CHANG-ES. XXIV. First Detection of a Radio Nuclear Ring and Potential LLAGN in NGC 5792

Abstract

We report the discoveries of a nuclear ring of diameter 10″ (∼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 Hα and 24 μm calibrations, we disentangle the thermal and nonthermal radio emission of the nuclear region and derive a star formation rate (SFR) of ∼0.4 M ☉ 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 μG. The radio ring surrounds a point-like faint radio core of S 6 GHz = (16 ± 4) μJy with polarized lobes at the center of NGC 5792, which suggests an LLAGN with an Eddington ratio of ∼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.

CHANG-ES. XXX. 10 kpc Radio Lobes in the Sombrero Galaxy

“…Figure 1(c) shows the linear polarization map obtained from our D-configuration L-band data, the map is superimposed with the magnetic field orientations and Stokes I contours. The vertically oriented cylindrical structure along the jet axis reveals the bipolar polarization lobes, which is AGN related (Yang et al 2022). These polarization segments are perpendicular to the bipolar polarization lobes, indicating the presence of a helical/toroidal magnetic field component traveling outward with the jet.…”

Effects of Magnetic Fields on Gas Dynamics and Star Formation in Nuclear Rings

“…Nuclear rings at the centers of barred galaxies are active in star formation (Mazzuca et al 2008;Ma et al 2018) and threaded by magnetic fields with a mean strength of ∼50-100 μG (Beck et al 2005;Yang et al 2022). To study how magnetic fields affect star formation in nuclear rings, we run MHD simulations of galactic centers.…”

“…Observations show that nuclear rings in real galaxies are quite strongly magnetized. Assuming energy equipartition between magnetic fields and cosmic rays (CRs), the average magnetic field strengths in nuclear rings inferred from radio synchrotron observations are estimated to be ∼55 μG for NGC 1097 (Beck et al 2005), ∼63 μG for NGC 1365 (Beck et al 2005), and ∼84 μG for NGC 5792 (Yang et al 2022), much stronger than in spiral arms of normal disk galaxies (Beck 2015). Beck et al (1999Beck et al ( , 2005 mapped radio continuum emission in barred galaxies and found that the magnetic fields are predominantly parallel to the dust lanes, while penetrating the nuclear rings with a large pitch angle (∼40°).…”

CHANG-ES XXXI—A Decade of CHANG-ES: What We Have Learned from Radio Observations of Edge-on Galaxies

“…On the left, polarization contours plus B-field orientations over a grayscale optical SDSS image of NGC 5792, from [39]. On the right, L-band total intensity contours over the B/L to C/C spectral index map of NGC 3628 [24].…”