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Published 1988 | public
Book Section - Chapter

Neutral Hydrogen and the Diffuse Interstellar Medium

Abstract

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.

Additional Information

© 1988 Springer-Verlag. This work was supported in part by National Science Foundation grant no. AST 85-13422, awarded to C. Heiles, and a Robert A. Millikan Fellowship to S.R. Kulkarni.

Additional details

Created:
August 22, 2023
Modified:
January 13, 2024