Giant Molecular Clouds in M 33 I – Bima All - Disk Survey

We present the first interferometric 12 CO (J = 1 → 0) map of the entire Hα disk of M33. The 13 ′′ diameter synthesized beam corresponds to a linear resolution of 50 pc, sufficient to distinguish individual giant molecular clouds (GMCs). From these data we generated a catalog of 148 GMCs with an expectation that no more than 15 of the sources are spurious. The catalog is complete down to GMC masses of 1.5 × 10 5 M ⊙ and contains a total mass of 2.3 × 10 7 M ⊙. Single dish observations of CO in selected fields imply that our survey detects ∼ 50% of the CO flux, hence that the total molecular mass of M33 is 4.5 × 10 7 M ⊙ , approximately 2% of the H I mass. The GMCs in our catalog are confined largely to the central region (R < 4 kpc). They show a remarkable spatial and kinematic correlation with overdense H I filaments; the geometry suggests that the formation of GMCs follows that of the filaments. The GMCs exhibit a mass spectrum dN/dM ∝ M −2.6±0.3 , considerably steeper than that found in the Milky Way and in the LMC. Combined with the total mass, this steep function implies that the GMCs in M33 form with a characteristic mass of ∼ 7 × 10 4 M ⊙. More than 2/3 of the GMCs have associated H II regions, implying that the GMCs have a short quiescent period. Our results suggest the rapid assembly of molecular clouds from atomic gas, with prompt onset of massive star formation. This paper presents the results of an interferometric survey in 12 CO (J = 1 → 0) of the entire Hα disk of M33. Observations were made with the Berkeley-Illinois-Maryland Association array. The proximity and low inclination of M33, an SA(s)cd spiral in the Local Group, permits direct and detailed comparisons of molecular, atomic, and ionized hydrogen gas on the scale of individual GMCs. The angular resolution of the BIMA survey is 13 ′′ , which corresponds to a linear resolution of 50 pc at the distance of M33 (0.85 Mpc, Lee, Freedman, & Madore 1993). This resolution is adequate to separate, but not to resolve, individual giant molecular clouds (GMCs) if they are similar in size to those in the Milky Way (Blitz 1993). A follow-up paper by (Rosolowsky, et al. 2003, Paper …