The Mass Distribution in the Elliptical Galaxy NGC 3377: Evidence for a 2 x 10**8 Solar Mass Black Hole

Abstract

We have measured the mass distribution in the central 35&#34; = 1.7 kpc of the E5 galaxy NGC 3377. Stellar rotation velocity and velocity dispersion profiles (seeing sigma_* = 0.20 - 0.56 arcsec) and V-band surface photometry (sigma_* = 0.20 - 0.26 arcsec) have been obtained with the Canada-France-Hawaii Telescope. NGC 3377 is kinematically similar to M32: the central kinematic gradients are steep. The apparent velocity dispersion rises from 95 +- 2 km/s at 1&#34; <= r <= 4&#34; to 178 +- 10 km/s at the center. To search for a central black hole, we derive three-dimensional velocity and velocity dispersion fields that fit our 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&#34; to M/L_V > 10. If the mass-to-light ratio of the stars is constant with radius, then NGC 3377 contains a central massive dark object (MDO), probably a black hole, of mass (1.8 +- 0.8) x 10**8 M_Sun. Several arguments suggest that NGC 3377 is likely to be nearly isotropic. However, flattened, anisotropic maximum entropy models can fit the data without an MDO. Therefore the MDO detection in NGC 3377 is weaker than those in M31, M32, or NGC 3115. The above MDO mass is 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 the mass and mass-to-light ratio. At 3&#34; <= r <= 35&#34;, M/L_V is constant at ~ 2.4. Therefore the inner parts of NGC 3377 are dominated by a normal old stellar population, not by halo dark matter.