H I clouds in the lower halo. I. The galactic all-sky survey pilot region

H. Alyson Ford, N. M. McClure-Griffiths, Felix J. Lockman, J. Bailin, M. R. Calabretta, P. M.W. Kalberla, T. Murphy, D. J. Pisano

Research output: Contribution to journalArticlepeer-review

34 Scopus citations


We have detected over 400 H I clouds in the lower halo of the Galaxy within the pilot region of the Galactic All-Sky Survey (GASS), a region of the fourth quadrant that spans 18° in longitude, 40° in latitude, and is centered on the Galactic equator. These clouds have a median peak brightness temperature of 0.6 K, a median velocity width of 12.8 km s-1, and angular sizes ≲1°. The motion of these clouds is dominated by Galactic rotation with a random cloud-to-cloud velocity dispersion of 18 km s-1. A sample of clouds likely to be near tangent points was analyzed in detail. These clouds have radii on the order of 30 pc and a median H i mass of 630 Modot;. The population has a vertical scale height of 400 pc and is concentrated in Galactocentric radius, peaking at R = 3.8 kpc. This confined structure suggests that the clouds are linked to spiral features, while morphological evidence that many clouds are aligned with loops and filaments is suggestive of a relationship with star formation. The clouds might result from supernovae and stellar winds in the form of fragmenting shells and gas that has been pushed into the halo rather than from a galactic fountain.

Original languageEnglish (US)
Pages (from-to)290-305
Number of pages16
JournalAstrophysical Journal
Issue number1
StatePublished - Nov 20 2008
Externally publishedYes


  • Galaxies: structure
  • Galaxy: halo
  • ISM: clouds
  • Ism: structure
  • Radio lines: ISM

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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