  We present sub-arcsecond resolution IRAM PdBI interferometry of eight
submillimeter galaxies at redshifts from 2 to 3.4, where we detect continuum at
1mm and/or CO lines at 3 and 1 mm. The CO 3-2/4-3 line profiles in five of the
sources are double-peaked, indicative of orbital motion either in a single
rotating disk or of a merger of two galaxies. The millimeter line and continuum
emission is compact; we marginally resolve the sources or obtain tight upper
limits to their intrinsic sizes in all cases. The median FWHM diameter for
these sources and the previously resolved sources, SMMJ023952-0136 and
SMMJ140104+0252 is less than or equal to 0.5" (4 kpc). The compactness of the
sources does not support a scenario where the far-IR/submm emission comes from
a cold, very extended dust distribution. These measurements clearly show that
the submillimeter galaxies we have observed resemble scaled-up and more gas
rich versions of the local Universe, ultra-luminous galaxy (ULIRG) population.
Their central densities and potential well depths are much greater than in
other redshift 2-3 galaxy samples studied so far. They are comparable to those
of elliptical galaxies or massive bulges. The SMG properties fulfill the
criteria of 'maximal' starbursts, in which most of the available initial gas
reservoir of 10^10-10^11 solar masses is converted to stars on a few dynamical
timescales.
