A Stellar Dynamical Mass Measurement of the Black Hole in NGC 3998 from Keck Adaptive Optics Observations

TL;DR

This paper measures the mass of the black hole in NGC 3998 using stellar dynamical modeling with adaptive optics and space telescope data, providing a precise estimate that differs from previous gas-based measurements.

Contribution

It presents a new stellar dynamical mass measurement of the black hole in NGC 3998, combining multiple high-resolution observations and advanced orbit-based models, and compares it with gas dynamical results.

Findings

Black hole mass: (8.1_{-1.9}^{+2.0}) x 10^8 M_sun

Stellar mass-to-light ratio: 5.0_{-0.4}^{+0.3} M_sun/L_sun

Stellar dynamical mass is nearly four times larger than previous gas dynamical estimate

Abstract

We present a new stellar dynamical mass measurement of the black hole in the nearby, S0 galaxy NGC 3998. By combining laser guide star adaptive optics observations obtained with the OH-Suppressing Infrared Imaging Spectrograph on the Keck II telescope with long-slit spectroscopy from the Hubble Space Telescope and the Keck I telescope, we map out the stellar kinematics on both small spatial scales, well within the black hole sphere of influence, and on large scales. We find that the galaxy is rapidly rotating and exhibits a sharp central peak in the velocity dispersion. Using the kinematics and the stellar luminosity density derived from imaging observations, we construct three-integral, orbit-based, triaxial stellar dynamical models. We find the black hole has a mass of M_BH = (8.1_{-1.9}^{+2.0}) x 10^8 M_sun, with an I-band stellar mass-to-light ratio of M/L = 5.0_{-0.4}^{+0.3} M_sun/L_sun (3-sigma uncertainties), and that the intrinsic shape of the galaxy is very round, but oblate. With the work presented here, NGC 3998 is now one of a very small number of galaxies for which both stellar and gas dynamical modeling have been used to measure the mass of the black hole. The stellar dynamical mass is nearly a factor of four larger than the previous gas dynamical black hole mass measurement. Given that this cross-check has so far only been attempted on a few galaxies with mixed results, carrying out similar studies in other objects is essential for quantifying the magnitude and distribution of the cosmic scatter in the black hole mass - host galaxy relations.