GIANT MOLECULAR CLOUD EVOLUTIONS IN THE NEARBY SPIRAL GALAXY M33

We present a giant molecular cloud (GMC) catalog of 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 H ii 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 H ii regions; Type C, with both H ii regions and <10 Myr old YSGs; and Type D, with both H ii 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 >105 M☉ 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.