This work sits alongside studies of Odd Radio Circles (ORCs) and giant radio halos, which are being actively investigated with ASKAP, MeerKAT, and other instruments. Neighboring papers describe radio jets, pulsars, quasars, and shock waves in clusters, but this paper stands out for discovering an object that combines the size of an ORC with an unusual two-shell morphology. This makes it a contribution to the growing family of ORC-like radio bubbles around nearby galaxies.
We used ASKAP EMU continuum data at 944 MHz with a resolution of 15″ and a sensitivity of 30 μJy/beam, supplemented by low-frequency MWA GLEAM-X images at 88–185 MHz. For the galaxy LEDA 217397, an optical spectrum was obtained with the ANU 2.3 m telescope (WiFeS), confirming a redshift of 0.040. Photometry in ten bands (g, r, i, z, J, H, Ks, W1, W2, W3) allowed us to build the spectral energy distribution and estimate the mass, star formation rate, and nuclear activity. The morphology is described as two circles of radius 114 kpc with centers separated by ~171 kpc.
The bubble has an integrated flux density of 56.8 ± 2.9 mJy at 944 MHz, corresponding to a luminosity of ~2.1×10²³ W/Hz at that frequency and ~1.4×10²³ W/Hz at 1.4 GHz. The spectral index between 154 and 944 MHz is −1.04 ± 0.04, and the spectral index map shows flattening toward the edge (from −1.3 in the center to −0.5…−0.6 at the periphery). The host galaxy is massive (log M*/M⊙ = 10.97 ± 0.09), forming almost no stars (SFR = 0.025 ± 0.083 M⊙/yr), and shows no signs of an active nucleus in the mid-infrared. There is no diffuse X-ray emission inside the bubble, and two faint eROSITA point sources are most likely background objects.
Unlike neighboring works, where ORC structures are often linked to shock waves from group or cluster mergers, here we find an object in a poor environment with almost perfectly round double shells. This challenges the universality of the merger scenario and supports the bipolar outflow model from an active nucleus. The work extends the class of ORC-like radio bubbles to closer and brighter objects, opening the possibility of detailed study of their physics.
The topic could develop through searches for similar double shells in ASKAP surveys and the future SKA. Comparison with simulations of jets and shock waves will help clarify how often relic active galactic nucleus bubbles take such a shape. It is possible that some known ORCs are actually double structures that appeared as single rings at low resolution.
The results will affect the interpretation of ORCs and giant radio halos, as well as the understanding of active galactic nucleus feedback in massive galaxies. The object will become a benchmark for planning deep observations with new radio telescopes.
Deep, high-resolution, broadband radio images are needed to search for a compact core or jets, polarimetry to map the magnetic field, spectroscopy of neighboring galaxies to check for a hidden group, and X-ray observations to detect hot gas.
This work is directly related to the unresolved problem of the origin of Odd Radio Circles (ORCs) and the role of active nuclei in shaping large-scale structures in the intergalactic medium. The absence of diffuse X-ray emission and the poor environment challenge models that require a dense medium or a recent merger, and highlight the need to distinguish between the contributions of shock waves and relic jets.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The study analyzes ASKAP EMU data at 944 MHz and MWA GLEAM-X, revealing a diffuse radio structure about 400 kpc across, consisting of two overlapping shells around the massive quiescent elliptical galaxy LEDA 217397 (z=0.040). A steep spectral index of α≈−1.04 is measured, with a flattening toward the edges, and the luminosity, galaxy mass, star formation rate, and nuclear activity are estimated. The authors conclude that the structure is likely a bipolar nuclear outflow, a relic of past AGN activity.
Where its strength is
This work identifies a nearby, bright representative of ORC-like radio bubbles with an unusual double-shell morphology in a sparse environment, offering a detailed view of the physics of such structures and challenging the universality of the merger scenario.
Where it could go next
Worth trying
Why this may matter
Detailed studies of nearby ORC-like bubbles could shed light on feedback mechanisms from active galactic nuclei in quiescent galaxies and on the origin of the enigmatic odd radio circles.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.55, against a usual 0.64. It looks like few have drilled nearby.
The bubble's diameter is about 400,000 light-years, four times the diameter of our Galaxy. Yet its radio luminosity is comparable to that of an ordinary radio galaxy, and on the sky it covers an area dozens of times larger than the disk of the Moon.
In Stanisław Lem's novel Fiasco, giant bubbles left by the outflows of extraterrestrial civilizations are described. Here, a similar trace was likely left by a black hole at the center of a galaxy, but without any intelligent purpose.