Abstract
The sample of 566 molecular clouds identified in the CO(2–1) IRAM survey covering the disk of M 33 is explored in detail. The clouds were found using CPROPS and were subsequently catalogued in terms of their star-forming properties as non-star-forming (A), with embedded star formation (B), or with exposed star formation (C, e.g., presence of Hα emission). We find that the size-linewidth relation among the M 33 clouds is quite weak but, when comparing with clouds in other nearby galaxies, the linewidth scales with average metallicity. The linewidth and particularly the line brightness decrease with galactocentric distance. The large number of clouds makes it possible to calculate well-sampled cloud mass spectra and mass spectra of subsamples. As noted earlier, but considerably better defined here, the mass spectrum steepens (i.e., higher fraction of small clouds) with galactocentric distance. A new finding is that the mass spectrum of A clouds is much steeper than that of the star-forming clouds. Further dividing the sample, this difference is strong at both large and small galactocentric distances and the A vs. C difference is a stronger effect than the inner vs. outer disk difference in mass spectra. Velocity gradients are identified in the clouds using standard techniques. The gradients are weak and are dominated by prograde rotation; the effect is stronger for the high signal-to-noise clouds. A discussion of the uncertainties is presented. The angular momenta are low but compatible with at least some simulations. Finally, the cloud velocity gradients are compared with the gradient of disk rotation. The cloud and galactic gradients are similar; the cloud rotation periods are much longer than cloud lifetimes and comparable to the galactic rotation period. The rotational kinetic energy is 1–2% of the gravitational potential energy and the cloud edge velocity is well below the escape velocity, such that cloud-scale rotation probably has little influence on the evolution of molecular clouds.
Highlights
The sample of 566 molecular clouds identified in the CO(2–1) IRAM survey covering the disk of M 33 is explored in detail
Should there be a link between size or shape and the velocity gradient deduced from calculating the first moment, the link would be preserved in these tests
We have shown, for the first time to our knowledge, that molecular clouds rotate and that their rotation is very slow but measurable from our high-quality data
Summary
Recent years have seen a sharply increasing number of studies of molecular clouds in external galaxies (Wilson & Scoville 1990; Engargiola et al 2003; Rosolowsky et al 2003; Kawamura et al 2009; Gratier et al 2010a, 2012; Donovan Meyer et al 2013; Schinnerer et al 2013; Druard et al 2014; Corbelli et al 2017; Freeman et al 2017). Extragalactic work is quite complementary to the Galactic observations as the biases are not at all the same the spatial resolution is much poorer They identified 45 molecular clouds in M 33 and found velocity gradients to be very low, with nearly half in the retrograde direction. The fraction decreases with galactocentric distance: 2/3 of the CO emission is in clouds in the inner disk and 1/3 in the outer disk (Druard 2014)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
