GMC Evolutions in the Nearby Spiral Galaxy M33
  We present a Giant Molecular Cloud (GMC) catalog toward M33, containing 71
GMCs in total, based on wide field and high sensitivity CO(J=3-2) observations
with a spatial resolution of 100 pc using the ASTE 10 m telescope. Employing
archival optical data, we identify 75 young stellar groups (YSGs) from the
excess of the surface stellar density, and estimate their ages by comparing
with stellar evolution models. A spatial comparison among the GMCs, YSGs, and
HII regions enable us to classify GMCs into four categories: Type A showing no
sign of massive star formation (SF), Type B being associated only with HII
regions, Type C with both HII regions and <10 Myr-old YSGs and Type-D with both
HII regions and 10--30 Myr YSGs. Out of 65 GMCs (discarding those at the edges
of the observed fields), 1 (1%), 13 (20%), 29 (45%), and 22 (34%) are Types A,
B, C, and D, respectively. We interpret these categories as stages in a GMC
evolutionary sequence. Assuming that the timescale for each evolutionary stage
is proportional to the number of GMCs, the lifetime of a GMC with a mass >10^5
Mo is estimated to be 20--40 Myr. In addition, we find that the dense gas
fraction as traced by the CO(J=3-2)/CO(J=1-0) ratio is enhanced around SF
regions. This confirms a scenario where dense gas is preferentially formed
around previously generated stars, and will be the fuel for the next stellar
generation. In this way, massive SF gradually propagates in a GMC until gas is
exhausted.
