HST observations of NGC 4395, the least luminous Seyfert 1 nucleus - Evidence against the starburst hypothesis for broad-lined active galactic nuclei

Abstract

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 (fv ∝ vα), 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.