A submillimetre galaxy at z=4.76 in the LABOCA survey of the Extended
  Chandra Deep Field South
  We report on the identification of the highest redshift submm-selected source
currently known: LESSJ033229.4-275619. This source was detected in the Large
Apex BOlometer CAmera (LABOCA) Extended Chandra Deep Field South (ECDFS)
Submillimetre Survey (LESS), a sensitive 870-um survey (~1.2-mJy rms) of the
full 30'x30' ECDFS with the LABOCA camera on the Atacama Pathfinder EXperiment
(APEX) telescope. The submm emission is identified with a radio counterpart for
which optical spectroscopy provides a redshift of z=4.76. We show that the
bolometric emission is dominated by a starburst with a star formation rate of
~1000 Msun/yr, although we also identify a moderate luminosity Active Galactic
Nucleus (AGN) in this galaxy. Thus it has characteristics similar to those of
z~2 submm galaxies (SMGs), with a mix of starburst and obscured AGN signatures.
This demonstrates that ultraluminous starburst activity is not just restricted
to the hosts of the most luminous (and hence rare) QSOs at z~5, but was also
occurring in less extreme galaxies at a time when the Universe was less than
10% of its current age. Assuming that we are seeing the major phase of star
formation in this galaxy, then we demonstrate that it would be identified as a
luminous distant red galaxy at z~3 and that the current estimate of the space
density of z>4 SMGs is only sufficient to produce ~10% of the luminous red
galaxy population at these early times. However, this leaves open the
possibility that some of these galaxies formed through less intense, but more
extended star formation events. If the progenitors of all of the luminous red
galaxies at z~3 go through an ultraluminous starburst at z>4 then the required
volume density of z>4 SMGs will exceed that predicted by current galaxy
formation models by more than an order of magnitude.
