The True Nature of LINERs: weak AGN or strong stars?

General

Are LINERs, which since their discovery 1980 are generally assumed to be a large sub-population of galaxies with an active nucleus, actually powered by old stars in their post-AGB phase? If so, the “zoo” of AGN will be much easier to understand, which allows a much better modelling of physics and mechanisms involved in accretion onto black holes. This project will for the first time give a discriminative answer to this fundamental question by using the unique CALIFA dataset, that only makes this investigation possible.

Background

A sizable fraction of emission-line galaxies are so called LINERs, “Low Ionization Nuclear Emission Region” galaxies, where the central regions exhibit extended emission by low-ionization lines. LINERs are generally believed to be the result of a low-power active nucleus (AGN), and hence would constitute a substantial part of the AGN population. However, lately there has been doubt shed on this scenario: Alternatively, the emission regions could be powered by giant stars just after their asymptotic giant branch phase, where their emission is comparably hard. If this can be shown to be true, this would have substantial implications for our understanding of accretion mechanisms in AGN, which then would not also have to explain LINERs and hence allow for a much simpler picture.

Objectives

The goal of this project is to tackle this question with the unique data of CALIFA, which provides IFS for about 100 LINERs - the largest ever dataset of this kind. It provides unprecedented diagnostic opportunities, by allowing analysis of (i) spatially resolved ionisation source diagnostics (line ratios, BPT diagram), (ii) radial surface brightness profiles of old stars and line emission. These two combine the necessary ingredients which in existing datasets are never present at the same time. SDSS covers many galaxies and allows line-ratio-diagnostic, but does not have spatial resolution. The SAURON dataset has a preselection of early type galaxies and spatial resolution, but its limited wavelength range does not allow line-ratio-diagnostic.
The two scenarios of AGN powering and post-AGB star powering predict different spatial distributions for the line emission, since the illuminating sources are either a central point source (AGN powered) or distributed post-AGB stars (star powered). A comparison with the old stellar body for the regions with clear LINER signatures will clearly discriminate between the two scenarios.

BPT diagram:

Figure from Kewley et al. 2006