The Three-mm Ultimate Mopra Milky Way Survey (ThrUMMS): A New View of the Molecular Milky Way
Abstract
We describe a new mm-wave molecular-line mapping survey of the southern Galactic Plane, and its first data releases and science results. The Three-mm Ultimate Mopra Milky Way Survey (ThrUMMS) maps a 60°×2° sector of our Galaxy’s fourth quadrant, using a combination of fast mapping techniques with the Mopra radio telescope, simultaneously in the J=1→0 lines of 12CO, 13CO, C18O, and CN near 112 GHz at ∼arcminute and 0.3 km s-1 resolution, with 1.2 K/chan sensitivity for 12CO and 0.6 K/chan for the other transitions. The calibrated data cubes from these observations are made available to the community after processing through our pipeline. Here, we describe the motivation for ThrUMMS, the development of new observing techniques for Mopra, and how these techniques were optimised to the objectives of the survey. We showcase some sample data products and describe the first science results, on global variations in the iso-CO line ratios and on a detailed multiwavelength study of the GMCs near l=333°. The line ratios vary dramatically across the Galactic Plane, indicating a very wide range of optical depth and excitation conditions, from warm and translucent to cold and opaque. The population of cold clouds in particular have optical depths for 12CO easily exceeding 100 in some locations, and suggests that the fraction of the molecular mass in the Galactic disk that is in the coldest gas may be substantially underestimated. We compute robust column densities from the global data and derive a new conversion law from CO to molecular mass, indicating that global relationships in disk galaxies that depend on the CO→H2 mass conversion, such as star formation laws, may need to be recalibrated. Near l=333°, we have compared ThrUMMS data to HI, cm-continuum, and several Herschel and Spitzer bands to derive the overall mass and star formation properties of two complexes along the line of sight, in the Scutum-Centaurus and Norma arms. The RCW106 complex in particular is currently undergoing a ministarburst event, potentially shedding light on starburst physics in distant galaxies.
- Publication:
-
IAU General Assembly
- Pub Date:
- August 2015
- Bibcode:
- 2015IAUGA..2251488B