The Low End of the Supermassive Black Hole Mass Function: Constraining the Mass of a Nuclear Black Hole in NGC 205 via Stellar Kinematics

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 10⁶ 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 × 10⁴ M_☉ (1 σ confidence) and an upper limit of 3.8 × 10⁴ M_☉ (3 σ confidence). This upper limit is consistent with the extrapolation of the M_BH-σ relation to the M_BH < 10⁶ M_☉ 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 ~10⁸ yr or less in the nucleus, and T_coll ≈ 10¹¹ 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.