  An extremely deep 5 GHz radio observation is presented of the rich cluster
MS1054-03 at redshift z=0.83. 34 radio sources are detected down to a 32
micro-Jy (6 sigma), compared to about 25 expected from previous blank field
radio source count determinations; the sources giving rise to these excess
counts lie within 2 arcmins (700 kpc) of the cluster centre. Existing imaging
and spectroscopy has provided optical identifications for 21 of the radio
sources and redshifts for 11, of which 8 are confirmed cluster members. 4 of
these 8 confirmed cluster sources are associated with close galaxy pairs (10-25
kpc projected offset) of similar magnitude, implying that the radio source may
be triggered by an interaction. However, although MS1054-03 has a very high
fraction (17%) of on-going mergers (separations <~ 10 kpc), no radio emission
is detected towards any of these merger events, setting a mean upper limit of
10 Msun/yr for any star formation associated with these mergers. This supports
a hypothesis that low luminosity radio sources may be onset by initial weak
interactions rather than direct mergers. The host galaxies of the other four
confirmed cluster radio sources are all isolated, and show a range of
morphologies from early-type to Sc. A comparison between the emission line and
radio luminosities suggests that two of these four radio sources are
low-luminosity AGN, whilst for at least one of the other two the radio emission
is associated with on-going star formation. All of the radio sources associated
with the galaxy pairs appear more likely AGN than starburst origin. The overall
proportion of radio sources associated with AGN in this cluster (>75%) is
higher than at these flux density levels in the field (40-50%).
