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 ( \sim 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 \approx 3.9 pc ; the upper limit on its velocity dispersion is only 30 km s ^ { -1 } .
We thus derive an upper limit of \sim 6.2 \times 10 ^ { 6 } ~ { } M _ { \odot } 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 \sim 10 ^ { 4 } -10 ^ { 5 } M _ { \odot } .
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 } \approx 2 \times 10 ^ { -2 } to 2 \times 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 .