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Passive red galaxies frequently contain warm ionized gas and have spectrasimilar to low-ionization nuclear emission-line regions (LINERs). Here weinvestigate the nature of the ionizing sources powering this emission, bycomparing nuclear spectroscopy from the Palomar survey with larger aperturedata from the Sloan Digital Sky Survey. We find the line emission in themajority of passive red galaxies is spatially extended; the Halpha surfacebrightness profile depends on radius (r) as r^(-1.28). We detect strong lineratio gradients with radius in [N II]/Ha, [S II]/Ha, and [O III]/[S II],requiring the ionization parameter to increase outwards. Combined with arealistic gas density profile, this outward increasing ionization parameterconvincingly rules out AGN as the dominant ionizing source, and strongly favorsdistributed ionizing sources. Sources that follow the stellar density profilecan additionally reproduce the observed luminosity-dependence of the line ratiogradient. Post-AGB stars provide a natural ionization source candidate, thoughthey have an ionization parameter deficit. Velocity width differences amongdifferent emission lines disfavor shocks as the dominant ionization mechanism,and suggest that the interstellar medium in these galaxies contains multiplecomponents. We conclude that the line emission in most LINER-like galaxiesfound in large aperture (>100pc) spectroscopy is not primarily powered by AGNactivity and thus does not trace the AGN bolometric luminosity. However, theycan be used to trace warm gas in these red galaxies.

The Nature of LINER-like Emission in Red Galaxies