Disc colours in field and cluster spiral galaxies at 0.5 < z < 0.8
  We present a detailed study of the colours in late-type galaxy discs for ten
of the EDisCS galaxy clusters with 0.5 < z < 0.8. Our cluster sample contains
172 spiral galaxies, and our control sample is composed of 96 field disc
galaxies. We deconvolve their ground-based V and I images obtained with FORS2
at the VLT with initial spatial resolutions between 0.4 and 0.8 arcsec to
achieve a final resolution of 0.1 arcsec with 0.05 arcsec pixels, which is
close to the resolution of the ACS at the HST. After removing the central
region of each galaxy to avoid pollution by the bulges, we measured the V-I
colours of the discs. We find that 50% of cluster spiral galaxies have disc V-I
colours redder by more than 1 sigma of the mean colours of their field
counterparts. This is well above the 16% expected for a normal distribution
centred on the field disc properties. The prominence of galaxies with red discs
depends neither on the mass of their parent cluster nor on the distance of the
galaxies to the cluster cores. Passive spiral galaxies constitute 20% of our
sample. These systems are not abnormally dusty. They are are made of old stars
and are located on the cluster red sequences. Another 24% of our sample is
composed of galaxies that are still active and star forming, but less so than
galaxies with similar morphologies in the field. These galaxies are naturally
located in the blue sequence of their parent cluster colour-magnitude diagrams.
The reddest of the discs in clusters must have stopped forming stars more than
~5 Gyr ago. Some of them are found among infalling galaxies, suggesting
preprocessing. Our results confirm that galaxies are able to continue forming
stars for some significant period of time after being accreted into clusters,
and suggest that star formation can decline on seemingly long (1 to 5 Gyr)
timescales.
