The ultraviolet spectra of LINERs: A comparative study

Imaging studies have shown that similar to 25% of LINER galaxies display a compact nuclear UV source. As part of a program to study the nature of LINERs and their connection to the active galaxy phenomenon, we compare the Hubble Space Telescope UV (1150-3200 Angstrom) spectra of seven such UV-bright LINERs. Data for three of the galaxies (NGC 404, 4569, and 5055) are presented for the first time, while data for four others (M81, NGC 4594, NGC 4579, and NGC 6500) have been recently published. The spectra of NGC 404, 4569, and 5055 show clear absorption-line signatures of massive stars, indicating a stellar origin for the UV continuum. Similar features are probably present in NGC 6500. The same stellar signatures may be present but undetectable in NGC 4594, because of the low signal-to-noise ratio of the spectrum, and in M81 and NGC 4579, because of superposed strong, broad emission lines. The compact central UV continuum source that is observed in these galaxies is a nuclear star cluster rather than a low-luminosity active galactic nucleus (AGN), at least in some cases. Except for the two LINERs with broad emission lines (M81 and NGC 4579), the LINERs have weak or no detectable UV emission lines. The UV emission-line spectrum strength shows no relation to the UV continuum strength. Furthermore, at least four of the LINERs suffer from an ionizing photon deficit in the sense that the ionizing photon flux inferred from the observed far-UV continuum is insufficient to drive the optical H I recombination lines. Examination of the nuclear X-ray flux of each galaxy shows a high X-ray-to-UV ratio in the four "UV photon-starved" LINERs. In these four objects, a separate component, emitting predominantly in the extreme-UV, is the likely ionizing agent and is perhaps unrelated to the observed nuclear UV emission. Future observations can determine whether the UV continuum in LINERs is always dominated by a starburst or, alternatively, that there are two types of UV-bright LINERs: starburst dominated and AGN dominated.