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We have used the Hubble Space Telescope to obtain ultraviolet spectra and optical images of the nucleus of NGC 4395, the least luminous and nearest known type 1 Seyfert. The spectrum closely resembles those of normal Seyfert nuclei; narrow emission lines span a wide range of ionization, and the permitted lines have broad wings. If parameterized by a power law (fnu_ is proportional to νalpha^), the essentially featureless continuum has a wavelength-dependent spectral index of - 1 to - 2. The nucleus is spatially unresolved in the optical images obtained through broad-band filters; we set an upper limit of 0. 05" (~0. 7 pc) for the diameter of the continuum source. These data, together with optical and near-infrared spectra that reveal no stellar absorption lines whatsoever, constitute strong evidence against the starburst hypothesis for the origin of the activity. A cluster of stars having the requisite characteristics is highly unlikely, while a single or binary star would require unprecedented properties. The continuum cannot be dominated by a young supernova, given the object's exceedingly low luminosity and lack of optical variability. In addition, the broad components of the permitted emission lines are probably not produced by a supernova remnant embedded in a star cluster. Since the nucleus of NGC 4395 resembles those of other Seyfert 1 galaxies in most respects, we conclude that luminous type 1 Seyferts and QSOs are also powered by nonstellar phenomena, consistent with the standard black hole model.
Filippenko et al. (Tue,) studied this question.