THE LOW END OF THE SUPERMASSIVE BLACK HOLE MASS FUNCTION: CONSTRAINING THE MASS OF A NUCLEAR BLACK HOLE IN NGC 205 VIA STELLAR KINEMATICS

摘要

Hubble Space Telescope (HST) images and spectra of the nucleated dwarf elliptical galaxy NGC 205 are combined with three-integral axisymmetric dynamical models to constrain the mass M_(BH) of a putative nuclear black hole. This is only the second attempt, after M33, to use resolved stellar kinematics to search for a nuclear black hole with mass below 10~6 solar masses. We are unable to identify a best-fit value of M_(BH) in NGC 205; however, the data impose a upper limit of 2.2 x 10~4 solar mass (1 σ confidence) and an upper limit of 3.8 x 10~4 solar mass (3 σ confidence). This upper limit is consistent with the extrapolation of the M_(BH)-σ relation to the M_(BH) < 10~6 solar mass regime. If we assume that NGC 205 and M33 both contain nuclear black holes, the upper limits on M_(BH) in the two galaxies imply a slope of ～5.5 or greater for the M_(BH)-σ relation. We use our three-integral models to evaluate the relaxation time and stellar collision time in NGC 205; T_r is ～10~8 yr or less in the nucleus, and T_(coll) ≈ 10~(11) yr. The low value of T_r is consistent with core collapse having already occurred, but we are unable to draw conclusions from nuclear morphology about the presence or absence of a massive black hole.