Following the suggestion of Ho et al. (1993), I have adopted the working hypothesis that the nuclei classified as transition objects represent composite systems consisting of contributions from both a LINER and an H II region component. Let us now consider this hypothesis in more detail. The physical nature of this subclass of emission-line nuclei has important consequences for the overall demographics of LINERs and AGNs, since numerically these objects rival LINERs (Table 2) (1).
It should come as no surprise that spectra gathered from any fixed-aperture survey will unavoidably integrate spatially distinct regions in some objects, as the physical scale projected by the spectrograph aperture varies with distance. Several examples of composite Seyfert/H II region systems have been recognized in the literature. In cases where the angular extent of the system is sufficiently large, the ``active'' component can be separated from the off-nuclear star-forming component (e.g., Edmunds & Pagel 1982; Véron-Cetty & Véron 1985; Shields & Filippenko 1990). Where only spatially integrated spectra are available, the composite nature has been identified through either decomposition of the line profiles (Véron et al. 1981; Heckman et al. 1983; Kennicutt, Keel, & Blaha 1989) or consideration of the line ratios (Keel 1984; Ho et al. 1993; Boer 1994). The typical distances of the transition objects and LINERs in the Palomar survey, however, are essentially identical. If spatial resolution is the main factor, then an interesting prediction, testable by high-resolution imaging, is that transition objects should show resolved star-forming regions surrounding a central LINER source (unless the star formation occurs in a compact, centrally located cluster). In fact, of the galaxies in the Hubble Space Telescope (HST) ultraviolet imaging survey of Maoz et al. (1995; also see Maoz in these proceedings) that show bright emission, three (NGC 4569, NGC 4736, and NGC 5055) have optical spectra resembling those of transition objects (Ho et al. 1996c), and all three exhibit resolved structure in addition to a central core. Although the true nature of the ultraviolet emission can only be assessed through follow-up spectroscopy at comparable angular resolution, its morphology strongly suggests that we are witnessing star formation encircling an active nucleus.
Another intriguing possibility, suggested by the distribution of galaxy axial ratios (Section 6.3), is that transition objects are simply LINERs whose inclinations are such that circumnuclear star-forming regions happen to be projected along the line of sight. Such a scenario favors a geometry in which the star formation in the environment of the nucleus is preferentially confined to a disk-like or ring-like configuration, as appears to be a common situation, especially for galaxies of early Hubble types (Phillips 1996). As the emission-line strengths of LINERs in most instances are in fact weaker than those of giant H II regions, an appropriate mixture of the two components easily accounts for the spectra of transition objects. The average excitation of transition objects, as measured by [O III] / H, is lower than that of LINERs. Within the framework discussed here, this finding is to be expected, given the high metal abundance (and hence low excitation) of nuclear H II regions (Ho et al. 1996d).
Distance and orientation effects probably can account for most transition objects, but that cannot be the whole story, since the distributions of Hubble types and absolute luminosity for transition objects are actually slightly different compared to those of LINERs (Fig. 4), in the sense that the former contain some members of later morphological types. If geometry (aperture and inclination effects) alone were the sole determining factor in whether a given nucleus is perceived as a LINER or a transition object, such differences would not be expected. Could it be that some of the transition objects in fact do not harbor an AGN (LINER) component? Indeed, models attempting to explain LINERs entirely in terms of stellar photoionization (Filippenko & Terlevich 1992; Shields 1992) succeed best when matched to objects whose [O I] strengths (relative to H) are relatively weak (2). If hot stars alone contribute to the ionization in these sources, and if the stars are not restricted to a centrally unresolved cluster, this alternative model can be tested through high-resolution imaging.
While the composite nature of transition objects demonstrates the spatial and temporal juxtaposition of star formation and the AGN phenomenon, it does not imply, much less prove, a direct causal or evolutionary connection between these two disparate physical processes. Stars continuously form at some level in the centers of many galaxies (Ho et al. 1996d), and in early-type spirals, the ``hot-spot'' H II regions can be particularly intense (e.g., Phillips 1996). It has been argued (e.g., Weedman 1983) that the remnants of by-gone massive stars might evolve into a compact configuration at the nucleus, possibly in the form of a single object such as a massive black hole. But until such a scenario can be proven to happen in nature, one must be cautious about unduly ascribing significance to a possibly fortuitous coexistence of two unrelated phenomena.
2 Objects originally named ``weak-[O I] LINERs''
by Filippenko &
Terlevich (1992) and
Ho & Filippenko (1993)
were renamed ``transition objects'' by
Ho et al. (1993).
1 Note that the fraction of transition objects is much higher than that given by Ho et al. (1993), whose estimate was based on heterogeneous data taken from the literature. Back.
2 Objects originally named ``weak-[O I] LINERs'' by Filippenko & Terlevich (1992) and Ho & Filippenko (1993) were renamed ``transition objects'' by Ho et al. (1993). Back.