A Low-Mass Central Black Hole in the Bulgeless Seyfert 1 Galaxy NGC 4395

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 ~6.2 × 10 6 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 ~10 4 -10 5 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 ≈ 2 × 10 -2 to 2 × 10 -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.