Double-peaked broad emission lines in NGC 4450 and other LINERs

Spectra taken with the Hubble Space Telescope (HST) reveal that NGC 4450 em its Balmer emission lines with displaced double peaks and extremely high ve locity wings. This characteristic line profile, previously seen in a few ne arby LINERs and in a small fraction of broad-line radio galaxies, can be in terpreted as a kinematic signature of a relativistic accretion disk. We can reproduce the observed profile with a model for a disk with a radial range of 1000-2000 gravitational radii and inclined by 27 degrees along the line of sight. The small-aperture HST data also allow us to detect, for the fir st time, the featureless continuum at optical wavelengths in NGC 4450; the nonstellar nucleus is intrinsically very faint, with M-B = -11.2 mag for D = 16.8 Mpc. We have examined the multiwavelength properties of NGC 4450 collectively wi th those of other low-luminosity active nuclei that possess double-peaked b road lines and find a number of common features. These objects are all clas sified spectroscopically as "type 1" LINERs or closely related objects. The nuclear luminosities are low, both in absolute terms and relative to the E ddington rates. All of them have compact radio cores, whose strength relati ve to the optical nuclear emission places them in the league of radio-loud active nuclei. The broadband spectral energy distributions of these sources are most notable for their deficit of ultraviolet emission compared to tho se observed in luminous Seyfert 1 nuclei and quasars. The double-peaked bro ad-line radio galaxies Arp 102B and Pictor A have very similar attributes. We discuss how these characteristics can be understood in the context of ad vection-dominated accretion onto massive black holes.