Giant molecular clouds and massive star formation in the Southern Milky Way

Abstract

The Columbia University–Universidad de Chile CO Survey of the southern Milky Way is used to separate the CO(1–0) emission of the fourth Galactic quadrant within the solar circle into its dominant components, giant molecular clouds (GMCs). After the subtraction of an axisymmetric model of the CO background emission in the inner southern Galaxy, 92 GMCs are identified, and for 87 of them the twofold distance ambiguity is solved. Their total molecular mass is M(H2) = 1.14 ± 0.05 × 108 M, accounting for around 40% of the molecular mass estimated from an axisymmetric analysis of the H2 volume density in the Galactic disk, M(H2)disk = 3.03 × 108 M. The large-scale spiral structure in the southern Galaxy, within the solar circle, is traced by the GMCs in our catalog; three spiral arm segments, the Centaurus, Norma, and 3 kpc expanding arm, are analyzed. After fitting a logarithmic spiral arm model to the arms, tangent directions at 310◦, 330◦, and 338◦, respectively, are found, consistent with previous values from the literature. A complete CS(2–1) survey toward IRAS point-like sources with far-IR colors characteristic of ultracompact H ii regions is used to estimate the massive star formation rate per unit H2 mass (MSFR) and the massive star formation efficiency () for GMCs. The average MSFR for GMCs is 0.41 ± 0.06 L/M, and for the most massive clouds in the Norma arm it is 0.58 ± 0.09L/M. Massive star formation efficiencies of GMCs are, on average, 3% of their available molecular mass.