Gravitational Stability of Circumnuclear Disks in Elliptical Galaxies

TL;DR

This study investigates the gravitational stability of circumnuclear gas disks in elliptical galaxies, revealing that galaxy mass, black hole mass, and stellar density profiles influence disk stability and star formation activity.

Contribution

It provides a detailed analysis of how black hole mass and stellar density profiles affect the gravitational stability of CN disks in ellipticals, explaining observed star formation trends.

Findings

Higher mass galaxies have more stable CN disks due to larger black holes.

Cored stellar profiles lead to more unstable CN disks in massive ellipticals.

Black hole mass and stellar density profile jointly determine disk stability.

Abstract

A significant fraction of nearby elliptical galaxies are known to have high density gas disks in their circumnuclear (CN) region (0.1 to a few kpc). Yet, ellipticals, especially luminous ones, show little signs of recent star formation (SF). To investigate the possible cause of the dearth of SF in these systems, we study the gravitational stability of CN gas disks embedded within the potentials of both the stellar bulge and the central massive black hole (BH) in ellipticals. We find that CN disks in higher mass galaxies are generally more stable than those in lower mass galaxies, because higher mass galaxies tend to have more massive BHs and more centrally concentrated stellar density profiles. We also consider the case in which the central stellar density profile has a core, which is often observed for ellipticals whose total stellar mass is higher than about 10^11 Msun. Such a cored stellar density profile leads to more unstable CN disks than the power-law density profile characteristic of less massive galaxies. However, the more massive BHs in high-mass galaxies act to stabilize the CN disk. Our results demonstrate that the gravitational potentials of both the central BH and the stellar component should be taken into account when studying the properties of CN disks, as their stability is sensitive to both the BH mass and the stellar density profile. Our results could explain the observed trend that less luminous ellipticals have a greater tendency to exhibit ongoing SF than giant ellipticals.