Revealing the intermediate-mass black hole at the heart of the dwarf galaxy NGC 404 with sub-parsec resolution ALMA observations

TL;DR

This study uses high-resolution ALMA observations to measure the mass of the intermediate-mass black hole in NGC 404, revealing a ~5x10^5 solar mass black hole through detailed molecular gas and stellar kinematic analysis.

Contribution

It provides the first high-resolution molecular gas kinematic measurement of the black hole mass in NGC 404, resolving previous discrepancies and emphasizing the importance of accounting for molecular gas in such estimates.

Findings

Black hole mass estimated at approximately 5.5 x 10^5 solar masses.

High-resolution ALMA observations reveal a rotating molecular gas disc around the black hole.

Consistent results between molecular gas and stellar kinematic measurements.

Abstract

We estimate the mass of the intermediate-mass black hole at the heart of the dwarf elliptical galaxy NGC 404 using Atacama Large Millimeter/submillimeter Array (ALMA) observations of the molecular interstellar medium at an unprecedented linear resolution of ~0.5 pc, in combination with existing stellar kinematic information. These ALMA observations reveal a central disc/torus of molecular gas clearly rotating around the black hole. This disc is surrounded by a morphologically and kinematically complex flocculent distribution of molecular clouds, that we resolve in detail. Continuum emission is detected from the central parts of NGC 404, likely arising from the Rayleigh-Jeans tail of emission from dust around the nucleus, and potentially from dusty massive star-forming clumps at discrete locations in the disc. Several dynamical measurements of the black hole mass in this system have been made in the past, but they do not agree. We show here that both the observed molecular gas and stellar kinematics independently require a ~5x10$^5$ Msun black hole once we include the contribution of the molecular gas to the potential. Our best estimate comes from the high-resolution molecular gas kinematics, suggesting the black hole mass of this system is 5.5$^{+4.1}_{-3.8}\times$10$^5$ Msun (at the 99% confidence level), in good agreement with our revised stellar kinematic measurement and broadly consistent with extrapolations from the black hole mass - velocity dispersion and black hole mass - bulge mass relations. This highlights the need to accurately determine the mass and distribution of each dynamically important component around intermediate-mass black holes when attempting to estimate their masses.