Science Diagnostics

Here we outline the diagnostic measurements that are required to address the science questions.

Distinguishing How Molecular Clouds Form

The assembly time for a giant molecular cloud is approximately the radius (~100-500 pc) of a cluster of small clouds divided by the speed (~5 km/s, turbulent or gravitational) at which these clouds come together, or 20-100 Myr. Since this is comparable to, or greater than, the estimated ages of giant clouds, we will observe at least as many such clusters as we observe giant molecular clouds along each line of sight (i.e. ~1–10). The survey area needs to include about 10-100 giant molecular clouds and a similar number of “forming” clouds to provide a reasonable sample size. We discussed above the four theories current for giant cloud formation. These result in different morphological and kinematic signatures for the gas.

1. If formation occurs by the gravitational collapse of a cluster of small clouds (i.e. scenario i), our observations will show either a roughly spherical distribution of small clouds or possibly a filamentary distribution with the filaments following ballooned magnetic field lines out of the galactic plane (the Parker instability), with velocity characteristics of infall. In addition, our measurements determine the mass inside any cluster radius. We can then compare gravitational (i.e. virial) velocities with the observed velocity dispersion of the clouds, and they should be comparable.
2. If the giant clouds form by random (no gravity) collisional coagulation of small clouds (i.e. ii), then the velocity field of the cluster clouds will look more random and less systematic than infall and their velocities will exceed virial speeds.
3. If they are formed in wind or supernova-driven shells (i.e. iii), a shell-like morphology will be apparent.
4. If they form by converging flows in a turbulent medium (i.e. iv), we should see an overall turbulent velocity field, but local to the formation sites the velocities will be coherent (converging) and not random, and the speeds will be super-virial.

Detecting and Measuring the Dark Gas

Dark gas consists primarily of hydrogen molecules, with other molecules (such as CO) having too low an abundance to be detectable. It is signposted by the presence of C and C⁺, but without either CO or H, so differentiating it from fully molecular (CO), atomic (C, H) and ionized (C⁺) gas. It can also be indicated by gamma ray emission, when atomic and fully molecular gas is absent.