Detection of magnetic fields toward M17 through the H I Zeeman effect

C. L. Brogan, T. H. Troland, D. A. Roberts, R. M. Crutcher

Research output: Contribution to journalArticlepeer-review


We have carried out VLA Zeeman observations of H I absorption lines toward the H II region in the M17 giant molecular cloud complex. The resulting maps have 60″ × 45″ spatial resolution and 0.64 km s-1 velocity separation. The H I absorption lines toward M17 show between 5 and 8 distinct velocity components, which vary spatially in a complex manner across the source. We explore possible physical connections between these components and the M17 region based on calculations of H I column densities, line-of-sight magnetic field strengths, as well as comparisons with a wide array of previous optical, infrared, and radio observations. In particular, an H I component at the same velocity as the southwestern molecular cloud (M17 SW; ∼20 km s-1) seems to originate from the edge-on interface between the H II region and M17 SW in unshocked photodissociation region (PDR) gas. We have detected a steep enhancement in the 20 km s-1 H I column density and line-of-sight magnetic field strengths (Blos) toward this boundary. A lower limit for the peak 20 km s-1 H I column density is NHI/Ts ≥ 5.6 × 1019 cm-2 K-1, whereas the peak Blos is ∼ -450 μG. In addition, blended components at velocities of 11-17 km s-1 appear to originate from shocked gas in the PDR between the H II region and an extension of M17 SW, which partially obscures the southern bar of the H II region. The peak NHI/T2 and Blos for this component are ≥ 4.4 × 1019 cm-2 K-1 and ∼ +550 μG, respectively. Comparison of the peak magnetic fields detected toward M17 with virial equilibrium calculations suggest that ≈1/2 of M17 SW's total support comes from its static magnetic energy, while the other half of its support is supplied by the turbulent kinetic energy (including MHD waves).

Original languageEnglish (US)
Pages (from-to)304-322
Number of pages19
JournalAstrophysical Journal
Issue number1 PART 1
StatePublished - Apr 10 1999


  • H II regions
  • ISM: clouds
  • ISM: individual (M17)
  • ISM: magnetic fields
  • Radio lines: ISM

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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