A new estimate of the mass of molecular gas in the Galaxy and its implications

The amount of gas in the Galaxy and its distribution, both in space and between its various components, are among the 'constants' of the Galaxy. Clearly, this knowledge is a prerequisite for models of star formation, the chemical evolution of the interstellar medium and the manner of evolution of the Galaxy as a whole. The 21 cm hydrogen-line observations have led to a rather precise derivation of the distribution of atomic hydrogen (HI), but the way in which the important, clumpy, molecular hydrogen (H2) is distributed has been the subject of much discussion; it is this component that is the principal concern of the present work. A variety of data, have been examined including that from cosmic γ-rays, X-rays, infrared and millimetre-wave astronomy to compute the manner in which the surface density of H2 varies with Galactocentric distance. The conclusion is that, in the inner Galaxy, the surface density is much less than previously thought. The total mass of H2 in the inner Galaxy is estimated to be ca. 6.1 x 108 Msub(solar masses), a value only some 60% of that in atomic hydrogen. (author)