# Improved Dynamical Constraints on the Masses of the Central Black Holes in Nearby Low-mass Early-type Galactic Nuclei And the First Black Hole Determination for NGC 205

## TL;DR

This study refines black hole mass estimates in three nearby low-mass galaxies, notably detecting the smallest central black hole ever in NGC 205, using advanced dynamical modeling and stellar population analysis.

## Contribution

First detection of a black hole in NGC 205 with improved dynamical models, establishing the lowest mass black hole in a galaxy to date.

## Key findings

- NGC 205 hosts a black hole of approximately 6,800 solar masses.
- Black hole masses in NGC 5102 and NGC 5206 are consistent with previous estimates.
- Color--mass-to-light ratio relations vary significantly among nuclei, especially with young stellar populations.

## Abstract

We improve the dynamical black hole (BH) mass estimates in three nearby low-mass early-type galaxies--NGC 205, NGC 5102, and NGC 5206. We use new \hst/STIS spectroscopy to fit the star formation histories of the nuclei in these galaxies, and use these measurements to create local color--mass-to-light ratio (\ml) relations. We then create new mass models from \hst~imaging and combined with adaptive optics kinematics, we use Jeans dynamical models to constrain their BH masses. The masses of the central BHs in NGC 5102 and NGC 5206 are both below one million solar masses and are consistent with our previous estimates, $9.12_{-1.53}^{+1.84}\times10^5$\Msun~and $6.31_{-2.74}^{+1.06}\times10^5$\Msun~(3$\sigma$ errors), respectively. However, for NGC 205, the improved models suggest the presence of a BH for the first time, with a best-fit mass of $6.8_{-6.7}^{+95.6}\times10^3$\Msun~(3$\sigma$ errors). This is the least massive central BH mass in a galaxy detected using any method. We discuss the possible systematic errors of this measurement in detail. Using this BH mass, the existing upper limits of both X-ray, and radio emissions in the nucleus of NGC 205 suggest an accretion rate $\lesssim$$10^{-5}$ of the Eddington rate. We also discuss the color--\mleff~relations in our nuclei and find that the slopes of these vary significantly between nuclei. Nuclei with significant young stellar populations have steeper color--\mleff~relations than some previously published galaxy color--\mleff~relations.