The radio properties of HERGs with intermediate radio powers. (arXiv:2002.07820v1 [astro-ph.GA])

The radio properties of HERGs with intermediate radio powers. (arXiv:2002.07820v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Pierce_J/0/1/0/all/0/1">Jonathon C. S. Pierce</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tadhunter_C/0/1/0/all/0/1">Clive N. Tadhunter</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Morganti_R/0/1/0/all/0/1">Raffaella Morganti</a>

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.

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.

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