Stellar collisions in galactic centers: black hole growth and color gradients

We study the effects of stellar collisions, particularly on feeding massive black holes (BHs) and color gradients, in realistic galactic centers. We find that the mass released by stellar collisions is not sufficient to account for the present BH mass in galactic centers, especially in bright galaxies. This study, together with the study by Magorrian & Tremaine (1999) on tidal disruption of stars by massive BHs, implies that the material for BH growth (especially in galaxies brighter than ~10^9 Lsun) can only come from other sources, for example, the mass released by stellar evolution in the initial ~1 Gyr of the galaxy's lifetime, or the gas that sinks to the galactic center in a galaxy merger. We also analyze how the color of a stellar system is affected by collisions of stars. We find that collisions between main-sequence stars cannot cause observable color gradients in the visible bands at projected radius R>0.1" in M31, M32 and other nearby galactic centers. This result is consistent with the lack of an observable color gradient in M32 at R>0.1". At even smaller radii, the color differences caused by collisions between main-sequence stars are at most 0.08 mag at R=0.02". The averaged blueing due to stellar collisions in the region R<0.1" of M32 should not be larger than 0.06 mag in color index U-V and 0.02 mag in V-I. The observed blueing in the center of the galaxy M31 (in a 0.14"x0.14" box) must be caused by some mechanism other than collisions between main-sequence stars.