  We study the nature of rapidly star-forming galaxies at z=2 in cosmological
hydrodynamic simulations, and compare their properties to observations of
sub-millimetre galaxies (SMGs). We identify simulated SMGs as the most rapidly
star-forming systems that match the observed number density of SMGs. In our
models, SMGs are massive galaxies sitting at the centres of large potential
wells, being fed by smooth infall and gas-rich satellites at rates comparable
to their star formation rates (SFR). They are not typically undergoing major
mergers that significantly boost their quiescent SFR, but they still often show
complex gas morphologies and kinematics. Our simulated SMGs have stellar masses
of log M*/Mo~11-11.7, SFRs of ~180-500 Mo/yr, a clustering length of 10 Mpc/h,
and solar metallicities. The SFRs are lower than those inferred from far-IR
data by a factor of 3, which we suggest may owe to one or more systematic
effects in the SFR calibrations. SMGs at z=2 live in ~10^13 Mo halos, and by
z=0 they mostly end up as brightest group galaxies in ~10^14 Mo halos. We
predict that higher-M* SMGs should have on average lower specific SFRs, less
disturbed morphologies, and higher clustering. We also predict that deeper
far-IR surveys will smoothly join SMGs onto the massive end of the SFR-M*
relationship defined by lower-mass z=2 galaxies. Overall, our simulated rapid
star-formers provide as good a match to available SMG data as merger-based
scenarios, offering an alternative scenario that emerges naturally from
cosmological simulations.
