Odd radio circles as supernovae remnants in the intragroup medium

Abstract

A measurable fraction (∼8 per cent) of recently discovered arcmin-size circular diffuse radio sources termed as odd radio circles or ORCs can be supernovae remnants in the intragroup medium, within the Local Group and its immediate neighbour groups of galaxies. This estimate is based on the optical detection rate of the intragroup supernovae events in the nearby (z ∼ 0.1−0.2) galaxy groups. A rate of about 5400 intragroup supernovae per million year is expected within the local and its immediate neighbour groups of galaxies. For a radio detectability period of about 104 yr, on average 1.3 intragroup medium supernovae remnants per 1000 deg2 are expected to be detected in the radio surveys with a sensitivity that led to discovery of ORCs. The angular size, surface brightness, and radio flux of the supernova remnants up to a distance of ∼3 Mpc in the intragroup medium can be expected to be similar to the five known ORCs. The intragroup supernovae remnants are not residing in the dense and cold interstellar medium of the galaxies but evolving in low-density (10−4−10−5 cm−3) warm medium (105−106 K) in galactic haloes or beyond, and may find their progenitors in the diffuse stellar light associated with various tidal streamers surrounding the Milky Way and other nearby galaxies