Bondi accretion in the finite luminous region of elliptical galaxies

TL;DR

This paper extends the classical Bondi accretion model to finite regions in elliptical galaxies, analyzing how boundary conditions, galaxy gravity, and radiation influence accretion rates onto central black holes.

Contribution

It introduces a finite boundary condition in Bondi accretion and examines the effects of galaxy gravity and radiation on accretion rates in elliptical galaxies.

Findings

Final radius kinetic energy is negligible.

Accretion rate approximates Bondi value when boundary is 2-3 orders larger than semi-Bondi radius.

Maximum accretion rate increases with galaxy density and decreases with radiation.

Abstract

The classical Bondi model is adopted to study accretion onto the finite luminous region around the central massive black hole (MBH) in an elliptical galaxy. Unlike Bondi (1952), we define the boundary conditions at a certain finite radius ($r_f$) instead of at the infinity and examine the variation of solutions for a simple case. In the following, we consider the special case of an MBH at the center of a Hernquist galaxy and involve the gravity and luminosity of its own galaxy. Our results in the first part show that kinetic energy at the final radius is ignorable even for not so far away from the center. Moreover, the mass accretion rate will be approximately equal to its Bondi value if the final radius ($r_f$) becomes about 2-3 orders of magnitude larger than semi-Bondi radius, i.e. $GM/c_{sf}^2$ (where $M$ and $c_{sf}$ are the mass of the central object and the sound speed at $r_f$). In the second part, adding the two extra forces of gravity and radiation in the momentum equation let us know that the maximum possible of accretion rate increases with a greater characteristic linear density of galaxy and lower radiation. e.g.: