Abstract
Abstract We report on a multi-parameter analysis of giant molecular clouds (GMCs) in the nearby spiral galaxy M 33. A catalog of GMCs identifed in 12CO(J = 3–2) was used to compile associated 12CO(J = 1–0), dust, stellar mass, and star formation rate. Each of the 58 GMCs are categorized by their evolutionary stage. Applying the principal component analysis on these parameters, we construct two principal components, PC1 and PC2, which retain 75% of the information from the original data set. PC1 is interpreted as expressing the total interstellar matter content, and PC2 as the total activity of star formation. Young (< 10 Myr) GMCs occupy a distinct region in the PC1–PC2 plane, with lower interstellar medium (ISM) content and star formation activity compared to intermediate-age and older clouds. Comparison of average cloud properties in different evolutionary stages imply that GMCs may be heated or grow denser and more massive via aggregation of diffuse material in their first ∼ 10 Myr. The PCA also objectively identified a set of tight relations between ISM and star formation. The ratio of the two CO lines is nearly constant, but weakly modulated by massive star formation. Dust is more strongly correlated with the star formation rate than the CO lines, supporting recent findings that dust may trace molecular gas better than CO. Stellar mass contributes weakly to the star formation rate, reminiscent of an extended form of the Schmidt–Kennicutt relation with the molecular gas term substituted by dust.
Highlights
The transition from gas to stars is a complex process involving various spatial and temporal scales
A correlation between molecular gas and star formation rate (SF R) is non-existent when molecular gas is traced by the 12CO(J = 1 − 0) line but slightly more correlated when using higher density tracers like the 12CO(J = 3 − 2). This is consistent with previous studies which find that molecular lines which trace denser or warmer molecular gas is more strongly correlated with star formation (Komugi et al 2007; Muraoka et al 2007; Iono et al 2009; Muraoka et al 2016), presumably because they trace gas that is spatially and temporally closer to star forming molecular gas
The correlation between SF R and dust mass is much more pronounced than the relation for CO
Summary
The transition from gas to stars is a complex process involving various spatial and temporal scales. The SK relation is known to break down at the scale of individual GMCs (Onodera et al 2010) when using diffuse molecular gas tracers like the 12CO(J = 1 − 0) emission. Type B are young GMCs which are associated with relatively small HII regions but not with young stellar groups (YSGs), and are at a stage approximately 3-7 Myr after the first formation of massive OB stars. Type D GMCs have HII regions and relatively old (10-30 Myr) stellar groups, and are 20-40 Myr in age (see Miura et al 2012).
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