Discovery of kpc-scale line emission in barred galaxies, not linked to AGN or star formation

Abstract

We present an analysis of the optical line emission from nearby barred galaxies, and in particular look at the radial range occupied by the bar. Inmany cases, this region ismarked by what we term a 'star formation desert', with a marked deficit of HII regions in optical narrow-band H α imaging. Here we present long-slit spectroscopy revealing that such regions do have line emission, but that it is low-level, spatially smooth and almost ubiquitous. The relative strengths of the H α and the spectrally adjacent [N II] lines in the regions are completely discrepant from those associated with star formation regions, and more closely match expectations for 'LINER' regions. We quantify the total line emission from these extended, kpc-scale regions, and determine the spurious contribution it would make to the determined star formation rate of these galaxies if interpreted as normal H α emission. We concur with previous studies that link this 'LINER' emission to old stellar populations, e.g. post-asymptotic giant branch stars, and propose strongly barred early-type spirals as a prime location for further tests of such emission.