Molecular clouds in the Galaxy

The J = 1 → 0 emission of CO has been surveyed in the galactic plane between ℓ = -10° and +90° with a U beam sampling every degree. Molecular clouds emitting in the CO line are plentiful over the inner region of the Galaxy. Their greatest number occurs in the galactic nucleus and at a radius of 5.5 kpc—a distribution similar to radio H II regions and galactic γ-ray emission but very different from that previously derived for atomic hydrogen. The total mass in molecular clouds is found to be 1-3 x 10^9 and each typically has 10^5 M_⊙, an H_2 density of 700 cm^(-3), and a temperature near 7 K. These results suggest that most of the interstellar medium in the interior of the Galaxy is molecular H_2