Astronomers Reveal Best Image Yet of Mysterious ORCs in Space

Press Release from SARAO

The MeerKAT Stokes-I image of ORC1, superimposed on optical data from the Dark Energy Survey DR1(Abbott et al. 2018), both spanning the same field of view. A square root transfer function was applied to the radio data of the field of view and this grayscale image was then adjusted for contrast. An image of the radio data confined to the ORC region was assigned mint green. This region was blended with the greyscale radio continuum field of view image and the DES optical image of the same field, so that faint radio sources outside the ORC appear as faint grey diffuse patches, often surrounding their host galaxies. The filters used in the DES image were assigned turquoise, magenta, yellow and red, with the result that DES sources mainly appear in this image as white. The layering schema employed is described in English (2017). This figure is optimised to convey the structure of the ORC, and quantitative information should be taken from Figure 1 or from the FITS files in the Supplementary Information.

Astronomy’s newest mystery objects, odd radio circles or ORCs, have been pulled into sharp focus by an international team of astronomers using the world’s most capable radio telescopes. First revealed by the ASKAP radio telescope, owned and operated by Australia’s national science agency CSIRO, odd radio circles quickly became objects of fascination. Theories on what caused them ranged from galactic shockwaves to the throats of wormholes.

A new detailed image, captured by the South African Radio Astronomy Observatory’s MeerKAT radio telescope and published in Monthly Notices of the Royal Astronomical Society (DOI 10.1093/mnras/stac701), is providing researchers with more information to help narrow down those theories.

There are now three leading theories to explain what causes ORCs:

  • They could be the remnant of a huge explosion at the centre of their host galaxy, like the merger of two supermassive black holes;
  • They could be powerful jets of energetic particles spewing out of the galaxy’s centre; or
  • They might be the result of a starburst ‘termination shock’ from the production of stars in the galaxy.

To date, ORCs have only been detected using radio telescopes, with no signs of them when researchers have looked for them using optical, infrared, or X-ray telescopes.

Dr Jordan Collier of the Inter-University Institute for Data-Intensive Astronomy, who compiled the image from MeerKAT data said continuing to observe these odd radio circles will provide researchers with more clues.

“People often want to explain their observations and show that it aligns with our best knowledge. To me, it’s much more exciting to discover something new, that defies our current understanding,” Dr Collier said. The rings are enormous – about a million light-years across, which is 16 times bigger than our own galaxy. Despite this, odd radio circles are hard to see.

(Left) The ASKAP image of ORC1, adapted from Norris et al. (2021b) at 944 MHz. The resolution is 11 arcsec, and the rms sensitivity is 25 𝜇Jy/beam. (Right) The new Meerkat image of ORC1 at 1284 MHz. The resolution is 6 arcsec, and the rms sensitivity is 2.4 𝜇Jy/beam

Professor Ray Norris from Western Sydney University and CSIRO, one of the authors on the paper, said astronomers have revealed only five odd radio circles in space. “We know ORCs are rings of faint radio emissions surrounding a galaxy with a highly active black hole at its centre, but we don’t yet know what causes them, or why they are so rare,”

Dr Fernando Camilo, Chief Scientist of the South African Radio Astronomy Observatory, which built and operates MeerKAT, said that “the ORC project is a great example of the clever use of MeerKAT by its users, playing to its strengths: ASKAP observes large swaths of the sky and can discover relatively rare types of objects; MeerKAT can then follow up to study them in greater detail.”

To understand ORCs scientists will need access to more sensitive radio telescopes such as those of the SKA Observatory. Over 12 countries including South Africa, Australia, the UK, France, Canada, China and India are supporting the SKAO

“No doubt the SKA telescopes, once built, will find many more ORCs and be able to tell us more about the lifecycle of galaxies,” Professor Norris said. “Until the SKA becomes operational, ASKAP and MeerKAT will revolutionise our understanding of the Universe faster than ever before.”

ASKAP is located in Wajarri Yamatji country in Western Australia. On the other hand, MeerKAT is located in the Northern Cape province of South Africa.

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