ar X iv : a st ro - p h / 97 11 01 4 v 2 4 N ov 1 99 7 Detection of Magnetic Fields Toward M 17 through the H i Zeeman Effect

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 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 seems to originate from the edge-on interface between the H ii region and M17 SW, in un-shocked PDR gas. We have detected a steep enhancement in the 20 km s 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 × 10 19 cm/K while the peak Blos is ∼ −450 μG. In addition, blended components at velocities of 11−17 km s 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/Ts and Blos for this component are ≥ 4.4× 10 cm/K and ∼ +550 μG, respectively. Comparison of the peak magnetic fields detected toward M17 with virial equilibrium calculations suggest that ≈ 1/3 of M17 SW’s total support comes from its static magnetic field and the other 2/3 from its turbulent kinetic energy which includes support from Alfvén waves. Subject headings: H ii regions — ISM:clouds — ISM:individual (M17) — ISM:magnetic fields — radio lines:ISM