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; Kelly E Whalen

doi:10.1038/s41586-023-06752-8

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

Nature. 2024 Jan;625(7995):459-462. doi: 10.1038/s41586-023-06752-8. Epub 2024 Jan 8.

Authors

Affiliations

¹ Department of Astronomy and Astrophysics, University of California, La Jolla, CA, USA. acoil@ucsd.edu.
² Department of Astronomy and Astrophysics, University of California, La Jolla, CA, USA.
³ Department of Physics, Rhodes College, Memphis, TN, USA.
⁴ Space Telescope Science Institute, Baltimore, MD, USA.
⁵ Department of Astronomy, University of Wisconsin-Madison, Madison, WI, USA.
⁶ Department of Physics and Astronomy, Bates College, Lewison, ME, USA.
⁷ Center for Computational Astrophysics, Flatiron Institute, New York, NY, USA.
⁸ Centre for Astrophysics Research, University of Hertfordshire, Hatfield, UK.
⁹ Department of Physics and Astronomy, Dartmouth College, Hanover, NH, USA.
¹⁰ Department of Physics and Astronomy, Siena College, Loudonville, NY, USA.
¹¹ Department of Physics and Astronomy, University of Kansas, Lawrence, KS, USA.
¹² Department of Astronomy, University of Florida, Gainesville, FL, USA.

PMID: 38191936
DOI: 10.1038/s41586-023-06752-8

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.