MID-INFRARED SPECTROSCOPY OF LINERS - THE AGN CONNECTION

low ionization nuclear emissionline regions (liners), which exist in the nuclei of a large fraction of galaxies, may be the weakest manifestation of the quasar/agn phenomenon. alternatively, the emission lines may be produced in shocked gas or hii regions associated with clusters of unusuallyhot stars. choosing between these alternatives is required in order to correctly place liners on (or remove them from) the agn map. since liners are so common, this will have farreaching implications for the number of agns at the lowest luminosities, their evolution, and the xray background. we propose to use the iso sws to observe five liner galaxies in which we have recently discovered bright compact ultraviolet nuclear sources in hst images, and two liners in which nuclear uv sources are absent. all seven of our targets display similar optical line ratios independent of the presence or absence of the central uv source. by searching for ir differences among the two types of object, our proposed iso observations will determine whether different excitation mechanisms are operating in the uvbright and uvdark liners, or whether the uvdark liners are obscured sources. the ir measurements will provide important and previouslyunavailable diagnostics for determining the excitation mechanisms in each of the two types of liners. we will complement our iso spectral measurements with uv and xray spectroscopy of our sources by hst and asca. the iso observations will, for the first time, provide comprehensive multiwavelength data for a sample of liner galaxies and new and unique information about the nature of the ionizing sources in these objects. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]