A Stellar Dynamical Measurement of the Black Hole Mass in the Maser Galaxy NGC 4258

TL;DR

This paper measures the black hole mass in NGC 4258 using stellar dynamical models constrained by Hubble Space Telescope data, achieving results consistent with maser-based measurements and validating stellar dynamical methods.

Contribution

It provides a stellar dynamical mass estimate of the black hole in NGC 4258 that closely agrees with maser-based measurements, supporting the reliability of stellar dynamical techniques.

Findings

Black hole mass estimated at (3.3 ± 0.2) × 10^7 solar masses

Stellar dynamical mass within 15% of maser-based mass

Validation of stellar dynamical methods for black hole mass measurement

Abstract

We determine the mass of the black hole at the center of the spiral galaxy NGC 4258 by constructing axisymmetric dynamical models of the galaxy. These models are constrained by high spatial resolution imaging and long-slit spectroscopy of the nuclear region obtained with the {\em Hubble Space Telescope}, complemented by ground-based observations extending to larger radii. Our best mass estimate is $\MBH = (3.3 \pm 0.2) \times 10^7 \MSun $ for a distance of 7.28 Mpc (statistical errors only). This is within 15% of $ (3.82\pm 0.01) \times 10^7 \MSun$, the mass determined from the kinematics of water masers (rescaled to the same distance) assuming they are in Keplerian rotation in a warped disk. The construction of accurate dynamical models of NGC 4258 is somewhat compromised by an unresolved active nucleus and color gradients, the latter caused by variations in the stellar population and/or obscuring dust. These problems are not present in the $\sim 30$ other black hole mass determinations from stellar dynamics that have been published by us and other groups; thus, the relatively close agreement between the stellar dynamical mass and the maser mass in NGC 4258 enhances our confidence in the black hole masses determined in other galaxies from stellar dynamics using similar methods and data of comparable quality.