Gas Kinematics and the Black Hole Mass at the Center of the Radio Galaxy NGC 4335

We investigate the kinematics of the central gas disk of the radio-loud elliptical galaxy NGC 4335, derived from Hubble Space Telescope (HST) long-slit spectroscopic observations of Hα + [N II] along three parallel slit positions. The observed mean velocities are consistent with a rotating thin disk. We model the gas disk in the customary way, taking into account the combined potential of the galaxy and a putative black hole with mass M•, as well as the influence on the observed kinematics of the point-spread function and finite slit width. This sets a 3 σ upper limit of 108 M⊙ on M•. The velocity dispersion at r ≲ 0.″5 is in excess of that predicted by the thin rotating disk model. This does not invalidate the model if the excess dispersion is caused by localized turbulent motion in addition to bulk circular rotation. However, if instead the dispersion is caused by the black hole (BH) potential then the thin disk model provides an underestimate of M•. A BH mass M• ∼ 6 × 108 M⊙ is inferred by modeling the central gas dispersion as due to an isotropic spherical distribution of collisionless gas cloudlets. The stellar kinematics for NGC 4335 are derived from a ground-based (William Herschel Telescope/ISIS) long-slit observation along the galaxy major axis. A two-integral model of the stellar dynamics yields M• ≳ 3 × 109 M⊙. However, there is reason to believe that this model overestimates M•. Reported correlations between black hole mass and inner stellar velocity dispersion σ predict M• to be ≥5.4 × 108 M⊙ in NGC 4335. If our standard thin disk modeling of the gas kinematics is valid, then NGC 4335 has an unusually low M• for its velocity dispersion. If, on the other hand, this approach is flawed and provides an underestimate of M•, then black hole masses for other galaxies derived from HST gas kinematics with the same assumptions should be treated with caution. In general, a precise determination of the M•-σ relation and its scatter will benefit from (1) joint measurements of M• from gas and stellar kinematics in the same galaxies and (2) a better understanding of the physical origin of the excess velocity dispersion commonly observed in nuclear gas disks of elliptical galaxies.