The ionization source in the nucleus of M84

We have obtained new Hubble Space Telescope observations of M84, a nearby m assive elliptical galaxy whose nucleus contains a approximate to 1.5 x 10(9 ) M. dark compact object, which presumably is a supermassive black hole. Ou r Space Telescope Imaging Spectrograph spectrum provides the first clear de tection of emission lines in the blue (e.g., [O II] lambda 3727, H beta, an d [O III] lambda lambda 4959, 5007), which arise from a compact region appr oximate to 0 ".28 across centered on the nucleus. Our Near-Infrared Camera and Multiobject Spectrometer images exhibit the best view through the promi nent dust lanes evident at optical wavelengths and provide a more accurate correction for the internal extinction. The relative fluxes of the emission lines we have detected in the blue together with those recently detected i n the wavelength range 6295-6867 Angstrom by Bower et al. indicate that the gas at the nucleus is photoionized by a nonstellar process instead of hot stars. Stellar absorption features from cool stars at the nucleus are very weak. We update the spectral energy distribution of the nuclear point sourc e and find that although it is roughly flat in most bands, the optical-to-U V continuum is very red, similar to the spectral energy distribution of BL Lac. Thus, the nuclear point source seen in high-resolution optical images by Bower et al. is not a star cluster but is instead a nonstellar source. A ssuming isotropic emission from this source, we estimate that the ratio of bolometric luminosity to Eddington luminosity is similar to 5 x 10(-7). How ever, this could be underestimated if this source is a misaligned BL Lac ob ject, which is a possibility suggested by the spectral energy distribution and the evidence of optical variability we describe.