5.1. The origin of strong [Fe II] lines
One of the issues currently inspiring much discussion and gnashing of teeth is the origin of the very strong [Fe II] lines in some sources, particularly compared with their H I or H2 emission. We have already touched on this question in the context of supernova remnants, where it is unclear whether the line strengths require actual grain destruction and/or gas-phase enrichment of the iron from SN debris, or whether the presence of a deep, partially ionized zone (caused either by shocks or by power-law photoionization) is sufficient (see Section 3.3). At the Galactic Center, the [Fe II]/H I line ratios are intermediate between those for H II regions and SNRs (DePoy 1992).
In spatially unresolved sources such as starbursts in galaxies and AGN's, there is a slightly different "[Fe II] conundrum." Even if we do not fully understand the cause of the strong [Fe II] emission in SNRs, a number of investigators have assumed that it is reasonable to consider such emission to be a tracer of the presence of SNRs. The case for a strong link between SNRs and the infrared [Fe II] lines is supported by the spatial correlation of [Fe II] emission with SN remnants in M82 (Greenhouse et al. 1991). Additional input to this issue comes from observations of the near-infrared H2 lines in Seyfert galaxies and starbursts. The ratios of the H2 and the [Fe II] lines are generally taken as evidence for shocks (Forbes & Ward 1993), with more recent work apparently putting to rest earlier claims of "fluorescent" H2 ratios (see Moorwood & Oliva 1990 for a discussion). However, the issue is far from completely settled (e.g., Mouri, Kawara, & Taniguchi 1993). For example, there may be a shock without SNRs, for example a large-scale wind shock driven by an active nucleus (Kawara, Nishida, & Taniguchi 1988; Lester & Thompson 1990). Or possibly both the [Fe II] and H2 lines are produced by photoionization, by a power-law continuum source rich in high-energy photons. Probably the strongest evidence for power-law ionization is observations of coronal lines such as [Si VI] 1.96 µm (Oliva & Moorwood 1990; Greenhouse et al. 1993).
5.2. Curiosities at the Galactic Center
The last few years have seen dramatic new developments in infrared spectroscopic studies of the Galactic Center. High velocity (v 500 km s-1) components in the 2.06 µm He I line, first detected over a decade ago (Hall, Kleinmann, & Scoville 1982), were hard to understand as originating from the diffuse, low-density ambient gas (Geballe et al. 1991). A new point of view, which has emerged from recent high spatial resolution spectroscopic mapping, is that these lines arise in the stellar winds of a cluster of hot, exotic, mass-losing objects akin to Wolf-Rayet stars (Krabbe et al. 1991). The UV radiation from these stars ionizes the adjacent material, but the stellar winds also drive fast shocks into the adjacent material, producing a hot, wind-swept "cavity" (Lutz, Krabbe, & Genzel 1993). The conditions in this cavity produce a variety of [Fe III] lines, whose ratios constrain the temperature and density of the gas (see their Fig. 6). The extent to which this cluster of hot stars is a paradigm for the central regions of other galaxies is unclear, but it does provide a sobering reminder that the Galactic Center is a special environment which may host many different kinds of unusual phenomena (and not "merely" black holes!).
5.3. Infrared spectroscopy of AGN
We have already mentioned infrared spectroscopy of AGN in the context of line intensity ratios of [Fe II], H I, and H2. There are other interesting applications of infrared spectroscopy to AGN. One is to look at infrared H I lines in Seyfert 2 galaxies, where all the optical lines are relatively narrow, and search for a reddened broad-line component, or "buried" Seyfert 1 (e.g., Hines 1991). In the current paradigm of "unified" theories for AGN, such objects are viewed nearly "edge-on," so that the narrow-line gas (located farther from the central engine) is seen directly, whereas the inner, broad-line region is hidden by a dusty torus (e.g., Antonucci 1993). In the near-infrared the extinction is lower, so the broad-line region is more easily seen and therefore the broad wings become more apparent in the line profiles as one goes to longer and longer wavelengths. A second use of infrared spectroscopy is to observe the familiar "optical" lines, such as [O III] 5007 Å and H, in high-redshift objects (Hill, Thompson, & Elston 1993).