The Remnants of Intergalactic Supernovae

Intergalactic type-Ia supernovae (SNe-Ia) have been discovered recently in rich galaxy clusters, likely the descendants of an intergalactic stellar population found in recent years through a variety of tracers. We estimate the observational signatures of the associated SN remnants (SNRs) in the unusual intracluster medium (ICM) environment. If SNe-Ia still have a circumstellar medium (CSM) at the time of explosion, then their remnants are visible in the optical for ~100-1000 years, with properties similar to young galactic SNRs. In contrast with galactic SNRs, in which the ejecta from the explosion interacts with the ISM, intracluster SNRs become undetectable in the optical once their ejecta passes beyond the CSM and enters the hot and tenuous ICM. If SNe-Ia have a CSM, there should be ~150 young SNRs in the Virgo cluster, with L(H-alpha)~10^{35} erg/s and angular size ~0.1''. We investigate the possibility that members of this SNR population may have recently been detected, but incorrectly identified as intergalactic HII regions. Alternatively, if optical intergalactic SNRs do not exist in Virgo, this will be evidence that SNe-Ia are devoid of a CSM, with implications for progenitor scenarios. Regardless of the presence of a CSM, about 10 older radio SNRs per square degree should be detectable in Virgo, with fluxes of ~0.1 mJy at 1 GHz. Their angular sizes, morphologies, and lack of optical association with distant galaxies can distinguish them from the much more numerous background population. Their detection would provide a measurement of the intracluster SN rate. Observations toward the site of SN1980I, a possibly intergalactic Virgo SN-Ia, can test the existence of a CSM by comparison to our early-time predictions for intergalactic SNR development.