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 emission in the LINER galaxy NGC 4203. The emission-line profile is suggestive of a relativistic accretion disk and is reminiscent of double-peaked transient Balmer emission observed in a handful of other LINERs. The very broad line emission thus constitutes clear qualitative evidence for a black hole, and spatially resolved narrow-line emission in NGC 4203 can be used to constrain its mass, MBH</=6x106 M middle dot in circle, at 99.7% confidence. This value implies a ratio of black hole mass to bulge mass of less, similar7x10-4 in NGC 4203, which is less by a factor of approximately 3-9 than the mean ratio obtained for other galaxies. The availability of an independent constraint on central black hole mass makes NGC 4203 an important testbed for probing the physics of weak active galactic nuclei. Assuming MBH near the detection limit, the ratio of the observed luminosity to the Eddington luminosity is approximately 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 observed double-peaked line emission. Follow-up observations will make it possible to improve the black hole mass estimate and study variability in the nuclear emission.