Improved Dynamical Constraints on the Mass of the central Black Hole in NGC 404

TL;DR

This study uses HST data and stellar dynamical modeling to constrain the black hole mass in NGC 404, establishing it as the lowest-mass galaxy with such dynamical limits, and finds evidence of nuclear variability and complex stellar populations.

Contribution

It provides the first dynamical upper limit on the black hole mass in NGC 404, incorporating stellar population modeling and addressing gas kinematics complexities.

Findings

Black hole mass upper limit of 1.5×10^5 M_sun at 3σ confidence

Detection of nuclear variability over 15 years in multiple wavelengths

Identification of a 1 Gyr old, counter-rotating stellar population in the nucleus

Abstract

We explore the nucleus of the nearby 10$^9M_{\odot}$~early-type galaxy (ETGs), NGC~404, using Hubble Space Telescope (HST)/STIS spectroscopy and WFC3 imaging. We first present evidence for nuclear variability in UV, optical, and infrared filters over a time period of 15~years. This variability adds to the already substantial evidence for an accreting black hole at the center of NGC~404. We then redetermine the dynamical black hole mass in NGC~404 including modeling of the nuclear stellar populations. We combine HST/STIS spectroscopy with WFC3 images to create a local color-$M/L$~relation derived from stellar population modeling of the STIS data. We then use this to create a mass model for the nuclear region. We use Jeans modeling to fit this mass model to adaptive optics (AO) stellar kinematic observations from Gemini/NIFS. From our stellar dynamical modeling, we find a 3$\sigma$ upper limit on the black hole mass of $1.5\times10^5M_{\odot}$. Given the accretion evidence for a black hole, this upper limit makes NGC~404 the lowest mass central black hole with dynamical mass constraints. We find that the kinematics of H$_2$ emission line gas show evidence for non-gravitational motions preventing the use of gas dynamical modeling to constrain the black hole mass. Our stellar population modeling also reveals that the central, counter-rotating region of the nuclear cluster is dominated by $\sim$1~Gyr old populations.