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 '' (similar to 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 subarcsecond resolution. Using a mixture of H alpha and 24 mu m calibrations, we disentangle the thermal and nonthermal radio emission of the nuclear region and derive a star formation rate (SFR) of similar to 0.4 M-circle dot yr(-1). We find that the nuclear ring is dominated by nonthermal synchrotron emission. The synchrotron-based SFR is about three times 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 subarcsecond VLA images resolve six individual knots on the nuclear ring. The equipartition magnetic field strength B-eq of the knots varies from 77 to 88 mu G. The radio ring surrounds a point-like faint radio core of S-6 GHz = (16 +/- 4) mu Jy with polarized lobes at the center of NGC 5792, which suggests an LLAGN with an Eddington ratio of similar to 10(-5). This radio nuclear ring is reminiscent of the Central Molecular Zone of the Galaxy. Both of them consist of a nuclear ring and LLAGN.