Emission and Absorption in the M87 LINER

The nucleus of M87 displays a LINER spectrum at optical wavelengths, with anuclear disk of nebulosity that is resolved by the Hubble Space Telescope. Wepresent new results from optical and ultraviolet spectra of the central ~40pcas measured by HST. In contrast with previous results for the extended disk,the emission-line spectrum of the central region is best reproduced by amulti-component photoionization scenario, rather than shock heating. Thenebular properties as well as energetic considerations suggest a transition onscales of several tens of parsecs, from predominantly photoionization by acentral accretion source, to shock ionization within the disk. If this sourceis at all representative, it suggests that many LINERs may be composite interms of the energetic processes that give rise to the emission spectrum. Wealso report measurements of resonance-line absorption for the nucleus. Theabsorption spectrum, like the emission lines, is characterized by lowionization. The absorption line measurements coupled with independentconstraints suggests a total hydrogen column density of 10^19 - 10^20 cm^-2,outflowing from the galaxy center with a velocity of ~126 km/s. The kinematicsignature of an outflow, along with evidence that the absorber covers thepower-law continuum source but not the emission-line clouds, suggests that theabsorbing matter is related to accretion phenomena in the nucleus. The presenceof such an outflow resembles similar behaviour in luminous AGN, although thelow ionization that characterizes LINERs is probably indicative of a differentmode of accretion in these sources.Comment: Accepted for publication in the Astrophysical Journa