Technical Capabilities for the Surveys

Here we discuss the technical capabilities that are necessary to make the measurements required by the science diagnostics. These capabilities are met by the Mopra, Nanten2 and STO telescopes, as is summarised in Table 1 at the end.

Angular and Spectral Resolution Required

In order to contain sufficient mass to build a giant molecular cloud an atomic cloud needs to have a hydrogen column of order 10²¹ cm^-2, which corresponds to a diameter of about 7 pc at interstellar pressures. Small molecular clouds, which may be largely dark, need similar columns, but are cooler and denser and so may have sizes of order 1–2 pc. The giant clouds themselves have diameters of ~10–100 pc. Ensembles of small clouds that are moving at detectable (> 1 km/s) speeds to coalesce into giant clouds will have ensemble diameters of 100–500 pc. The survey area we have chosen goes through the molecular ring of our Galaxy at distances of typically 8 kpc, and therefore these sizes correspond to 1–3 arcminutes for the small clouds, 4–40 arcminutes for the giant clouds, and 1.5–7 degrees for the ensembles of small clouds. Therefore, we require 1 arcminute spatial resolution to resolve individual clouds and a survey area that extends at least 7 degrees in the Galactic plane—preferably across a spiral arm, where the molecular clouds are concentrated.

Line widths towards small clouds are ~1 km/s and towards giant clouds ~2–3 km/s, so we need ~0.2 km/s spectral resolution. Velocity information can generally be used to place the clouds along the line of sight, using the galactic rotation curve. In the survey direction, 1 km/s corresponds to about 30–40 pc .

Optimum Tracers to Use

We need to use emission features that trace all of the molecular and atomic gas. Carbon monoxide (CO) is well known to be the best tracer of most molecular gas because the molecular hydrogen itself needs to be warmer (>100 K) than the typical temperature in molecular clouds (T~10 K) to produce detectable emission. For the atomic gas the best tracers are the terahertz C⁺ and sub-millimetre C lines, when used in conjunction with atomic hydrogen cm-wavelength data. For the dark molecular gas the C⁺ and C lines are again the best tracers, but this time without the presence of atomic hydrogen gas or CO. Distinguishing between the four scenarios for cloud formation (see above) also requires us to measure the velocity fields of these species as well their spatial correspondence. The telescopes we are using all have the required spectral (~0.2 km/s) and spatial resolution (< 1 arcminute) needed to do this.