Giant molecular clouds in M33. I. BIMA all-disk survey

We present the first interferometric (CO)-C-12 (J = 1 --> 0) map of the entire Halpha 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 x 10(5) M-. and contains a total mass of 2.3 x 10(7) M-.. Single-dish observations of CO in selected fields imply that our survey detects similar to 50% of the CO flux, hence that the total molecular mass of M33 is 4.5 x 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 x 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.