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 Seyfertgalaxies, and it also lacks a bulge. We present an HST I-band image of itsnuclear region and Keck high-resolution (8 km/s) echelle spectra containing theCa II near-infrared triplet. In addition to the unresolved point source, thereis a nuclear star cluster of size r ~ 3.9 pc; the upper limit on its velocitydispersion is only 30 km/s. We thus derive an upper limit of 6.2x10^6 solarmasses for the mass of the compact nucleus. Based on the amount of spatiallyresolved light in the HST image, a sizable fraction of this is likely to residein stars. Hence, this estimate sets a stringent upper limit on the mass of thecentral black hole. We argue, from other lines of evidence, that the true massof the black hole is likely to be 10^4-10^5 solar masses. Although the blackhole is much less massive than those thought to exist in classical activegalactic nuclei, its accretion rate of L_bol/L_Edd ~ 2x10^-2 to 2x10^-3 isconsistent with the mass-luminosity relation obeyed by classical AGNs. This mayexplain why NGC 4395 has a high-excitation (Seyfert) emission-line spectrum;active galaxies having low-ionization spectra seem to accrete at significantlylower rates. NGC 4395, a pure disk galaxy, demonstrates that supermassive blackholes are not associated exclusively with bulges.Comment: To appear in ApJ (Letters). 5 page