Ionized gas extends over 40 kpc in an odd radio circle host galaxy

Alison L. Coil (), Serena Perrotta, David S. N. Rupke, Cassandra Lochhaas, Christy A. Tremonti, Aleks Diamond-Stanic, Drummond Fielding, James E. Geach, Ryan C. Hickox, John Moustakas, Gregory H. Rudnick, Paul Sell and Kelly E. Whalen
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Alison L. Coil: University of California
Serena Perrotta: University of California
David S. N. Rupke: Rhodes College
Cassandra Lochhaas: Space Telescope Science Institute
Christy A. Tremonti: University of Wisconsin-Madison
Aleks Diamond-Stanic: Bates College
Drummond Fielding: Flatiron Institute
James E. Geach: University of Hertfordshire
Ryan C. Hickox: Dartmouth College
John Moustakas: Siena College
Gregory H. Rudnick: University of Kansas
Paul Sell: University of Florida
Kelly E. Whalen: Dartmouth College

Nature, 2024, vol. 625, issue 7995, 459-462

Abstract: Abstract A new class of extragalactic astronomical sources discovered in 2021, named odd radio circles (ORCs)1, 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 galaxy2 but an order of magnitude smaller than the large-scale radio continuum emission. We detect an approximately 200 km s−1 velocity gradient across the [O ii] nebula, as well as a high velocity dispersion of approximately 180 km s−1. 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.

Date: 2024
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DOI: 10.1038/s41586-023-06752-8

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