The radio properties of HERGs with intermediate radio powers

Abstract

In the past decade, high sensitivity radio surveys have revealed that the local radio AGN population is dominated by moderate-to-low power sources with emission that is compact on galaxy scales. High-excitation radio galaxies (HERGs) with intermediate radio powers (22.5 $<$ log(L$_{\rm 1.4GHz}$) $<$ 25.0 W Hz$^{-1}$) form an important sub-group of this population, since there is strong evidence that they also drive multi-phase outflows on the scales of galaxy bulges. Here, we present high-resolution VLA observations at 1.5, 4.5 and 7.5 GHz of a sample of 16 such HERGs in the local universe ($z<0.1$), conducted in order to investigate the morphology, extent and spectra of their radio emission in detail, down to sub-kpc scales. We find that the majority (56 per cent) have unresolved structures at the limiting angular resolution of the observations ($\sim$0.3"). Although similar in the compactness of their radio structures, these sources have steep radio spectra and host galaxy properties that distinguish them from local low-excitation radio galaxies (LERGs) that are unresolved on similar scales. The remaining sources exhibit extended radio structures with projected diameters $\sim$1.4$-$19.0 kpc and a variety of morphologies: three double-lobed; two large-scale diffuse; one jetted and `S-shaped'; one undetermined. Only 19 per cent of the sample therefore exhibit the double-lobed/edge-brightened structures often associated with their counterparts at high and low radio powers: radio-powerful HERGs and Seyfert galaxies, respectively. Additional high-resolution observations are required to investigate this further, and to probe the $\lesssim$300 pc scales on which some Seyfert galaxies show extended structures.