The Mass Distribution in the Elliptical Galaxy NGC 3377: Evidence for a 2 × 10[TSUP]8[/TSUP] [ITAL]M[/ITAL][TINF][sun][/TINF] Black Hole

Abstract

This paper is a study of the mass distribution in the central 35'' 1.7 kpc of the E5 galaxy NGC 3377. Stellar rotation velocity and velocity dispersion profiles (seeing σ* = 020-056) and V-band surface photometry (σ* = 020-026) have been obtained with the Canada-France-Hawaii Telescope. NGC 3377 is kinematically similar to M32: the central kinematic gradients are steep. There is an unresolved central rise in rotation velocity to V = 110 ± 3 km s−1 (internal error) at r = 10. The apparent velocity dispersion rises from 95 ± 2 km s−1 at 10≤r<4'' to 178 ± 10 km s−1 at the center. To search for a central black hole, we derive three-dimensional velocity and velocity dispersion fields that fit the above observations and Hubble Space Telescope surface photometry after projection and seeing convolution. Isotropic models imply that the mass-to-light ratio rises by a factor of ~4 at r < 2'', to M/LV 10. If the mass-to-light ratio of the stars, M/LV = 2.4 ± 0.2, is constant with radius, then NGC 3377 contains a central massive dark object (MDO), probably a black hole, of mass M• (1.8 ± 0.8) × 108 M⊙. Several arguments suggest that NGC 3377 is likely to be nearly isotropic. However, flattened anisotropic maximum entropy models can fit the present data without an MDO. Therefore, the MDO detection in NGC 3377 is weaker than those in our Galaxy, M31, M32, and NGC 3115. The above masses are corrected for the E5 shape of the galaxy and for the difference between velocity moments and velocities given by Gaussian fits to the line profiles. We show that the latter correction does not affect the strength of the MDO detection, but it slightly reduces M• and M/LV. At 3'' r 35'', M/LV is constant at ~2.4. Therefore, the inner parts of NGC 3377 are dominated by a normal old stellar population. In this elliptical galaxy, as in the bulge-dominated galaxies NGC 3115 and NGC 4594, halo dark matter is unimportant over a significant range in radius near the center.