The changing ISM of massive elliptical galaxies and cosmic evolution of radio galaxies and quasars

TL;DR

This paper investigates how the expansion of massive elliptical galaxies affects their interstellar medium (ISM) and the evolution of radio galaxies and quasars, highlighting changes in accretion modes and black hole spin over cosmic time.

Contribution

It provides a quantitative analysis of the ISM evolution in elliptical galaxies and links these changes to the shift from cold to hot accretion modes and black hole spin-down across cosmic epochs.

Findings

ISM density and pressure decrease with cosmic time.

Shift from cold disk to hot ADAF accretion mode occurs at low redshifts.

Black hole spin-down timescale is approximately 0.2 Gyr for a $10^9 M_\\odot$ black hole.

Abstract

The recently discovered apparent dramatic expansion in the effective radii of massive elliptical galaxies from $z \simeq 2$ to $z \simeq 0.1$ has been interpreted in terms of either galaxy mergers or the rapid loss of cold gas due to AGN feedback. In examining the latter case we have quantified the extent of the expansion, which is uncertain observationally, in terms of the star formation parameters and time of the expulsion of the cold gas. In either case, the large global decrease in stellar density should translate into a major drop in the ISM density and pressure, and a much steeper radial decline in those quantities with cosmic epoch. These cosmological changes are expected to have a major influence on the gas accretion mode, which will shift from `cold' thin disk accretion at high redshifts toward `hot' Bondi fed ADAF accretion at low redshifts. The decline of angular momentum inflow would then lead to a spin down of the black hole, for which we have calculated more precise time scales; a value of about 0.2 Gyr is typical for a $10^9 M_\odot$ central black hole. These results have implications for the different cosmological evolutionary patterns found for the luminosity functions of powerful and weak radio galaxies.