Very Large Array observations of the 8 o'clock arc lens system: Radio
  emission and a limit on the star-formation rate
  The 8 o'clock arc is a gravitationally lensed Lyman Break Galaxy (LBG) at
redshift z=2.73 that has a star-formation rate (SFR) of 270 solar-mass/year
(derived from optical and near-infrared spectroscopy). Taking the magnification
of the system ~12 and the SFR into account, the expected flux density of any
associated radio emission at 1.4 GHz is predicted to be just 0.1 mJy. However,
the lens system is found to be coincident with a radio source detected in the
NRAO Very Large Array (VLA) Sky Survey with a flux density of ~5 mJy. If this
flux density is attributed to the lensed LBG then it would imply a SFR ~11000
solar-mass/year, in contrast with the optical and near-infrared derived value.
We want to investigate the radio properties of this system, and independently
determine the SFR for the LBG from its lensed radio emission. We have carried
out new high resolution imaging with the VLA ain A and B-configurations at 1.4
and 5 GHz. We find that the radio emission is dominated by a radio-loud AGN
associated with the lensing galaxy. The radio-jet from the AGN partially covers
the lensed arc of the LBG, and we do not detect any radio emission from the
unobscured region of the arc down to a 3 sigma flux-density limit of 108
micro-Jy/beam. Using the radio data, we place a limit of <750 solar-mass/year
for the SFR of the LBG, which is consistent with the results from the optical
and near-infrared spectroscopy. We expect that the sensitivity of the Expanded
VLA will be sufficient to detect many high redshift LBGs that are
gravitationally lensed after only a few hours of observing time. The high
angular resolution provided by the EVLA will also allow detailed studies of the
lensed galaxies and determine if there is radio emission from the lens.
