Constraining the Nature of Two Ly-alpha Emitters detected by ALMA at z =
  4.7
  We report optical VLT FORS2 spectroscopy of the two Ly-alpha emitters (LAEs)
companions to the quasi-stellar object (QSO) - sub-millimetre galaxy (SMG)
system BRI1202-0725 at z = 4.7, which have recently been detected in the
[CII]158um line by the Atacama Large Millimetre/Sub-millimetre Array (ALMA). We
detect Ly-alpha emission from both sources and so confirm that these Ly-alpha
emitter candidates are physically associated with the BRI1202- 0725 system. We
also report the lack of detection of any high ionisation emission lines (N V,
Si IV, C IV and He II) and find that these systems are likely not photoionised
by the quasar, leaving in situ star formation as the main powering source of
these LAEs. We also find that both LAEs have Ly-alpha emission much broader
(1300 km/s) than the [CII] emission and broader than most LAEs. In addition,
both LAEs have roughly symmetric Ly-alpha profiles implying that both systems
are within the HII sphere produced by the quasar. This is the first time that
the proximity zone of a quasar is probed by exploiting nearby Ly-alpha
emitters. We discuss the observational properties of these galaxies in the
context of recent galaxy formation models.
