The prevalence of supernova remnants among unidentified Galactic radio sources

Motivated by a desire to identify new Crab-like and composite supernova remnants, we have mapped nine galactic radio sources selected on the basis of existing stringent upper limits on their hydrogen recombination line fluxes. We have found in this small sample one new Crab-like remnant, one new composite remnant, at least one, and probably two, new shell-like remnants, and a compact H II region along with the expected collection of extragalactic objects. Follow-up observations of the composite source, G16.73+0.08, show that its core is highly polarized and has a spectral index of -0.15 from 90 cm to 2 cm; with a 6 cm flux of only 100 mJy, this remnant contains the faintest Crab-like component observed to date. Maps of the remaining extended sources are also presented. One implication of these results is that there are several hundred SNRs in the Galaxy which are detectable with current instruments but which have yet to be identified.