Magnetically Dominated Strands of Cold Hydrogen in the Riegel-Crutcher Cloud

We present new high-resolution (100″) neutral hydrogen (H I) self-absorption images of the Riegel-Crutcher cloud obtained with the Australia Telescope Compact Array and the Parkes Radio Telescope. The Riegel-Crutcher cloud lies in the direction of the Galactic center at a distance of 125 ± 25 pc. Our observations resolve the very large, nearby sheet of cold hydrogen into a spectacular network of dozens of hairlike filaments. Individual filaments are remarkably elongated, being up to 17 pc long with widths of less than ∼0.1 pc. The strands are reasonably cold, with spin temperatures of ∼40 K and in many places appearing to have optical depths larger than 1. Comparing the H I images with observations of stellar polarization, we show that the filaments are very well aligned with the ambient magnetic field. We argue that the structure of the cloud has been determined by its magnetic field. In order for the cloud to be magnetically dominated the magnetic field strength must be >30 μG. © 2006. The American Astronomical Society. All rights reserved.