Evidence for a black hole and accretion disk in the liner NGC 4203

We present spectroscopic observations from the Hubble Space Telescope that reveal for the first time the presence of a broad pedestal of Balmer line e mission in the LINER galaxy NGC 4203. The emission-line profile is suggesti ve of a relativistic accretion disk and is reminiscent of double-peaked tra nsient Balmer emission observed in a handful of other LINERs. The very broa d line emission thus constitutes clear qualitative evidence for a black hol e, and spatially resolved narrow-line emission in NGC 4203 can be used to c onstrain its mass, M-BH less than or equal to 6 x 10(6) M-circle dot, at 99 .7% confidence. This value implies a ratio of black hole mass to bulge mass of less than or similar to 7 x 10(-4) in NGC 4203, which is less by a fact or of similar to 3-9 than the mean ratio obtained for other galaxies. The a vailability of an independent constraint on central black hole mass makes N GC 4203 an important testbed for probing the physics of weak active galacti c nuclei. Assuming M-BH near the detection limit, the ratio of the observed luminosity to the Eddington luminosity is similar to 10(-4). This value is consistent with advection-dominated accretion and hence with scenarios in which an ion torus irradiates an outer accretion disk that produces the obs erved double-peaked line emission. Follow-up observations will make it poss ible to improve the black hole mass estimate and study variability in the n uclear emission.