2.4. Ultraviolet Emission and Constraints on Shock Excitation
The nonstellar nature of LINERs might be revealed through the presence of a central compact source responsible for the photoionizing continuum. The UV band is preferred over the optical because it minimizes contamination from old stars, although it is much more adversely affected by dust extinction. Two imaging surveys performed with the HST (Maoz et al. 1995; Barth et al. 1998) find that LINERs in fact do contain compact UV emission, but in only 20%-25% of the cases. By itself, however, this result is ambiguous. Are the central UV sources in most LINERs obscured by dust, are they in the ``off'' state of a duty cycle most of the time as suggested by Eracleous et al. (1995), or do the majority of LINERs simply lack a pointlike ionizing source because they are not AGNs after all? There is some indication that the sources detected in the UV tend to be in more face-on galaxies than the undetected sources (Barth et al. 1998). Moreover, as discussed below, LINERs seem to be intrinsically weak in the UV, and this may further contribute to the low detection rates. Mere morphological information, of course, cannot specify definitively the physical origin of the UV emission. For example, the point sources could be simply very compact nuclear star clusters. Indeed, follow-up spectroscopy indicates that the bulk of the UV emission in some sources comes from young massive stars (Maoz et al. 1998). Others, on the other hand, exhibit featureless, power-law continua as expected for an energetically significant AGN component (M81: Ho et al. 1996; NGC 4579: Barth et al. 1996; M87: Tsvetanov et al. 1998).
Collisional ionization by shocks has been considered a plausible energy source for LINERs since the discovery of these objects (Fosbury et al. 1978; Heckman 1980). Dopita and Sutherland (1995) recently showed that the diffuse radiation field generated by fast (v 150-500 km s-1) shocks can reproduce the optical narrow emission lines seen in both LINERs and Seyferts. In their models, LINER-like spectra are realized under conditions in which the precursor H II region of the shock is absent, as might be the case in gas-poor environments. The postshock cooling zone attains a much higher equilibrium electron temperature than a photoionized plasma; consequently, a robust prediction of the shock model is that it should produce a higher excitation spectrum, most readily discernible in the UV, than photoionization models. In all the cases studied so far, the UV spectra are inconsistent with the fast-shock scenario because the observed intensities of the high-excitation lines such as C IV 1549 and He II 1640 are much weaker than predicted (Barth et al. 1996, 1997; Nicholson et al. 1998; Maoz et al. 1998). [The case of M87 presented by Dopita et al. (1997) is irrelevant to the present discussion because those observations explicitly avoided the nucleus of the galaxy.] The data, however, cannot rule out contributions from slower shocks (v 150 km s-1), although the viability of shock ionization in luminous AGNs has been criticized on energetic grounds by Laor (1998).