4 Molecular Hydrogen

図 5: Molecular Hydrogen

Column density of molecular hydrogen inferred from the intensity of the J =1-0 spectral line of carbon monoxide, a standard tracer of the cold, dense parts of the interstellar medium. Such gas is concentrated in the spiral arms in discrete "molecular clouds." Most molecular clouds are sites of star formation. The molecular gas is pre-dominantly ${\rm H_2}$ , but ${\rm H_2}$ is difficult to detect directly at interstellar conditions and CO, the second most abundant molecule, is observed as a surrogate. The column densities were derived on the assumption of a constant proportionality between the column density of ${\rm H_2}$ and the intensity of the CO emission.

$$115 {\rm GHz} =0.00260689 {\rm m} = 2.6 {\rm mm}$$