  We have obtained new Hubble Space Telescope (HST) observations of M84, a
nearby massive elliptical galaxy whose nucleus contains a 1.5 X 10^9 Msun dark
compact object, which presumably is a supermassive black hole. Our Space
Telescope Imaging Spectrograph (STIS) spectrum provides the first clear
detection of emission lines in the blue (e.g., [O II] 3727, Hbeta, and [O III]
4959, 5007), which arise from a compact region 0.28" across centered on the
nucleus. Our Near Infrared Camera and Multi-Object Spectrometer (NICMOS) images
exhibit the best view through the prominent 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 detected in the wavelength range 6295 - 6867 AA by Bower et al.
(1998, ApJ, 492, L111) 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 source and find that although it is roughly
flat in most bands, the optical to UV continuum is very red, similar to the
spectral energy distribution of BL Lac. Thus, the nuclear point source seen in
high-resolution optical images (Bower et al. 1997, ApJ, 483, L33) is not a star
cluster but is instead a nonstellar source. Assuming isotropic emission from
this source, we estimate that the ratio of bolometric luminosity to Eddington
luminosity is 5 X 10^(-7). However, this could be underestimated if this source
is a misaligned BL Lac object, which is a possibility suggested by the spectral
energy distribution and the evidence of optical variability we describe.
