Diffuse LINER-type emission from extended disc regions of barred galaxies

TL;DR

This study investigates the diffuse LINER-like emission in the extended disc regions of barred galaxies, finding it is likely caused by p-AGB stars and not recent star formation, with implications for understanding galaxy evolution.

Contribution

It provides new spectroscopic data for 34 galaxies and demonstrates that diffuse emission in SFD regions is predominantly due to p-AGB stars, not star formation.

Findings

Diffuse emission detected in all galaxies studied.

Emission line ratios exclude recent star formation as the source.

Emission properties are consistent with p-AGB star excitation.

Abstract

We present a spectroscopic analysis of the central disc regions of barred spiral galaxies, concentrating on the region that is swept by the bar but not including the bar itself (the `Star Formation Desert' or SFD region). New spectroscopy is presented for 34 galaxies, and the full sample analysed comprises 48 SBa - SBcd galaxies. These data confirm the full suppression of star formation within the SFD regions of all but the latest type (SBcd) galaxies. However, diffuse [NII] and H alpha line emission is detected in all galaxies. The ubiquity and homogeneous properties of this emission from SBa - SBc galaxies favour post-Asymptotic Giant Branch (p-AGB) stars as the source of this line excitation, rather than extreme Blue Horizontal Branch stars. The emission-line ratios strongly exclude any contribution from recent star formation, but are fully consistent with recent population synthesis modelling of p-AGB emission by other authors, and favour excitation dominated by ambient gas of approximately solar abundance, rather than ejecta from the AGB stars themselves. The line equivalent widths are also larger than those observed in many fully passive (e.g. elliptical) galaxies, which may also be a consequence of a greater ambient gas density in the SFD regions.