A Shocked Wind Interpretation of an Odd Radio Circle

Abstract Odd radio circles (ORCs) are a new class of extragalactic object, with large rings of faint radio continuum emission typically spanning hundreds of kiloparsecs; their origins are unknown. Previous optical spectroscopy of the central galaxy in ORC4, a classic isolated ORC, revealed spatially extended ionized gas with strong O ii emission and line ratios consistent with LINER emission. We present new Keck/KCWI+KCRM integral field spectroscopy covering multiple strong optical emission lines to measure the extent, morphology, and spatially resolved kinematics and line ratios of the ionized and neutral gas in the ORC4 central galaxy. We find that O ii is the strongest optical emission line in this massive, old galaxy, and the O ii emission is detected to larger radial extent than the other optical lines. The gas kinematics show strong spatial asymmetries, high velocity gradients (>100 km s −1 ), and high velocity dispersion (∼200 km s −1 ). The emission-line ratios are most consistent with shock models with shock velocities of ∼200–300 km s −1 and are not fit well by stellar or active galactic nucleus photoionization models. These findings are consistent with a model in which the gas in the ORC4 central galaxy is the result of shock ionization in and around the central galaxy, likely due to mixing and cooling of gas associated with the event that created the large-scale radio ring of emission that identified this source as an ORC.