Giant Molecular Clouds in M33. II. High‐Resolution Observations

Abstract

We present 12 CO ( J = 1 → 0) observations of 45 giant molecular clouds (GMCs) in M33 made with the BIMA array. The observations have a linear resolution of 20 pc, sufficient to measure the sizes of most GMCs in the sample. We place upper limits on the specific angular momentum of the GMCs and find the observed values to be nearly an order of magnitude below the values predicted from simple formation mechanisms. The velocity gradients across neighboring, high-mass GMCs appear preferentially aligned on scales less than 500 pc. If the clouds are rotating, 40% are counterrotating with respect to the galaxy. GMCs require a braking mechanism if they form from the large-scale radial accumulation of gas. These observations suggest that molecular clouds form locally out of atomic gas, with significant braking by magnetic fields to dissipate the angular momentum imparted by galactic shear. The observed GMCs share basic properties with those found in the Galaxy, such as similar masses, sizes, and line widths, as well as a constant surface density of 120 M ☉ pc -2 . The size-line width relationship follows Δ V ∝ r 0.45±0.02 , consistent with that found in the Galaxy. The cloud virial masses imply that the CO-to-H 2 conversion factor has a value of 2 × 10 20 H 2 cm -2 (K km s -1 ) -1 and does not change significantly over the disk of M33, despite a change of 0.8 dex in the metallicity.