  We have detected the CO(2-1) transition from the submillimetre galaxy (SMG)
LESSJ033229.4-275619 at z=4.755 using the new Compact Array Broadband Backend
system on the Australian Telescope Compact Array. These data have identified a
massive gas reservoir available for star formation for the first time in an SMG
at z~5. We use the luminosity and velocity width (FWHM of 160 km/s) of the
CO(2--1) line emission to constrain the gas and dynamical mass of
Mgas~1.6x10^10 Msun and Mdyn(<2kpc)~5x10^10 (0.25/sin^2(i)) Msun, respectively,
similar to that observed for SMGs at lower redshifts of z~2-4, although we note
that our observed CO FWHM is a factor of ~3 narrower than typically seen in
SMGs. Together with the stellar mass we estimate a total baryonic mass of
Mbary~1x10^11 Msun, consistent with the dynamical mass for this young galaxy
within the uncertainties. Dynamical and baryonic mass limits of high-redshift
galaxies are useful tests of galaxy formation models: using the known z~4-5
SMGs as examples of massive baryonic systems, we find that their space density
is consistent with that predicted by current galaxy formation models. In
addition, these observations have helped to confirm that z~4-5 SMGs possess the
baryonic masses and gas consumption timescales necessary to be the progenitors
of the luminous old red galaxies seen at z~3. Our results provide a preview of
the science that ALMA will enable on the formation and evolution of the
earliest massive galaxies in the Universe.
