2003ApJ...588L..13F

NGC 4395 is one of the least luminous and nearest known type 1 Seyfert galaxies, and it also lacks a bulge. We present a Hubble Space Telescope (HST) I-band image of its nuclear region and Keck high-resolution (∼8 km.s^–1) echelle spectra containing the Ca II near-infrared triplet. In addition to the unresolved point source, there is a nuclear star cluster of size r~3.9 pc; the upper limit on its velocity dispersion is only 30 km.s^–1. We thus derive an upper limit of ∼6.2x10⁶ M_☉for the mass of the compact nucleus. Based on the amount of spatially resolved light in the HST image, a sizable fraction of this is likely to reside in stars. Hence, this estimate sets a stringent upper limit on the mass of the central black hole. We argue, from other lines of evidence, that the true mass of the black hole is likely to be ∼10⁴-10⁵ M_☉. Although the black hole is much less massive than those thought to exist in classical active galactic nuclei (AGNs), its accretion rate of L_bol/L_Edd~2x10^–2 to 2x10^–3 is consistent with the mass-luminosity relation obeyed by classical AGNs. This may explain why NGC 4395 has a high-excitation (Seyfert) emission-line spectrum; active galaxies having low-ionization spectra seem to accrete at significantly lower rates. NGC 4395, a pure disk galaxy, demonstrates that supermassive black holes are not associated exclusively with bulges.