The LLAGN interpretation of LINERs was initially motivated by their significant X-ray emission (Ferland & Netzer 1983; Halpern & Steiner 1983). Although LLAGNs are found via radio and X-ray observations in the majority of LINERs (Dudik et al. 2005, 2009; Nagar et al. 2005; Filho et al. 2006; Flohic et al. 2006; González-Martín et al. 2009), they are not powerful enough to photoionize the gas in their vicinity on ∼100 pc (i.e., a few arcseconds in nearby galaxies) scales on which the characteristic emission lines are detected (Flohic et al. 2006; Eracleous et al. 2010a, and references therein). Imaging studies of LINERs have found extended line-emitting regions and complex circumnuclear dust morphologies that might obscure and further prevent the LLAGNs from fully ionizing the surrounding gas (Barth et al. 1999; Pogge et al. 2000; Simões Lopes et al. 2007; González Delgado et al. 2008; González-Martín et al. 2009; Masegosa et al. 2011). Wolf-Rayet stars (i.e.,“warmers”; Terlevich & Melnick 1985) could successfully mimic the X-ray emission produced by a LLAGN, as well as provide the hard ionizing photons necessary to explain the relative intensities of the observed optical emission lines. If Wolf-Rayet stars were the primary source of ionizing photons for LINER-like emission lines, then most LINERs would be in the immediate post-starburst phase, which is unlikely given their high occurrence rate (Ho et al. 1997b) and stellar populations (Cid Fernandes et al. 2004; González Delgado et al. 2004). Compact starbursts containing hot O stars offer an alternative explanation for the relative intensities of LINER emission lines (Filippenko & Terlevich 1992; Shields 1992), but have difficulty explaining the broad Balmer emission wings often seen in LINER spectra (Filippenko 1996), since this would require long-lived supernova remnants. Moreover, the census of stellar populations in LINERs by González Delgado et al. (2004) and Cid Fernandes et al. (2004) does not show a high incidence of compact, nuclear starbursts. pAGB stars and planetary nebulae (Binette et al. 1994; Taniguchi et al. 2000; Yan & Blanton 2012; Belfiore et al. 2016) are more plausible stellar-based models, but are applicable only to a subset of LINERs due to their inability to explain large Hα equivalent widths (Ho et al. 2003). Shock models can adequately describe the off-nuclear optical and ultraviolet (UV) emission-line spectra of some LINERs, such as M87, obtained with the Hubble Space Telescope (HST; see Dopita et al. 1996, 1997; Sabra et al. 2003). Meanwhile the UV spectra of some LINER nuclei, such as NGC 4579, do not have emission line ratios that are well described by shock excitation, and yet show high ionization lines that would require a hard extreme UV source, such as the continuum from an active galactic nucleus (AGN), or fast shocks (Barth et al. 1996; Dopita et al. 2015). Successful shock models could have a wide range of shock velocities (Filippenko 1996), but the gas must be continuously shocked to maintain LINER-like ratios. The shocks could potentially be driven by radio jets, which are fairly common in LINERs and whose kinetic power is considerably higher than the electromagnetic luminosity of the LLAGNs (Filho et al. 2002; Nagar et al. 2005; Maoz 2007). Alternatively, the shocks could result from supernovae or winds from either young or evolved stars.
Citation: Shields 1992
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