Almost two decades ago, Heckman (1980) found quite a large amount of Active Galactic Nuclei (AGN) with spectroscopic properties which depart from those of either ``normal'' HII regions or AGN. Their principal signature was the enhancement of the low ionization lines and, therefore, the name proposed for these new objects was: ``Low Ionization Nuclear Emission Regions'' (LINER galaxies or Liners). These nuclei were described as systems with optical spectra dominated by lines arising from low ionization states, with moderate emission line luminosity (comparable to Giant HII Regions), and with line widths approaching those of the Narrow Line Regions (NLR) of Seyfert galaxies. In some of them, a compact nuclear radiosource was also detected. In that early work Heckman noticed that a sequence between Liners and AGN could be drawn in which Liners occupy the low luminosity end. However, whereas AGN were broadly recognized as regions ionized by a power-law continuum (i.e. where lines result from soft X-ray ionization by the AGN, Halpern & Steiner 1983), Liners were best explained in the context of shock ionization (Fosbury et al. 1978; Dopita et al. 1996), and photoinization by post-AGB stars in the case of weak Liners in Ellipticals (Binette et al. 1994). Another problem in relating Liners to other AGN was the overabundance of Liner characteristics in early type spiral nuclei (Keel 1983a, b).
The puzzling physical nature of these objects has been pointed out by many authors. Among them, the results by Filippenko & Halpern (1984) on the Liner galaxy NGC 7213 are particularly interesting. These authors noticed that broad permitted lines were also present in the optical spectrum, which suggested the possibility that at least in some Liners the source of ionization could be a power-law continuum. On the other hand, in the work by Filippenko & Terlevich (1992) on weak OI Liners, they invoked stellar photoionization as the origin of the emission, stressing the complicated nature of Liners and opening the discussion of whether their nature has any physical meaning in terms of class of galaxies (such as Seyfert galaxies). More recent surveys at optical frequencies by Ho, Filippenko & Sargent (1997) have not solved these controversial issues. Their extensive survey of Emission Line Nuclei lead them to similar results as those in the original work by Heckman. Around 20% of the nuclei of Spirals have Liner characteristic, many with broad Balmer emission lines (the so called Liner 1 using the analogy with Seyferts). The most important new result of these surveys is that Liners constitute not a class, but rather a variety of objects ranging from Liners with properties similar to Seyfert galaxies, to (``transition'') Liners with properties more similar to HII regions.
Normaly we would expect that information coming from all spectral ranges could help to disentangle and perhaps taxonomize the Liner family. But new data at different wavelengths have provoked even more discussion. The study at UV frequencies by Maoz et al. (1998) has shown that the UV spectrum is more consistent with Starburst than with a non-thermal nature. Moreover, the lack of detection of UV sources in most of the Liners searched for by Barth et al. (1998) cast further doubt about the AGN nature of most Liners. However, different possibilities have been explored to reconcile these results with the view of Liners as quiescent AGN (see ``The Physics of LINERS in View of Recent Observations'', 1996). Eracleous, Livio & Binette (1995) have developed a very suggestive hypothesis to explain the lack of a compact UV source as a consequence of a duty cycle variability in the UV source which is only switched on 20% of the time. Other more conservative explanation developed by Binette, Wilson & Storchi-Bergmann (1996) has been to understand the Liner optical spectrum as a result of soft X-ray ionization from an AGN. In both cases the UV spectrum cannot be fully explained.
We want to emphasize that most of the work on Liners has been concentrated in optical selected samples, and the results from FIR selected samples have been overlooked. The work by Kim et al. (1995) and Veilleux et al. (1995) on Luminous FIR galaxies and more recently by Kim et al. (1998) and Kim and Sanders (1998) allows us to draw some important conclusions from both set of data. In particular, the reported evidence that the IR source of energy in Liner-like spectra (38% of the ULIGs) is of stellar origin rather than AGNs.
The above considerations motivated us to compile a multifrequency catalog of Liners selected from different sources in the literature. The main purpose of this catalog is to construct a large sample of Liners, as unbiased as possible, and to provide a basis for multifrequency statistical analysis. In the next section it will be shown that this catalog does not unduly favor optical selected samples as it includes all the objects classified as Liners in the current literature.