We present the first results from our ongoing Australia Telescope
Compact Array survey of 12CO(1-0) in Atacama Large Millimeter
Array (ALMA)-identified submillimetre galaxies (SMGs) in the Extended
Chandra Deep Field South. Strong detections of 12CO(1-0)
emission from two SMGs, ALESS 122.1 (z = 2.0232) and ALESS 67.1 (z =
2.1230), were obtained. We estimate gas masses of Mgas
˜ 1.3 × 1011 M⊙ and
Mgas ˜ 1.0 × 1011 M⊙
for ALESS 122.1 and ALESS 67.1, respectively, adopting
αCO = 1.0. Dynamical mass estimates from the kinematics
of the 12CO(1-0) line yields Mdyn sin2
I = (2.1 ± 1.1) × 1011 M⊙ and
(3.2 ± 0.9) × 1011 M⊙ for ALESS
122.1 and ALESS 67.1, respectively. This is consistent with the total
baryonic mass estimates of these two systems. We examine star formation
efficiency, using the LFIR versus L^' }_{CO(1-0)} relation
for samples of local ultraluminous infrared galaxies (ULIRGs) and
Luminous Infrared Galaxies (LIRGs), and more distant star-forming
galaxies, with 12CO(1-0) detections. We find some evidence of
a shallower slope for ULIRGs and SMGs compared to less luminous systems,
but a larger sample is required for definite conclusions. We determine
gas-to-dust ratios of 170 ± 30 and 140 ± 30 for ALESS
122.1 and ALESS 67.1, respectively, showing that ALESS 122.1 has an
unusually large gas reservoir. By combining the 38.1 GHz continuum
detection of ALESS 122.1 with 1.4 and 5.5 GHz data, we estimate that the
free-free contribution to radio emission at 38.1 GHz is 34 ± 17
μJy, yielding a star formation rate (1400 ± 700
M⊙ yr-1) consistent with that from the
infrared luminosity.