2019MNRAS.483.5135W

We present a catalogue of Giant Molecular Clouds (GMCs) in M33, extracted from cold dust continuum emission. Our GMCs are identified by computing dendrograms. We measure the spatial distribution of these clouds, and characterize their dust properties. Combining these measured properties with CO(J = 2-1) and 21 cm H I data, we calculate the gas-to-dust ratio (GDR) of these clouds, and from this compute a total cloud mass. In total, we find 165 GMCs with cloud masses in the range of 10⁴-10⁷ M_☉. We find that radially, log₁₀(GDR) = -0.043(±0.038) R [kpc] + 1.88(±0.15), a much lower GDR than found in the Milky Way, and a correspondingly higher α_CO factor. The mass function of these clouds follows a slope proportional to M^–2.84, steeper than many previous studies of GMCs in local galaxies, implying that M33 is poorer at forming massive clouds than other nearby spirals. While we can rule out interstellar pressure as the major contributing factor, we are unable to disentangle the relative effects of metallicity and H I velocity dispersion. We find a reasonably featureless number density profile with galactocentric radius, and weak correlations between galactocentric radius and dust temperature/mass. These clouds are reasonably consistent with Larson's scaling relationships, and many of our sources are co-spatial with earlier CO studies. Massive clouds are identified at large galactocentric radius, unlike in these earlier studies, perhaps indicating a population of CO-dark gas dominated clouds at these larger distances.