ASCA observations of "type 2" liners: Evidence for a stellar source of ionization

We present ASCA observations of low-ionization nuclear emission-line region s (LINERs) without broad H alpha emission in their optical spectra. The sam ple of " type 2 " LINERs consists of NGC 404, 4111, 4192, 4457, and 4569. W e have detected X-ray emission from all the objects except for NGC 404; amo ng the detected objects are two so-called transition objects (NGC 4192 and NGC 4569), which have been postulated to be composite nuclei having both an H II region and a LINER component. The images of NGC 4111 and NGC 4569 in the soft (0.5-2 keV) and hard (2-7 keV) X-ray bands are extended on scales of several kiloparsecs. The X-ray spectra of NGC 4111, NGC 4457, and NGC 45 69 are well fitted by a two-component model that consists of soft thermal e mission with kT similar to 0.65 keV and a hard component represented by a p ower law (photon index similar to 2) or by thermal bremsstrahlung emission (kT several keV). The extended hard X-rays probably come from discrete sour ces, while the soft emission most likely originates from hot gas produced b y active star formation in the host galaxy. We have found no clear evidence for the presence of active galactic nuclei (AGNs) in the sample. Using bla ck hole masses estimated from host galaxy bulge luminosities, we obtain an upper limit on the implied Eddington ratios less than 5 x 10(-5). If an AGN component is the primary ionization source of the optical emission lines, then it must be heavily obscured with a column density significantly larger than 10(23) cm(-2), since the observed X-ray luminosity is insufficient to drive the luminosities of the optical emission lines. Alternatively, the o ptical emission could be ionized by a population of exceptionally hot stars . This interpretation is consistent with the small [O I] lambda 6300/H alph a ratios observed in these sources, the ultraviolet spectral characteristic s in the cases where such information exists, and the X-ray results reporte d here. We also analyze the X-ray properties of NGC 4117, a low-luminosity Seyfert 2 galaxy serendipitously observed in the field of NGC 4111.