  We present deep Hubble Space Telescope NICMOS near-infrared and WFPC2 optical
imaging of a small region in the core of the distant rich cluster Cl0939+4713
(z=0.41). We compare the optical and near-infrared morphologies of cluster
members and find apparent small-scale optical structures within the galaxies
which are absent in the near-infrared. We conclude that strong dust obscuration
is a common feature in the late-type galaxies in distant clusters. We then
concentrate on a sample of ten faint radio galaxies lying within our NICMOS
field and selected from a very deep 1.4-GHz VLA map of the cluster with a
1sigma flux limit of 9uJy. Using published data we focus on the spectral
properties of the eight radio-selected cluster members and show that these
comprise a large fraction of the post-starburst population in the cluster. The
simplest interpretation of the radio emission from these galaxies is that they
are currently forming massive stars, contradicting their classification as
post-starburst systems based on the optical spectra. We suggest that this star
formation is hidden from view in the optical by the same obscuring dust which
is apparent in our comparison on the optical and near-infrared morphologies of
these galaxies. We caution that even in the restframe optical the effects of
dust cannot be ignored when comparing samples of distant galaxies to
low-redshift systems, particularly if dust is as prevelant in distant galaxies
as appears to be the case in our study.

Astrophysics