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Neutral Hydrogen and the Diffuse Interstellar Medium

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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 109 M⊙ of HI (Henderson et al. 1982)*. Estimates for the total amount of H2 range from 3.5 x 109 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 H2 in mass beyond Galactocentric radius 8 kpc (Blitz et al. 1983). Unlike H2, 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.

Keywords

  • Column Density
  • Scale Height
  • Faraday Rotation
  • Galactic Plane
  • Zeeman Splitting

These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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  • DOI: 10.1007/978-1-4612-3936-9_3
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© 1988 Springer-Verlag New York Inc.

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Kulkarni, S.R., Heiles, C. (1988). Neutral Hydrogen and the Diffuse Interstellar Medium. In: Verschuur, G.L., Kellermann, K.I. (eds) Galactic and Extragalactic Radio Astronomy. Astronomy and Astrophysics Library. Springer, New York, NY. https://doi.org/10.1007/978-1-4612-3936-9_3

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  • DOI: https://doi.org/10.1007/978-1-4612-3936-9_3

  • Publisher Name: Springer, New York, NY

  • Print ISBN: 978-0-387-97735-5

  • Online ISBN: 978-1-4612-3936-9

  • eBook Packages: Springer Book Archive