The Mopra Southern Galactic Plane CO Survey – the formation of molecular clouds

This site provides information about the molecular gas survey of the southern galactic plane being conducted with the Mopra millimetre-wave telescope in Coonabarabran, New South Wales, Australia. The survey is designed to map the distribution and dynamics of the carbon monoxide molecule (CO) along the Fourth Quadrant of the Galaxy. We plan to map from l=265° to 005°, b=-1° to +1°, covering 200 sq. degrees. The principal scientific motivation is to understand how molecular clouds are formed in space. The data will be combined with similar survey data collected on the sub-millimetre neutral carbon line (C) with the Nanten2 telescope in Chile, and the terahertz neutral and ionized carbon lines (C and C+) from the balloon-borne Stratospheric Terahertz Observatory (STO) and the ice-bound High Elevation Antarctic Telescope (HEAT), both in Antarctica. With these surveys we will be able to build a picture of the distribution and motion of the principal forms of carbon in the interstellar medium. Taken together, these three species can provide us with a picture of the molecular medium of our our Galaxy.

With the data we aim to answer a longstanding question: how do molecular clouds form in the interstellar medium? This process plays a key, but so far poorly understood, role in the life cycle of our Galaxy, as it is from these molecular clouds that stars will be born.

To understand molecular cloud formation we must first find all of the molecular gas. An important, but unseen component, is the “dark” gas. Lacking the primary tracers found in atomic gas (hydrogen atoms) and fully-developed molecular clouds (carbon monoxide), this “dark” gas can be seen primarily via the emission from atoms of neutral and singly-ionized carbon.

Interestingly, this dark gas may also be associated with the emission of gamma rays, produced when cosmic ray particles interact with gas nuclei in the interstellar medium. Currently we cannot reconcile the distribution measured for the gas using conventional tracers with that for the gamma rays in many sources. This work will test the hypothesis that the difference between these results arises from the presence of dark gas.

To address these questions we need to map the atomic and molecular gas across a spiral arm of our Galaxy, thus determining the distribution and dynamics of all the gas in all its states – including the dark component. We will use these telescopes that our research team have developed, in Australia, Chile and Antarctica, to trace the element carbon in molecular, neutral and ionized gas. These novel telescopes are able to look through hard-to-see windows in the atmosphere to measure these spectral diagnostics. Together, they make it possible to trace carbon in these forms (and hence all of the gas) over large areas of sky, and with sufficient spatial and spectral resolution to isolate individual emitting gas clouds and to determine their motions through the Galaxy.

In the pages linked below we provide further information on this program as follows:

The Team Mopra public website is at



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Last Updated 18th June 2013