Jaffe stellar density profile is embedded in a dark matter halo such that the total density distribution follows a $r^{-3}$ profile at large radii; the stellar dynamical quantities can be expressed in a fully analytical way. The hydrodynamical properties of the flow are set by imposing that the gas temperature at infinity is proportional to the virial temperature of the stellar component. The isothermal and adiabatic (monoatomic) cases can be solved analytically, in the other cases we explore the accretion solution numerically. As non-adiabatic accretion inevitably leads to an exchange of heat with the ambient, we also discuss some important thermodynamical properties of the polytropic Bondi accretion, and provide the expressions needed to compute the amount of heat exchanged with the environment, as a function of radius. The results can be useful for the subgrid treatment of accretion in numerical simulations, as well as for the interpretation of observational data.