Giant Molecular Cloud Evolutions in the Nearby Spiral Galaxy M33

Abstract

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 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.

Alma Observations of the Antennae Galaxies. I. A New Window on a Prototypical Merger

“…It is interesting to note that Kawamura et al (2009) estimate a 20-30 Myr lifetime for GMCs in regions with recent star formation for their LMC data set, and Miura et al (2012) find the range to be 20-40 Myr in M33, both slightly larger than our estimate of ≈10 Myr.…”

“…5. Using this framework we conclude that the maximum age spread for a cluster complex (or equivalently, the lifetime of a GMC in a star formation region) is ≈10 Myr, which is slightly lower than estimates by Kawamura et al (2009;20-30 Myr) and Miura et al (2012;20-40 Myr). This estimate is based on the fact that there are many regions with a simultaneous mixture of objects in Stages 1 (protoclusters) through 4 (young clusters), but no regions that we are currently aware of that have objects in Stages 1 through 5 (intermediate/old clusters).…”

“…Other relevant papers are Kawamura et al (2009) and Miura et al (2012). Kawamura et al (2009;see Yamaguchi et al 2001 for an earlier version of related work) used NANTEN observations of molecular gas observations in the Large Magellanic Clouds and developed a model with four evolutionary stages (no star formation, H ii regions, H ii regions and <10 Myr young stellar groups, and H ii regions and 10-30 Myr young stellar groups).…”

“…Assuming the timescales for each stage are proportional to the number of objects, they find that the lifetime of a GMC is 20-30 Myr. Miura et al (2012) used the ASTE telescope to observe CO (3-2) for 71 GMCs in M33 with 100 pc resolution, and compared with ages for young stellar groups (YSG) from optical data. They define a four stage sequence (no star formation, H ii regions, H ii regions, and young stellar ages [<10 Myr YSGs], and H ii regions and intermediate stellar ages ).…”

“…These include selection and completeness effects, the possibility that many of the clusters may be destroyed with time (e.g., see Whitmore et al 2007), the fact that the star formation rate is not constant, and low-number statistics. Nonetheless, if we cautiously forge ahead we can at least make order-of-magnitude estimates, as has been done by Kawamura et al (2009) and Miura et al (2012).…”