Cold dust in (some) high-z supernova host galaxies

We present deep submillimetre photometry for 14 galaxies at z = 0.5 that are hosts of Type la supernovae, with the aim of examining the evolution of dust mass and extinction in normal galaxies. We combine these results with our previous observations of 17 z similar to 0.5 SN1a hosts to look for any evolution in the dust content of normal galaxies between z = 0 and 0.5. The average observed-frame 850-1 mu m flux of SN1a hosts in the full sample, excluding two bright individually detected objects, is 0.44 +/- 0.22 mJy. This flux level is consistent with there being little or no evolution in the dust content, or optical extinction, of normal galaxies from z = 0 to 0.5. One galaxy, the host of SN1996cf, is detected individually, and we also present a deep Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) image for this object. It appears to be an edge-on disc system, similar to the submillimetre bright host of SN1997ey. We thus examine the dust properties of these and one other individually detected object. Flux ratios and limits of 450-850 mu m suggest that the dust in the two brightest submillimetre sources, SN1996cf and SN1997ey, is cold, T similar to 20 K, implying that they contain a substantial mass of dust similar to 10(9) M-circle dot The presence of two bright (F-850 > 7 mJy) submillimetre sources at z similar to 0.5 in a sample of ostensibly normal galaxies is surprising, and has important implications. It supports the idea that a substantial part of the cosmic infrared background (CIB) may be produced at z < 1, while also suggesting that 'foreground' objects such as these may be a significant 'contaminant' in submillimetre surveys. Finally, we examine the overall submillimetre luminosity distribution at z = 0.5 implied by our results, and conclude that either there is substantial evolution in the submillimetre luminosity function from z = 0 to 0.5, or our submillimetre-detected sources are somehow not representative of the bulk of galaxies at this redshift.