A very low nuclear-luminosity class of low-ionization nuclear emission-line region galaxies (LINERs) was identified by Heckman (1980). Spectroscopically, they resemble Seyfert 2 galaxies, except that the low-ionization lines, e.g., [O I] 6300 and [N II] 6548, 6583, are relatively strong. LINERs are very common, and might be present at detectable levels in nearly half of all spiral galaxies (Ho, Filippenko, and Sargent 1994). A sample LINER spectrum is shown in the Figure.

Figure 2.1. The optical spectrum of the LINER NGC 1052 is shown, with important emission lines identified (Ho, Filippenko, and Sargent 1993). Some strong absorption lines that arise in the host galaxy rather than the AGN itself are also identified. This spectrum can be compared with the spectrum shown in Figure 1.1 in section S of the glossary and the QSO spectrum shown in Figure 2.2 in section Q. Important differences between Seyfert 2s and LINERs are apparent: the [O III] 5007 / H flux ratio is much larger in Seyfert 2s (in NGC 1667, the weak H line is obscured by blending with the stellar H absorption line) than in LINERs, and low-ionization lines ([N II] 6716, 6731, [S II] 6548, 6853, [O II] 3727, and [O I] 6300) are all relatively prominent in LINER spectra. Data courtesy of A. V. Filippenko. The y-axis units are F (ergs s-1 cm-2 Å-1).

The [O III] / H flux ratio is often used to distinguish Seyfert galaxies from other types of emission-line galaxies. The criterion that the flux ratio [O III] / H > 3 in AGNs is not a robust indicator, however, because this flux ratio is also typical of low-metallicity HII regions. Indeed, LINER, Seyfert-galaxy, and HII-region spectra cannot be unambiguously distinguished from one other on the basis of any single flux ratio from any pair of lines. However, Baldwin, Phillips, and Terlevich (1981) have shown that various types of objects with superficially similar emission-line spectra (i.e., characteristic of a 10⁴ K gas) can be distinguished by considering the intensity ratios of two pairs of lines; the relative strengths of various lines are a function of the shape of the ionizing continuum, and they therefore can be used to distinguish between, for example, blackbody and power-law ionizing spectra. Figure 2.3 is an example of a ``BPT'' (for Baldwin, Phillips, and Terlevich) diagram which demonstrates how LINERs can be distinguished from normal HII regions and normal AGNs (Seyferts and QSOs) on the basis of the [O III] 5007 / H, [N II] 6583 / H, and [S II] 6716, 6731 / H flux ratios. Here it is seen that the Seyfert 2s have high values of each ratio. H II regions define a locus of lower values which does not overlap with the region of parameter space occupied by the Seyferts. The LINERs can be distinguished from the Seyfert 2s by their low values of [O III] 5007 / H relative to [N II] 6583 / H, and from the H II regions by their larger values of [N II] 6583 / H.

Some models indicate that the emission-line spectra of LINERS are consistent with photoionization by a Seyfert-like continuum which is very dilute. The presence of strong [O I] 6300 is especially indicative of a power-law ionizing spectrum, because the ionization potential of O⁰ is nearly identical to that of H⁰; the [O I] line, which is collisionally excited, will only occur in a zone which has a sufficiently high electron density and temperature to excite the upper level. With a stellar input spectrum, these conditions only occur within the H⁺ Strömgren sphere, where the O⁰ abundance is negligible. However, a gas ionized by a relatively flat power-law spectrum has an extended partially ionized zone where the [O I] emission arises.

The relationship between LINERs and AGNs is not completely clear. Some, but by no means all, LINERs appear to be simply very low-luminosity Seyfert galaxies. LINER-type spectra can also be produced in cooling flows, in starburst-driven winds, and in shock-heated gas (Heckman 1987, Filippenko 1992).

Adapted from B.M. Peterson An Introduction to Active Galactic Nuclei, Cambridge University Press, (1997)