Ionized Gas Extended Over 40 kpc in an Odd Radio Circle Host Galaxy

Coil, Alison L., Perrotta, Serena, Rupke, David S. N., Lochhaas, Cassandra, Tremonti, Christy A., Diamond-Stanic, Aleks, Fielding, Drummond, Geach, Jim, Hickox, Ryan C., Moustakas, John, Rudnick, Gregory H., Sell, Paul and Whalen, Kelly E. (2024) Ionized Gas Extended Over 40 kpc in an Odd Radio Circle Host Galaxy. Nature, 625. 459–462. ISSN 0028-0836
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A new class of extragalactic astronomical sources discovered in 2021, named Odd Radio Circles (ORCs, Norris et al. 2021), are large rings of faint, diffuse radio continuum emission spanning ~1 arcminute on the sky. Galaxies at the centers of several ORCs have photometric redshifts of z~0.3-0.6, implying physical scales of several 100 kiloparsecs in diameter for the radio emission, the origin of which is unknown. Here we report spectroscopic data on an ORC including strong [OII] emission tracing ionized gas in the central galaxy of ORC4 at z=0.4512. The physical extent of the [OII] emission is ~40 kpc in diameter, larger than expected for a typical early-type galaxy (Pandya et al, 2017) but an order of magnitude smaller than the large-scale radio continuum emission. We detect a ~200 km/s velocity gradient across the [OII] nebula, as well as a high velocity dispersion of ~180 km/s. The [OII] equivalent width (EW, ~50 Ang) is extremely high for a quiescent galaxy. The morphology, kinematics, and strength of the [OII] emission are consistent with the infall of shock ionized gas near the galaxy, following a larger-scale, outward moving shock driven by a galactic wind. Both the extended optical and radio emission, while observed on very different scales, may therefore result from the same dramatic event.


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