Measuring the mass of the central black hole in the bulgeless galaxy NGC
  4395 from gas dynamical modeling
  NGC 4395 is a bulgeless spiral galaxy, harboring one of the nearest known
type 1 Seyfert nuclei. Although there is no consensus on the mass of its
central engine, several estimates suggest it to be one of the lightest massive
black holes (MBHs) known. We present the first direct dynamical measurement of
the mass of this MBH from a combination of two-dimensional gas kinematic data,
obtained with the adaptive optics assisted near infrared integral field
spectrograph Gemini/NIFS, and high-resolution multiband photometric data from
Hubble Space Telescope's Wide Field Camera 3 (HST/WFC3). We use the photometric
data to model the shape and stellar mass-to-light ratio (M/L) of the nuclear
star cluster. From the Gemini/NIFS observations, we derive the kinematics of
warm molecular hydrogen gas as traced by emission through the H$_2$ 1--0 S(1)
transition. These kinematics show a clear rotational signal, with a position
angle orthogonal to NGC 4395's radio jet. Our best fitting tilted ring models
of the kinematics of the molecular hydrogen gas contain a black hole with mass
$M=4_{-3}^{+8}\times 10^5$ M$_\odot$ (3$\sigma$ uncertainties) embedded in a
nuclear star cluster of mass $M=2 \times 10^6$ M$_\odot$. Our black hole mass
measurement is in excellent agreement with the reverberation mapping mass
estimate of Peterson et al. (2005), but shows some tension with other mass
measurement methods based on accretion signals.
