We present an Atacama Large Millimeter/submillimeter Array (ALMA) Cycle
0 survey of 126 submillimeter sources from the LABOCA ECDFS
Submillimeter Survey (LESS). Our 870 μm survey with ALMA (ALESS) has
produced maps ~3× deeper and with a beam area ~200× smaller
than the original LESS observations, doubling the current number of
interferometrically-observed submillimeter sources. The high resolution
of these maps allows us to resolve sources that were previously blended
and accurately identify the origin of the submillimeter emission. We
discuss the creation of the ALESS submillimeter galaxy (SMG) catalog,
including the main sample of 99 SMGs and a supplementary sample of 32
SMGs. We find that at least 35% (possibly up to 50%) of the detected
LABOCA sources have been resolved into multiple SMGs, and that the
average number of SMGs per LESS source increases with LESS flux density.
Using the (now precisely known) SMG positions, we empirically test the
theoretical expectation for the uncertainty in the single-dish source
positions. We also compare our catalog to the previously predicted
radio/mid-infrared counterparts, finding that 45% of the ALESS SMGs were
missed by this method. Our ~1.''6 resolution allows us to measure a size
of ~9 kpc × 5 kpc for the rest-frame ~300 μm emission region in
one resolved SMG, implying a star formation rate surface density of 80 M
⊙ yr-1 kpc-2, and we
constrain the emission regions in the remaining SMGs to be