The M87 Black Hole Mass from Gas-dynamical Models of Space Telescope
  Imaging Spectrograph Observations
  The supermassive black hole of M87 is one of the most massive black holes
known and has been the subject of several stellar and gas-dynamical mass
measurements; however the most recent revision to the stellar-dynamical black
hole mass measurement is a factor of about two larger than the previous
gas-dynamical determinations. Here, we apply comprehensive gas-dynamical models
that include the propagation of emission-line profiles through the telescope
and spectrograph optics to new Space Telescope Imaging Spectrograph
observations from the Hubble Space Telescope. Unlike the previous gas-dynamical
studies of M87, we map out the complete kinematic structure of the
emission-line disk within about 40 pc from the nucleus, and find that a small
amount of velocity dispersion internal to the gas disk is required to match the
observed line widths. We examine a scenario in which the intrinsic velocity
dispersion provides dynamical support to the disk, and determine that the
inferred black hole mass increases by only 6%. Incorporating this effect into
the error budget, we ultimately measure a mass of M_BH = (3.5^{+0.9}_{-0.7}) x
10^9 M_sun (68% confidence). Our gas-dynamical black hole mass continues to
differ from the most recent stellar-dynamical mass by a factor of two,
underscoring the need for carrying out more cross-checks between the two main
black hole mass measurement methods.
