A 10,000-solar-mass black hole in the nucleus of a bulgeless dwarf galaxy

The motions of gas and stars in the nuclei of nearby galaxies have demonstrated that massive black holes are common¹ and that their masses correlate with the stellar velocity dispersion σ_★ of the bulge^2-4. This correlation suggests that massive black holes and galaxies influence each other's growth^5-7. Dynamical measurements are less reliable when the sphere of influence is unresolved; thus, it remains unknown whether this correlation exists in galaxies much smaller than the Milky Way. Light echoes from photoionized clouds around accreting black holes^8,9, in combination with the velocity of these clouds, yield a direct mass measurement that circumvents this difficulty. Here we report an exceptionally low reverberation delay of 83 ± 14 min between variability in the accretion disk and Hα emission from the nucleus of the dwarf galaxy NGC 4395. Combined with the Hα velocity dispersion σ_line = 426 ± 1 km s^-1, this lag determines a mass of about 10,000 M_⊙ for the black hole (M_BH). This mass is among the smallest central black hole masses reported, near the low end of expected masses for heavy `seeds'^10-12, and the best direct mass measurement for a galaxy of this size. Despite the lack of a bulge, NGC 4395 is consistent with the M_BH-σ_★ relation, indicating that the relation need not originate from hierarchical galaxy assembly nor from black hole feedback.