Extragalactic nature of G227.1 + 1.0

Abstract

Green and Gull¹ have presented evidence that G227.1 + 1.0, one of the small-diameter radio sources in the Molonglo survey of Clark and Crawford², is a Crab-like supernova remnant. If this is the case, the object is an extremely important one, as Green and Gull emphasize¹. Such remnants are rare and are valuable as probes of the particle acceleration by pulsars. The very small diameter (∼ 1′) of G227.1 + 1.0 implies that it is one of the youngest of these remnants, perhaps only a few hundred years old. It could then represent the first known example of a Phase I Crab-like remnant³; that is, one for which the age is less than the slowing timescale of the underlying pulsar. Such a remnant would be uniquely important in helping to establish by observation an evolutionary picture of these objects (see ref. 4 for a theoretical discussion). Furthermore, the direction of G227.1 + 1.0—towards the galactic anti-centre—makes it a good candidate for optical study, as the usual problems of crowding and high extinction in the plane should be reduced. Here we report the discovery of a bright ( V = 17.4), diffuse optical object within a few arc seconds of the centre of the radio source, but we present spectroscopic evidence that it is a luminous external galaxy at z = 0.073. We argue, furthermore, that the original radio emission represents not a supernova remnant, but a radio halo surrounding this galaxy, similar to that around M87.