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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 an HST I-band image of its nuclear region and Keck high-resolution (8 km/s) 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. We thus derive an upper limit of 6.2x10^6 solar masses 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 solar masses. Although the black hole is much less massive than those thought to exist in classical active galactic nuclei, 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.