We present deep submillimetre photometry for 14 galaxies at z=0.5 that are hosts of type 1a 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 \sim 0.5 SN1a hosts to look for any evolution in the dust content of normal galaxies between z=0 and z=0.5 .
The average observed frame 850 \mu m flux of SN1a hosts in the full sample , excluding 2 bright individually detected objects , is 0.44 \pm 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 z=0.5 .
One galaxy , the host of SN1996cf , is detected individually , and we also present a deep HST STIS image for this object .
It appears to be an edge on disk system , similar to the submm bright host of SN1997ey .
We thus examine the dust properties of these and one other individually detected object .
450-to-850 \mu m flux ratios and limits suggest that the dust in the two brightest submm sources , SN1996cf and SN1997ey , is cold , T \sim 20K , implying that they contain a substantial mass of dust \sim 10 ^ { 9 } M _ { \odot } .
The presence of two bright ( F _ { 850 } > 7mJy ) submm sources at z \sim 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 submm surveys .
Finally , we examine the overall submm luminosity distribution at z=0.5 implied by our results , and conclude that either there is substantial evolution in the submm luminosity function from z=0 to 0.5 , or our submm detected sources are somehow not representative of the bulk of galaxies at this redshift .