Estimating masses of Keplerian disk systems: the case of AGN in NGC 4258

TL;DR

This paper develops a formula linking disk and central masses in AGN with Keplerian disks, and applies it to NGC 4258 to confirm the black hole's dominant role in disk rotation.

Contribution

It introduces a phenomenological formula relating disk and central masses in Keplerian accretion disks and applies it to real data from NGC 4258.

Findings

The central black hole dominates the disk rotation in NGC 4258.

The accretion disk mass exceeds 100 solar masses.

The formula accurately estimates the mass ratio in Keplerian systems.

Abstract

The Keplerian motion of accretion disks in Active Galactic Nuclei (AGN) is usually believed to be generated by a heavy central mass. We investigate accreting disk systems with polytropic gas in Keplerian rotation and obtain a phenomenological formula that relates the Keplerian angular frequency to the ratio of disk and central masses. Central mass approaches the Keplerian value, if the inner boundary of a disk is close to the minimal stable orbit of a black hole. These results are applied to NGC 4258, the unique AGN with a finely measured Keplerian rotation curve of the central disk, with the conclusion that its rotation curve is, in fact, determined by the central black hole. The mass of the accretion disk exceeds 100 solar masses.