Neutral Hydrogen and the Diffuse Interstellar Medium

Neutral atomic hydrogen (HI) is an important component of the interstellar medium (ISM). The Galaxy has been estimated to contain about 4.8 x 10^9 M_⊙ of HI (Henderson et al. 1982)*. Estimates for the total amount of H_2 range from 3.5 x 10^9 M_⊙ (Sanders et al. 1984), nearly equal to the HI estimate, to a value that is only 25% of the HI estimate (Bloemen et al. 1986). Galactic HI constitutes about 4.4% of the mass of the visible matter (Bahcall et al. 1983). The mean surface density distribution of HI is roughly constant from about 4 kpc to 20 kpc; however, HI dominates H_2 in mass beyond Galactocentric radius 8 kpc (Blitz et al. 1983). Unlike H_2, HI is not concentrated in a small number of giant clouds. Estimates of the "filling factor", the fraction of the Galactic interstellar space occupied by hydrogen, range from 20% to 90%. These factors establish the preeminence of atomic hydrogen in the dynamics and evolution of the ISM