  We report a CO(3-2) detection of 23 molecular clouds in the extended
ultraviolet (XUV) disk of the spiral galaxy M83 with ALMA. The observed 1kpc^2
region is at about 1.24 times the optical radius (R25) of the disk, where
CO(2-1) was previously not detected. The detection and non-detection, as well
as the level of star formation (SF) activity in the region, can be explained
consistently if the clouds have the mass distribution common among Galactic
clouds, such as Orion A -- with star-forming dense clumps embedded in thick
layers of bulk molecular gas, but in a low-metallicity regime where their outer
layers are CO-deficient and CO-dark. The cloud and clump masses, estimated from
CO(3-2), range from 8.2x10^2 to 2.3x10^4 Msun and from 2.7x10^2 to 7.5x10^3
Msun, respectively. The most massive clouds appear similar to Orion A in star
formation activity as well as in mass, as expected if the cloud mass structure
is universal. The overall low SF activity in the XUV disk could be due to the
relative shortage of gas in the molecular phase. The clouds are distributed
like chains up to 600 pc (or longer) in length, suggesting that the trigger of
cloud formation is on large scales. The universal cloud mass structure also
justifies the use of high-J CO transitions to trace the total gas mass of
clouds, or galaxies, even in the high-z universe. This study is the first
demonstration that CO(3-2) is an efficient tracer of molecular clouds even in
low-metallicity environments.
