The Black Hole Mass and the Stellar Ring in NGC 3706
  We determine the mass of the nuclear black hole ($M$) in NGC 3706, an early
type galaxy with a central surface brightness minimum arising from an apparent
stellar ring, which is misaligned with respect to the galaxy's major axis at
larger radii. We fit new HST/STIS and archival data with axisymmetric orbit
models to determine $M$, mass-to-light ratio ($\Upsilon_V$), and dark matter
halo profile. The best-fit model parameters with 1$\sigma$ uncertainties are $M
= (6.0^{+0.7}_{-0.9}) \times 10^8\ M_{\scriptscriptstyle \odot}$ and
$\Upsilon_V = 6.0 \pm 0.2\ M_{\scriptscriptstyle \odot}\ L_{{\scriptscriptstyle
\odot},V}^{-1}$ at an assumed distance of 46 Mpc. The models are inconsistent
with no black hole at a significance of $\Delta\chi^2 = 15.4$ and require a
dark matter halo to adequately fit the kinematic data, but the fits are
consistent with a large range of plausible dark matter halo parameters. The
ring is inconsistent with a population of co-rotating stars on circular orbits,
which would produce a narrow line-of-sight velocity distribution (LOSVD).
Instead, the ring's LOSVD has a small value of $|V|/\sigma$, the ratio of mean
velocity to velocity dispersion. Based on the observed low $|V|/\sigma$, our
orbit modeling, and a kinematic decomposition of the ring from the bulge, we
conclude that the stellar ring contains stars that orbit in both directions. We
consider potential origins for this unique feature, including multiple tidal
disruptions of stellar clusters, a change in the gravitational potential from
triaxial to axisymmetric, resonant capture and inclining of orbits by a binary
black hole, and multiple mergers leading to gas being funneled to the center of
the galaxy.
