Distribution of molecular clouds in the galaxy: H2 as a major component of the interstellar medium

An extensive survey of molecular clouds in the galaxy is presented. Observations of CO J = 1 - 0 emission are interpreted to determine the galactic distribution of molecules and the total number, size and mass of molecular clouds. Two major data sets were obtained: (1) a 1060 point grid in 12CO covering/ = 3560 to 700 in steps of one degree and absolute value b (Vertical Barapprox. 1.60 in steps of 0.20 and, (2) a 1006 point strip map in 12CO at or near b = 00, from I = 100 to 510 sampled at intervals of 0.0330 or 0.0670. The radial distribution of CO emission at R < R/sub sun mass/ indicates an intense concentration of molecular gas in the galactic center (R < 1 kpc), where the total integrated intensity I/sub CO/, is equal to 50 K km s-1, a minimum between 1.5 and 3.5 kpc (I/sub CO/ = 0.6), a peak at 6 kpc (I/sub CO/ = 3), then a decrease toward larger radius with an exponential scale length of 2.7 kpc. The shape of the H2 distribution is in sharp contrast to the flat HI. The total mass of H2 in the galactic center (R < 2kpc), is 5 x 108 M/sub sun mass/, a factor of 3 larger than HI. At R < 20 kpc the H2 mass is estimated to be 3.6 x 109 M/sub sun mass/. At R > 6 kpc the fraction of total disk mass in gas is approximately constant (approx. = 12%), suggesting that the star formation rate is proportional to the first power of the average gas density. The CO emissivity distribution represents the distribution of molecular clouds which is dominated by approximately 4000 large massive clouds with diameters greater than 20 pc and density 300 H2 cm-3, corresponding to masses, M(H2) > 105M/sub sun mass/. Collectively these massive clouds, which are referred to as Giant Molecular Clouds (GMC) contain most of the CO emission