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

Abstract

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_solar 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_solar 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_solar. 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_solar 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.

Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.