Ionized gas extends over 40 kpc in an odd radio circle host galaxy.
basic_science · Level V
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- Record sourced from PubMed, PMID 38191936.
- Also identified by DOI 10.1038/s41586-023-06752-8.
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Abstract
A new class of extragalactic astronomical sources discovered in 2021, named odd radio circles (ORCs)<sup>1</sup>, are large rings of faint, diffuse radio continuum emission spanning approximately 1 arcminute on the sky. Galaxies at the centres of several ORCs have photometric redshifts of z ≃ 0.3-0.6, implying physical scales of several 100 kpc in diameter for the radio emission, the origin of which is unknown. Here we report spectroscopic data on an ORC including strong [O II] emission tracing ionized gas in the central galaxy of ORC4 at z = 0.4512. The physical extent of the [O II] emission is approximately 40 kpc in diameter, larger than expected for a typical early-type galaxy<sup>2</sup> but an order of magnitude smaller than the large-scale radio continuum emission. We detect an approximately 200 km s<sup>-1</sup> velocity gradient across the [O II] nebula, as well as a high velocity dispersion of approximately 180 km s<sup>-1</sup>. The [O II] equivalent width (approximately 50 Å) is extremely high for a quiescent galaxy. The morphology, kinematics and strength of the [O II] emission are consistent with the infall of shock ionized gas near the galaxy, following a larger, outward-moving shock. Both the extended optical and radio emission, although observed on very different scales, may therefore result from the same dramatic event.