Odd radio circles (ORCs), first discovered by Norris and colleagues in ASKAP radio continuum data from the ‘Evolutionary Map of the Universe’ (EMU) Pilot Survey (800 – 1088 MHz; rms ∼30 𝜇Jy beam¯1), resemble rings or edge-brightened disks of radio emission that, so far, remain undetected at non-radio wavelengths. In their paper, Norris and colleagues presented four ORCs (each ∼60 arcsec in diameter), three of which were detected with ASKAP, including a pair of ORCs. The fourth ORC, which is notable for its central radio source, was discovered in 325 MHz radio data from the Giant Meterwave Radio Telescope (GMRT). Most notably, the two single ORCs each have an elliptical galaxy in their geometrical ring centre.
“ORC’s appear in radio images as circular edge-brightened discs about one arc-minute diameter, and do not seem to correspond to any known type of object.”
Odd Radio Circles – at first glance – look like supernova remnants (SNRs), so could they be formed by a giant blast wave from a transient event (e.g., a merging binary super-massive black hole (SMBH), a hyper-nova, or a 𝛾-ray burst) in the central elliptical galaxy many millions of years ago. In Norris et al. a wide range of possible formation scenarios are discussed, which are evaluated and expanded with each new ORC discovery.
Now, Koribalski and colleagues presented the discovery of Odd Radio Circle (ORC), ORC J0102–2450, with the Australian Square Kilometre Array Pathfinder (ASKAP) at 944 MHz, which makes it the third odd radio circle with an elliptical galaxy at its geometrical centre. They found that, ORC J0102– 2450, has a diameter of ∼70 arcsec or 300 kpc, if associated with the central elliptical galaxy DES J010224.33–245039.5 (𝑧 ∼ 0.27). Their study recently appeared on Journal Arxiv.
“This is unlikely a coincidence and bring us a step closer to determine the ORC formation mechanisms”— told Koribalski, first author of the study
They also discussed three ORC formation scenarios, two of which have the central galaxy as its basis. These are:
(1) A relic lobe of a giant radio galaxy seen end-on: One possible explanation is that ORCs are relic radio galaxy lobes seen end-on, possibly in a rare transition state where the central jet has switched off but the shells of fading lobes are still expanding into the surrounding intergalactic medium (IGM).
(2) A giant blast wave, possibly from a binary SMBH merger, resulting in a radio ring of 300 − 500 kpc diameter: ORCs could be the result of a giant blast wave in the central galaxy, producing a spherical shell/ring of radio emission of 300 − 500 kpc diameter. Such shells would appear as edge-brightened discs, similar to supernova remnants or planetary nebulae. The radio emission is presumably synchrotron emission from electrons accelerated by a shock. The edge-brightened emission is caused by the long path length through the limbs of the sphere, while the diffuse emission within the ring is caused by the front and back hemispheres of the shell. The thickness of the shell would determine the ratio of the ring emission brightness to the diffuse emission brightness. One way to produce such a spherical shock would be as the result of a binary SMBH merger in the central galaxy. In that case, they would expect to observe a largely tangential magnetic field in the ring, orthogonal to the velocity of the shock, similar to that of supernova remnants or cluster relics.
(3) Interaction Scenarios: which considers radio galaxy and IGM interactions, involving the companion galaxy, that may be able to create the observed ring structure.
“The discovery of further ORCs in the rapidly growing amount of wide-field radio continuum data from ASKAP and other telescopes will show if the above scenarios have any merit, contributing to exciting times in astronomy.”
Finally, they concluded that low-frequency LOFAR surveys at high-resolution (600) will be of particular interest, given the steep spectral index of known ORCs. Deep X-ray observations may also detect these energetic events as shown in the case of a giant relic radio galaxy by Tamhane et al. (2015).
About ASKAP: ASKAP is a new radio interferometer consisting if 36×12-m antennas, each equipped with wide-field Phased Array Feed (PAF), baselines out to 6.4 km, operating at frequencies from 700 MHz to 1.8 GHz.
Featured image: ORC J0102–2450. — ASKAP radio continuum contours overlaid onto an optical RGB colour image created from the Dark Energy Survey (DES) 𝑧𝑟𝑔-bands. The ASKAP radio contour levels are 0.045 (dark red), 0.065, 0.09, 0.12, 0.17, 0.22, 0.27, 0.4, 0.6 and 0.8 mJy beam¯1; the resolution is 1300 and the rms is ∼15𝜇Jy beam¯1. DES image cutouts of the three radio-detected galaxies and their average photometric redshifts are shown on the right side (top: background galaxy, middle: central galaxy, bottom: south-eastern galaxy) © Koribalski et al.
Reference: Baerbel S. Koribalski, Ray P. Norris, Heinz Andernach, Lawrence Rudnick, Stanislav Shabala, Miroslav Filipovic, Emil Lenc, “Discovery of a new extragalactic circular radio source with ASKAP: ORC J0102-2450”, ArXiv pp. 1-5, 2021. https://arxiv.org/abs/2104.13055
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