Giant Molecular Clouds and Massive Star Formation in the Southern Milky Way

Abstract

The Columbia -U. de Chile CO Survey of the Southern Milky Way is used for separating 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 two-fold distance ambiguity is solved. Their total molecular mass is M(H 2 ) = 1.14 ± 0.05 × 10 8 M ⊙ accounting for around 40% of the molecular mass estimated from an axisymmetric analysis of the H 2 volume density in the Galactic disk (Bronfman et al. 1988b) M(H 2 ) disk = 3.03 × 10 8 M ⊙ . The large scale spiral structure in the Southern Galaxy, within the solar circle, is traced by the GMCs in our catalog; 3 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 FIR colors characteristic of UC HII regions is used to estimate the massive star formation rate per unit H 2 mass (MSFR), and the massive star formation efficiency (ǫ) 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 ± 0.09 L ⊙ /M ⊙ . Massive star formation efficiencies of GMCs are on average 3% of their available molecular mass.

SEDIGISM: Structure, excitation, and dynamics of the inner Galactic interstellar medium

“…Of these, 126 clumps had assigned distances, and we find that 113 (i.e. ∼90%) are in agreement with those assigned by García et al (2014). We had placed all nine remaining clumps at the far distance, consistent with the fact that all of these are relatively isolated.…”

“…4 (ID number as listed in Tables 2 and A.1) and the number of leaves associated with them (in parentheses). Col. 7 lists the matches to the 12 CO(1-0) GMC catalogue presented by García et al (2014) (1) clumps located in the science demonstration field and find an overall agreement of ∼90%.…”

“…Using the 12 CO(1 -0) data of the Dame et al (2001) survey, García et al (2014) found a total of only 11 GMCs within the science demonstration field, which comprise 172 of the SCIMES clouds. However, the spatial resolution of the Dame et al (2001) survey is much coarser than that of the SEDIGISM survey (530 compared to 30 ).…”

“…We have done so using both the 12 CO(1 -0) and 13 CO(1 -0) data and found that the clouds from García et al (2014) are now sub-divided into smaller clouds, but they still tend to group several SEDIGISM clouds together. The grouping of numerous SEDIGISM clouds into larger GMCs as seen with 12 CO or lower transitions of 13 CO seems to confirm the existence of a large scale diffuse molecular gas, which connects the different peaks extracted from SEDIGISM.…”

“…28 radius). García et al (2014) have also determined distances to six of their lower-resolution 12 CO GMCs within the science demonstration field; these are associated with 131 ATLASGAL clumps. Of these, 126 clumps had assigned distances, and we find that 113 (i.e.…”