The galactic distribution (in radius and Z) of interstellar molecular hydrogen

Abstract

Observations of the galactic longitude and latitude distributions of lambda = 2.6 mm CO emission are presented. Analysis of these spectral-line data yields the large-scale distribution of molecular clouds in the galactic disk and their z-distribution out of the disk. Strong maxima in the number of molecular clouds occur in the galactic nucleus and at galactic radii 4 to 8 kpc. The peak at 4 to 8 kpc correlates well with a region of enhanced 100-MeV γ-ray emissivity. This correlation strongly supports the conclusion that the γ-rays are produced as a result of cosmic ray interactions in molecular H_2 clouds rather than in H(I). The width of the cloud layer perpendicular to the galactic plane between half-density points is 105 ± 15 pc near the 5.5-kpc peak. The total mass of molecular gas in the interior of the galaxy exceeds that of atomic hydrogen and is 3·10^9 M⊙ based on these observations.