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 hair-like 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 one. 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. Subject headings: ISM: structure — ISM: clouds — ISM: magnetic fields — radio lines: ISM Australia Telescope National Facility, CSIRO, PO Box 76, Epping NSW 1710, Australia; [email protected] School of Mathematics and Physics, University of Tasmania, Private Bag 21, Hobart TAS 7001, Australia; [email protected] Alfred P. Sloan Fellow; Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 Current Address: School of Physics, The University of Sydney A29, NSW 2006, Australia; [email protected] School of Physics, The University of Sydney A29, NSW 2006, Australia; [email protected] Astronomy Department, University of California, 601 Campbell Hall, Berkeley, CA 84720; [email protected] Jansky Fellow, National Radio Astronomy Observatory