Submillimeter Galaxies as Tracers of Mass Assembly at Large M
  Luminous submillimeter galaxies (SMGs) have been known for some time to produce a substantial fraction of the far-IR/submillimeter background. New observations show that these systems are not only very luminous, but also very massive. Here we present results from (a) a survey of SMGs' molecular line emission, which has confirmed Keck optical redshifts and revealed their high gas and dynamical masses; and (b) a detailed case study of the rest-frame optical emission from one SMG, which has revealed its high metallicity and old stellar population. We infer a high z~2.7 comoving number density of galaxies with baryonic masses greater than 10^11 solar masses; this is not accounted for in current semi-analytic models of galaxy evolution. 
R. Genzel, A.J. Baker, R.J. Ivison, F. Bertoldi, A.W. Blain, S.C.   Chapman, P. Cox, R.I. Davies, F. Eisenhauer, D.T. Frayer...