Evidence of a Supermassive Black Hole in the Galaxy NGC 1023 from the Nuclear Stellar Dynamics

Abstract

We analyze the nuclear stellar dynamics of the SB0 galaxy NGC 1023, utilizing observational data both from the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope and from the ground. The stellar kinematics measured from these long-slit spectra show rapid rotation (V~70 km s-1 at a distance of 0.1"=4.9 pc from the nucleus) and increasing velocity dispersion toward the nucleus (where σ=295+/-30 km s-1). We model the observed stellar kinematics assuming an axisymmetric mass distribution with both two and three integrals of motion. Both modeling techniques point to the presence of a central dark compact mass (which presumably is a supermassive black hole) with confidence greater than 99%. The isotropic two-integral models yield a best-fitting black hole mass of (6.0+/-1.4)×107 Msolar and mass-to-light ratio (M/LV) of 5.38+/-0.08, and the goodness of fit (χ2) is insensitive to reasonable values for the galaxy's inclination. The three-integral models, which nonparametrically fit the observed line-of-sight velocity distribution as a function of position in the galaxy, suggest a black hole mass of (3.9+/-0.4)×107 Msolar and M/LV of 5.56+/-0.02 (internal errors), and the edge-on models are vastly superior fits over models at other inclinations. The internal dynamics in NGC 1023 as suggested by our best-fit three-integral model shows that the velocity distribution function at the nucleus is tangentially anisotropic, suggesting the presence of a nuclear stellar disk. The nuclear line-of-sight velocity distribution has enhanced wings at velocities >=600 km s-1 from systemic, suggesting that perhaps we have detected a group of stars very close to the central dark mass. Based on observations 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. (AURA), under NASA contract NAS 5-26555.