3.4. Emission lines in giant elliptical galaxies
In more than half of the optically selected nearby gE galaxies, nuclear emission lines have been detected. The ionized gas typically has an extended distribution (1-10 kpc) and can have a plethora of different morphologies: flattened disk-like structures, filamentary structures etc. (; ). Most of these objects are genuine Liners () and show indication of an AGN (unresolved broad wings of H, strong radio or X-ray emission) (Alonso-Herrero et al. 2000). Their H luminosity is weak (3 1037 to 5 1040 erg s-1) (; ).
E galaxies contain dust, cool gas (H I, T < 102 K) and warm gas (H II, T ~ 104 K) which could be either external to the galaxy resulting, for example, from a cooling flow or the merging of two galaxies, or internal to the galaxy through stellar mass loss. , , ,  and  argued that the gas as well as the dust is of external origin in the majority of cases.
Extended X-ray emission from individual E galaxies due to hot interstellar gas has been reported; radiative cooling may () or may not () be important throughout most of this gas.  pointed out that small X-ray cooling flows may explain the presence of Liners in ordinary galaxies.  have isolated a class of luminous X-ray sources (LX > 1043.5 erg s-1) that they call ``cooling flow galaxies'' which are associated with E galaxies not located within a rich cluster and having an optical spectrum similar to the spectra of the dominant galaxies in cooling flow clusters.
Post-AGB stars present in E galaxies could provide sufficient ionizing radiation to account for the observed H luminosity in these galaxies and to reproduce the excitation level of the ionized gas ().
In conclusion, the Liners found in E galaxies could be a mixed population containing both AGN-type Liners ionized by a non-stellar central continuum and objects in which the ionization is provided by hot stars.
Liner nuclei are found not only in E galaxies, but also in S0s and early type spirals ; many of these galaxies are transition objects with spectra appearing to be intermediate between those of H II regions and Liners; their low ionization lines are stronger than those of the former, yet weaker than those of the latter; they have been called weak-[O I] Liners ().
Some of them are probably composite H II/Liner systems (Binette 1985; ); others could be photoionized by hot (> 45 000 K) main-sequence O stars (); however, , from near-infrared spectroscopy ([Fe II]1.2567µm and Pa) of a number of them, suggested that they could rather be due to ionization by aging starbursts in which the supernova remnants (SNRs) play an increasingly important role as the starburst ages and the hot stars fade while a high supernova rate persists: the Fe II line is predominantly excited by SNR-driven shocks and the P tracks H II regions excited by massive young stars. The weak X-ray emission of these objects seem to be of stellar origin ().
25% of the observed Liners hosted in a spiral display a compact (but not always unresolved) nuclear UV source; the two observerd galaxies with an unresolved nuclear UV source have broad H emission (). UV spectra show clear absorption-line signatures of massive stars in about half of them, indicating a stellar origin for the UV continuum; the stellar population in these objects could provide enough ionizing photons to explain the observed optical emission line flux thus showing that they are probably not associated with an AGN ().
Komossa et al. (1999) have investigated the X-ray properties of 13 S0 to Sb galaxies having a Liner spectrum; eight have been detected at rather low luminosity level; but three sources are best described by a single AGN-like power-law with X-ray luminosity above that expected from discrete stellar sources; these spectra most likely indicate the presence of low-luminosity AGNs.
Thus it appears that Liners found in spiral galaxies could be either AGNs or starbursts.