RAD@home discovery of extragalactic radio rings and odd radio circles: clues to their origins

TL;DR

This work identifies three rare extragalactic radio-ring systems discovered by RAD@home in low-frequency surveys, including the most distant and powerful ORC known to date. It tests three formation channels—bipolar superwind–driven twin rings, diverted backflows at cluster outskirts, and jet–gas/jet–galaxy interactions—within dense group/cluster environments. The results link ring morphologies to environmental density and dynamical interactions, providing crucial observational constraints on aging synchrotron plasmas and the mechanisms that revive or reshape them. The study also underscores the enduring value of human pattern recognition for identifying rare, complex radio structures that automated pipelines overlook, guiding future ML training and discovery strategies.

Abstract

We present three rare and striking extragalactic radio sources discovered through visual inspection of low-frequency continuum maps from LoTSS DR2 and TGSS by the RAD@home citizen-science collaboratory. The first, RAD J131346.9+500320, is the first clear Odd Radio Circle (ORC) identified in LoTSS. At photometric $z \sim$ 0.94, it hosts a pair of intersecting rings of ~300 kpc diameter, embedded in diffuse emission extending over ~800 kpc, making it both the most distant and most powerful ORC reported to date. Its steep spectrum $α_{54}^{144}=1.22\pm0.15$) points to a relic synchrotron origin. The second object, RAD J122622.6+640622, is a ~865 kpc giant radio galaxy whose southern jet is abruptly deflected, inflating a ~100 kpc limb-brightened ring, while the northern jet terminates in a compact hotspot-like feature. The third, RAD J142004.0+621715 (~440 kpc), shows a comparable ring at the end of its northern filamentary jet, along with a secondary filament parallel to its southern jet. All three systems lie in $\sim10^{14}M_\odot$ clusters or group-scale haloes, suggesting that environmental density gradients and possible jet-galaxy interactions play a central role in shaping these ring morphologies. These discoveries expand the zoo of extragalactic radio morphologies, highlight the diversity of pathways that can generate ring-like synchrotron structures, and demonstrate the continuing importance of human pattern recognition in identifying rare sources that escape current automated pipelines.

Figures (6)

• Figure 1: (a):LoTSS 144 MHz radio image with 6$^{\prime\prime}$ angular resolution of RAD J131346.9+500320, where only emission above $3\sigma$ is shown, and the $\sigma$ (rms noise) is $\sim$ 50 $\muup$Jy beam$^{-1}$. The white cross shows the location of the object's host galaxy. (b): Edge-enhanced radio map of RAD J131346.9+500320 (Sec. \ref{['sec:orc']}). The image was convolved with the standard $3\times3$ Sobel kernels in the horizontal ($G_x$) and vertical ($G_y$) directions, and the resulting pixel intensity is given by $\sqrt{G_x^2 + G_y^2}$, thereby highlighting sharp brightness gradients and filamentary structures. (c) LoTSS 144 MHz radio contours ($10^{-3} \times$ [0.15, 0.19, 0.26, 0.39, 0.49, 0.62, 0.81, 1.01, 1.41, 1.89, 2.54, 4.66, 8.63, 16.07, 21.96, 30.01] Jy beam$^{-1}$) at $6^{\prime\prime}$ resolution in white, overlaid on the BASS optical image. Two green circles highlight the rings; cyan dashed circles mark galaxies at similar redshifts (for more, see Tab. \ref{['tab:radorcenv']}). (d) Same as (c), but galaxies are colour-coded by redshift (see colour bar). The two large dashed green circles again indicate the rings.
• Figure 2: Left:LoTSS 6$^{\prime\prime}$ radio image of RAD J122622.6+640622 showing emission above $3\sigma$, where the $\sigma$ (rms noise) is $\sim$ 50 $\muup$Jy beam$^{-1}$. The green circle shows the location of host BCG. Right: LoTSS 6$^{\prime\prime}$ resolution radio contours in white (levels = $10^{-3} \times [0.24,\,0.27,\,0.33,\,0.45,\,0.72,\,1.30,\,2.55,\,5.25,\,23.57]$ Jy beam$^{-1}$) overlaid on the BASS RGB optical image. Galaxy 1 is a neighbouring system at a similar redshift as the host BCG, while Galaxies 2 and 3 are foreground objects unrelated to the source.
• Figure 3: Left:LoTSS 6$^{\prime\prime}$ radio image of RAD J142004.0+621715 showing emission above $3\sigma$, where the $\sigma$ (rms noise) is $\sim$ 66 $\muup$Jy beam$^{-1}$. The green circle shows the location of the host BCG. The white cross indicates the location of the edge-on disk galaxy as described in Sec. \ref{['sec:filamentring']} and shown in more detail in Fig. \ref{['fig:ring2_north']}. Right: LoTSS $6^{\prime\prime}$ resolution radio contours in white (levels = $10^{-3} \times [0.19,\,0.30,\,0.49,\,0.91,\,1.29,\,1.86,\,2.73,\,4.01,\,5.95,\,8.85,\,13.22,\,19.77,\,29.60,\,44.37]$ Jy beam$^{-1}$) overlaid on the BASS RGB optical image.
• Figure 4: Similar to the right sub-figure of Fig. \ref{['fig:rad-ring2']} but zoomed in on the northern ring region to highlight structural details around the companion galaxy ($RA = 14^{\rm h}20^{\rm m}06\fs87$, $Dec =+62^{\circ}18'19\farcs1$).
• Figure 5: A false colour radio image from LoTSS with 6$^{\prime\prime}$ angular resolution has been superposed with contours (yellow) of the optical image from BASS. Galaxies which are possible members of the cluster have been marked with a red circle.