MeerKAT discovery of 164 compact radio rings toward the Galactic Plane

TL;DR

This study leverages MeerKAT L-band surveys to blind-search for 164 compact ring-like radio structures in the Galactic Plane, uncovering a diverse population that includes potential circumstellar shells, HII regions, planetary nebulae, and background galaxies. By combining morphology, flux measurements, and multiwavelength footprint analyses (SIMBAD, 2MASS, GLIMPSE, and infrared data), the authors assess plausible formation scenarios—ranging from evolved-star mass loss to SNRs and gravitational lensing—while highlighting significant Galactic Center overdensities and the potential discovery space offered by SKA precursors. About half the rings have infrared counterparts, and approximately 60% receive tentative classifications, underscoring both the richness of the data and the need for targeted follow-up (spectral indices, multi-frequency radio data, and spectroscopy) to confirm physical natures. Overall, the work demonstrates MeerKAT’s power to reveal previously undetected compact radio structures and to probe missing Galactic radio-emitting objects, with implications for stellar evolution, ISM structure, and extragalactic phenomena.

Abstract

We report the discovery of 164 compact (radius < 1 arcmin) radio rings using MeerKAT 1.3 GHz data from the SARAO MeerKAT Galactic Plane Survey (l=2-60deg, 252-358deg, |b|<1.5deg) and the Galactic Centre mosaic, from a search aimed at identifying previously uncatalogued radio sources. Within this sample, approximately 19 per cent of the rings contain a central point radio source. A multiwavelength analysis reveals a striking diversity: about 40 per cent of the rings enclose an isolated infrared point source, 50 per cent exhibit an extended counterpart in the mid- or far-infrared, and several are only detected in the radio band. We found that 17 per cent of the rings in the sample are positionally coincident (within 5 arcsec) with known entries in SIMBAD, including unclassified infrared sources, spiral galaxies, young stellar objects and long-period variable candidates. Based on these matches and exploiting ancillary multiwavelength data and catalogues, we explore several formation scenarios for the rings, such as HII regions, planetary nebulae, mass-loss relics from evolved massive stars, supernova remnants, nova shells, galaxies, galaxy cluster lenses and odd radio circles. Tentative classifications are proposed for nearly 60 per cent of the sample. These results highlight the potential of MeerKAT to uncover previously undetected compact radio structures and, particularly, recover missing Galactic radio-emitting objects.

Figures (9)

• Figure 1: Top: sky distribution of the radio rings (SMGPS -- blue crosses, GC -- red crosses), overlaid on a 2D histogram representing the spatial density of extended sources in the SMGPS (colour scale), based on the catalogue by bor25. The histograms on top and right represent the marginal distribution of rings in $l$ and $b$. Bottom: histograms showing the distributions of ring angular radii (left) and flux densities (right) in the sample (cumulative histogram shown in grey). Vertical lines indicate the median values of each group.
• Figure 2: Examples of radio rings of type 1 (left), 2 (middle) and 3 (right). The name of each ring is shown in the top-left corner of each plot, with the morphological type indicated in the bottom-left. The scale bar represents 30 arcsec. Colour bar indicates surface brightness in units of mJy beam$^{-1}$.
• Figure 3: Spatial distribution of radio rings (red crosses) overlaid on the Galactic Centre mosaic hey2022. Major star-forming complexes and other prominent radio sources are marked for reference.
• Figure 4: Rings potentially associated with evolved massive stars. The MeerKAT L-band radio continuum is shown in blue colours, while infrared counterparts at different wavelengths (indicated in the top-left corner of each panel) are shown in greyscale. The contours have been chosen for each source to highlight the outline of the radio ring. The position of the coincident variable star is indicated as a white star.
• Figure 5: Examples of ring-like structures lacking multiwavelength counterparts. In the absence of further constraints, they may represent ORC candidates or compact SNRs with an unusually low brightness. RGB composite: R=70 $\mu$m, G=24 $\mu$m (except ring 109, where G=WISE 12 $\mu$m due to artifacts), B=8 $\mu$m. MeerKAT L-band radio continuum overlaid in turquoise.