The Remnants of Intergalactic Supernovae

Intergalactic type-Ia supernovae (SNe) have been discovered recently in rich galaxy clusters. These SNe are likely the descendants of an intergalactic stellar population, which has been discovered 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. We find that, if type-Ia SNe still have a circumstellar medium (CSM) at the time of explosion, then their remnants are visible in the optical for ∼ 10 years, with properties similar to young galactic SNRs. In contrast with galactic SNRs, in which the ejecta from the explosion interacts with the interstellar medium (ISM), intracluster SNRs become undetectable in the optical band once their ejecta passes beyond the CSM and enters the hot and tenuous ICM. If type-Ia SNe have a CSM, there should be ∼ 150 young SNRs in the nearby Virgo cluster, with an Hα luminosity of ∼ 10 erg s and an angular size ∼ 0.1. We investigate the possibility that members of this SNR population may have recently been detected, but incorrectly identified as intergalactic H II regions. Alternatively, if optical intergalactic SNRs do not exist in Virgo, this will constitute evidence that type-Ia SNe are devoid of a CSM, with implications for progenitor scenarios. Regardless of the presence of a CSM, about 10 older SNRs per square degree should be detectable in Virgo in the radio band, with fluxes of order 0.1 mJy at 1 GHz. Their angular sizes, ∼ 1, morphologies, and lack of optical association with distant galaxies can distinguish them from the much more numerous background population. Their detection would provide an accurate measurement of the intracluster SN rate. Deep pointed observations toward the site of SN1980I, a possibly intergalactic type-Ia event in Virgo, could test for the existence of a CSM by comparison to our predictions for the early time development of intergalactic SNRs. Subject headings: supernova remnants — supernovae: general — galaxies: clusters: general