CHANG-ES. XXIV. First Detection of A Radio Nuclear Ring and Potential
  LLAGN in NGC 5792
  We report the discoveries of a nuclear ring of diameter 10$\arcsec$
($\sim$1.5 kpc) and a potential low luminosity active galactic nucleus (LLAGN)
in the radio continuum emission map of the edge-on barred spiral galaxy
NGC~5792. These discoveries are based on the Continuum Halos in Nearby Galaxies
- an Expanded Very Large Array (VLA) Survey, as well as subsequent VLA
observations of sub-arcsecond resolution. Using a mixture of H$\alpha$ and 24
$\mu$m calibration, we disentangle the thermal and non-thermal radio emission
of the nuclear region, and derive a star formation rate (SFR) of $\sim
0.4~M_{\sun}$ yr$^{-1}$. We find that the nuclear ring is dominated by
non-thermal synchrotron emission. The synchrotron-based SFR is about three
times of the mixture-based SFR. This result indicates that the nuclear ring
underwent more intense star-forming activity in the past, and now its star
formation is in the low state. The sub-arcsecond VLA images resolve six
individual knots on the nuclear ring. The equipartition magnetic field strength
$B_{\rm eq}$ of the knots varies from 77 to 88 $\mu$G. The radio ring surrounds
a point-like faint radio core of $S_{\rm 6GHz}=(16\pm4)$ $\mu$Jy with polarized
lobes at the center of NGC~5792, which suggests an LLAGN with an Eddington
ratio $\sim10^{-5}$. This radio nuclear ring is reminiscent of the Central
Molecular Zone (CMZ) of the Galaxy. Both of them consist of a nuclear ring and
LLAGN.
