A catalogue of Galactic supernova remnants in the far-infrared: revealing ejecta dust in pulsar wind nebulae

We search for far-infrared counterparts of known supernova remnants (SNRs) in the Galactic plane (10 degrees < vertical bar l vertical bar < 60 degrees) at 70-500 mu m using the Herschel Infrared Galactic Plane Survey (Hi-GAL). Of 71 sources studied, we find that 29 (41 per cent) SNRs have a clear FIR detection of dust emission associated with the SNR. Dust from 8 of these is in the central region, and 4 indicate pulsar wind nebulae (PWNe) heated ejecta dust. A further 23 have dust emission in the outer shell structures which is potentially related to swept-up material. Many Galactic SNe have dust signatures but we are biased towards detecting ejecta dust in young remnants and those with a heating source (shock or PWN). We estimate the dust temperature and mass contained within three PWNe, G11.2-0.3, G21.5-0.9, and G29.7-0.3, using modified blackbody fits. To more rigorously analyse the dust properties at various temperatures and dust emissivity index beta, we use point process mapping (PPMAP). We find significant quantities of cool dust (at 20-40 K) with dust masses of M-d = 0.34 +/- 0.14 M-circle dot, M-d = 0.29 +/- 0.08 M-circle dot, and M-d = 0.51 +/- 0.13 M-circle dot for G11.2-0.3, G21.5-0.9, and G29.7-0.3, respectively. We derive the dust emissivity index for the PWN ejecta dust in G21.5-0.3 to be beta = 1.4 +/- 0.5 compared to dust in the surrounding medium where beta = 1.8 +/- 0.1.