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

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.