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Ionized gas extends over 40 kpc in an odd radio circle host galaxy
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Ionized gas extends over 40 kpc in an odd radio circle host galaxy
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Journal Article
Ionized gas extends over 40 kpc in an odd radio circle host galaxy
2024
Overview
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 galaxy
2
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.
We find that the optical emission due to ionized gas that is characteristic of a shock extends 40 kpc in diameter across the host galaxy of an odd radio circle.
