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

Abstract

We present 12CO(1->0) observations of 45 giant molecular clouds 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 counter-rotating 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 linewidths as well as a constant surface density of 120 M_sun pc^{-2}. The size-linewidth relationship follows Delta V ~ r^{0.45\pm 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 X 10^20 H2 cm^-2/(K km/s) and does not change significantly over the disk of M33 despite a change of 0.8 dex in the metallicity.