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Line, LINER, linest - from micro-AGN to ultra-luminous LINERs. One and the same?

This paper compares the optical spectra of a wide range of galaxies categorised as members of the Low Ionisation Nuclear Emission Region (LINER) class of active galactic nuclei (AGN). LINERs are defined by emission spectra with relatively faint high ionisation lines (compared to other AGN classes). The gas emission luminosity ranges from the weak flux emanating from some nearby galactic nuclei all the way to extremely luminous radio galaxies, where the line emission can completely dominate the host galaxy starlight component. In this study I analyse the Sloane Digital Sky Survey optical spectra of 15 LINERS identified in the course of the preparation of the new edition of the ZORROASTER AGN catalogue, spanning the largest possible luminosity range. I compare relative emission line strengths, focusing on uncommonly analysed ratios such as those involving [N I], line widths, profiles and even the spectral features of the host galaxy stellar continuum. The study identifies possible luminosity dependent trends in the spectral properties of the studied objects. Possible reasons are presented to rationalise these trends, and the paper concludes with a discussion regarding the uniformity of the LINER class.


Introduction
The second version of the ZORROASTER 1 online catalogue of optical spectra of active galactic nuclei (AGN), to be released in October 2013, will now include numerous narrow-line objects. In particular, the update contains a significant number of entries meeting the general criteria of a class of AGN referred to as LINER (Low Ionisation Nuclear Emission-line Region).
The LINER class was originally suggested by Heckman [1], and its main characteristics are the comparatively strong spectral lines corresponding to low ionisation features such as [

O I] and [N I].
Conversely, the high excitation [O III] lines are much weaker than found in most other types of AGN (e.g. Seyferts and starburst galaxies).
It is not clear that LINERs constitute a homogeneous class of objects. Various mechanisms have been suggested to explain the general features of the optical spectra, including i) photoionisation by an accretion disk surrounding a black hole (as in Seyfert galaxies), ii) photoionisation by stars (as in starburst galaxies) and iii) shock heated gas (as in some radio galaxies) [2,3,4]. None of these models have been completely discounted for all objects associated with this classification. It should hence be considered that several classes of AGN share similar spectral features, but are in fact quite disparate types of objects with very different physical characteristics.
It has become common to use line ratios as the defining parameter to distinguish LINERs from alternative AGN classes. In particular, two-dimensional logarithmic plots of one line ratio versus another offer a convenient systematic tool to automatically assign activity classes to large samples of galaxies [5,6]. However, by failing to carefully consider the peculiarities and full details of individual objects, this procedure is prone to misclassifications.
The line ratio based classification scheme also does not consider the wide range observed in LINER luminosities. It is well established that a lot of the otherwise unremarkable nuclei of nearby galaxies exhibit faint LINER emission spectra [7]. But highly luminous AGN like Arp 102B are also viewed as part of this class [8]. The luminosity dependence of LINER spectra is not properly established.
This paper investigates the spectra of 15 objects that satisfy the general LINER classification requirements according to the original definition [1], and explores possible line ratio vs. luminosity correlations. Their coordinates, redshifts and common names (where applicable) are listed in table 1.

Archival spectra
For the majority of objects analysed here, electronic spectra are available in the Sloan Digital Sky Survey (SDSS) database [9]. These cover the entire optical range, and successive data point in the spectrum corresponds to adjacent channels of wavelength intervals of ∆(log λ) = 0.0001. In addition to the SDSS objects, two well-studied AGN with LINER features (NGC 1275 and NGC 6240) were included in the sample as well. Archival spectra of these were also available in electronic form [10,11], and were converted into the SDSS format. All spectra are displayed in figure 1.

Bands
The 2013 version of the ZORROASTER database is set to include total fluxes for selected bands consisting of 11 adjacent channels. These are strategically chosen to include all the major emission

Results
The comparative emission line region luminosity L was parameterised through multiplication of the continuum-subtracted band fluxes by z 2 . The second column of  Table 3. Correlation tests for the line ratio vs. line luminosity graphs. The entries correspond to the slope and (in brackets) R 2 correlation parameter. Bold entries correspond to the strongest correlations, significant at the 95% level.

Discussion
Even a cursory visual inspection of the spectra displayed in figure 1    The fact that there seems to be a dependence of the line ratios on luminosity is not entirely surprising, as both the plotted parameters depend on the strength and spectral distribution of the excitational radiation and element abundances. It is however notable that some ratios, for example The complete lack of correlation in particular cases such as those illustrated in figure 3 suggests that these may be indicative of intrinsically different line forming mechanisms in some of the objects plotted here. These may paradoxically be optimal for differentiating between different sub-classes of LINERs undergoing distinct physical processes.

Conclusion
The study confirms that what we collectively refer to as LINERs are likely to be an inhomogeneous group of AGN, generated by a variety of very disparate processes. Specific plots of line ratios versus emission line region luminosities show promise of differentiating between the various types of objects. The achievement of this will however require the analysis of a vastly larger sample of LINERs, something that will soon be possible due to the imminent expansion of the ZORROASTER database.