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 _ { \mathrm { BH } } = ( 8.1 _ { -1.9 } ^ { +2.0 } ) \times 10 ^ { 8 } M _ { \odot } , with an I -band stellar mass-to-light ratio of M / L = 5.0 _ { -0.4 } ^ { +0.3 } M _ { \odot } / L _ { \odot } ( 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 .