Low Ionization Nuclear Emission Line Regions (LINERs) are found in ˜30% of all bright galaxies. The nuclear luminosities in these objects are such that they can be produced by a number of mechanisms and there have been heated debates on the nature of ionizing sources in LINERs. The variety of ionizing mechanisms suggested are low luminosity AGNs, starbursts, shocks, or any combination of these. We have studied Hubble Space Telescope (HST) ultraviolet (UV) spectra of seven LINERs having compact nuclear UV sources.
The picture emerging from this comparison is that the compact source observed in these LINER galaxies, at least in some cases, is a nuclear star cluster rather than a low-luminosity active galactic nucleus (AGN). In these cases, the UV luminosity is driven by tens of thousands of O-type stars, depending on the assumed extinction for these objects. The O-stars could be the high-mass end of a bound stellar population, similar to those seen in super star clusters. Our data do not exclude the possibility that a similar stellar continuum source could dominate in all the LINERs. Alternatively, there may be two types of UV-bright LINERs: those where the UV continuum is produced by a starburst, and those where it is nonstellar.
The “clearly-stellar”, weak [O I] emitters, LINERs have relatively weak X-ray emission, and their stellar populations probably provide enough ionizing photons to explain the observed optical emission-line flux. The other LINERs, strong [O I] emitters, have severe ionizing photon deficits, for reasonable extrapolations of their UV spectra beyond the Lyman limit, but have an X-ray/UV power ratio that is higher by two orders of magnitudes than that of the “clearly-stellar” LINERs. A component which emits primarily in the extreme-UV may be the main photoionizing agent in these objects.
Recent results show that nuclear-starburst and quasar-like activity are often intermingled. Our results extend this result to the lower luminosities of the LINERs.