The Remnants of Intergalactic Supernovae

Intergalactic type-Ia supernovae (SNe-Ia) have been discovered recently inrich galaxy clusters, likely the descendants of an intergalactic stellarpopulation found in recent years through a variety of tracers. We estimate theobservational signatures of the associated SN remnants (SNRs) in the unusualintracluster medium (ICM) environment. If SNe-Ia still have a circumstellarmedium (CSM) at the time of explosion, then their remnants are visible in theoptical for ~100-1000 years, with properties similar to young galactic SNRs. Incontrast with galactic SNRs, in which the ejecta from the explosion interactswith the ISM, intracluster SNRs become undetectable in the optical once theirejecta passes beyond the CSM and enters the hot and tenuous ICM. If SNe-Ia havea CSM, there should be ~150 young SNRs in the Virgo cluster, withL(H-alpha)~10^{35} erg/s and angular size ~0.1''. We investigate thepossibility that members of this SNR population may have recently beendetected, but incorrectly identified as intergalactic HII regions.Alternatively, if optical intergalactic SNRs do not exist in Virgo, this willbe evidence that SNe-Ia are devoid of a CSM, with implications for progenitorscenarios. Regardless of the presence of a CSM, about 10 older radio SNRs persquare 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 distantgalaxies can distinguish them from the much more numerous backgroundpopulation. Their detection would provide a measurement of the intracluster SNrate. Observations toward the site of SN1980I, a possibly intergalactic VirgoSN-Ia, can test the existence of a CSM by comparison to our early-timepredictions for intergalactic SNR development.Comment: ApJ, in press, 8 page