The NGC 404 Nucleus: Star Cluster and Possible Intermediate Mass Black Hole

TL;DR

This study investigates the nucleus of galaxy NGC 404, revealing a complex structure with a nuclear star cluster, distinct stellar populations, and evidence suggesting a possible intermediate mass black hole, using advanced spectroscopic and imaging techniques.

Contribution

It provides detailed kinematic and stellar population analysis of NGC 404's nucleus, and offers new constraints on the mass of a potential intermediate black hole in a nearby galaxy.

Findings

Nuclear star cluster mass estimated at 1.1x10^7 Msol.

Possible black hole mass constrained to <1x10^5 Msol or 4.5x10^5 Msol.

Evidence of a nuclear disk with orthogonal rotation and distinct stellar populations.

Abstract

We examine the nuclear morphology, kinematics, and stellar populations in nearby S0 galaxy NGC 404 using a combination of adaptive optics assisted near-IR integral-field spectroscopy, optical spectroscopy, and HST imaging. These observations enable study of the NGC 404 nucleus at a level of detail possible only in the nearest galaxies. The surface brightness profile suggests the presence of three components, a bulge, a nuclear star cluster, and a central light excess within the cluster at radii <3 pc. These components have distinct kinematics with modest rotation seen in the nuclear star cluster and counter-rotation seen in the central excess. Molecular hydrogen emission traces a disk with rotation nearly orthogonal to that of the stars. The stellar populations of the three components are also distinct, with half of the mass of the nuclear star cluster having ages of ~1 Gyr (perhaps resulting from a galaxy merger), while the bulge is dominated by much older stars. Dynamical modeling of the stellar kinematics gives a total nuclear star cluster mass of 1.1x10^7 Msol. Dynamical detection of a possible intermediate mass black hole is hindered by uncertainties in the central stellar mass profile. Assuming a constant mass-to-light ratio, the stellar dynamical modeling suggests a black hole mass of <1x10^5 Msol, while the molecular hydrogen gas kinematics are best fit by a black hole with mass of 4.5x10^5 Msol. Unresolved and possibly variable dust emission in the near-infrared and AGN-like molecular hydrogen emission line ratios do suggest the presence of an accreting black hole in this nearby LINER galaxy.