Giant molecular clouds in the Galaxy. I - The axisymmetric distribution of H2

A two-dimensional (l, b) survey of molecular clouds in the Galaxy is presented. Observations of CO (J = 1→0) emission at over 2500 positions between l = -4° and 170° are interpreted to determine the distribution of molecules at galactic radius, R, from 0 to 16 kpc. The radial distribution of CO surface brightness I<SUB>CO</SUB>, exhibits a sharp maximum in the central 1.5 kpc, a minimum at R = 1.5 - 3.5 kpc, and a ring peaking at 6 kpc. Between 6 and 15 kpc, I<SUB>CO</SUB> falls by two orders of magnitude, while 21 cm measurements show virtually constant H I surface density. The conversion of I<SUB>CO</SUB> to H<SUB>2</SUB> mass is discussed. All empirical measures of this conversion factor agree to better than a factor of 2. The H<SUB>2</SUB> density and surface density are presented as a function of R and z out to R = 16 kpc and compared with the H I densities. The total surface density of interstellar gas (H<SUB>2</SUB> + H I) as function of R is derived from these data. In particular, a detailed discussion of the distribution of interstellar gas in the galactic center region is presented.