The low density and magnetization of a massive galaxy halo exposed by a fast radio burst J. Xavier Prochaska1;2, Jean-Pierre Macquart3, Matthew McQuinn4, Sunil Simha1, Ryan M. Shannon5, Cherie K. Day5;6, Lachlan Marnoch6;7, Stuart Ryder7, Adam Deller5, Keith W. Bannister6, Shivani Bhandari6, Rongmon Bordoloi8, John Bunton6, Hyerin Cho9, Chris Flynn5, Elizabeth K. Mahony6, Chris Phillips6, Hao Qiu10, Nicolas Tejos11 1: University of California Observatories-Lick Observatory, University of California, 1156 High Street, Santa Cruz, CA 95064, USA 2: Kavli Institute for the Physics and Mathematics of the Universe, 5-1-5 Kashiwanoha, Kashiwa, 277-8583, Japan 3:International Centre for Radio Astronomy Research, Curtin University, Bentley WA 6102, Australia 4: Astronomy Department, University of Washington, Seattle, WA 98195, USA 5: Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn VIC 3122, Australia 6: Commonwealth Science and Industrial Research Organisation, Australia Telescope National Facility, P.O. Box 76, Epping, NSW 1710 Australia 7: Department of Physics and Astronomy, Macquarie University, NSW 2109, Australia 8: North Carolina State University, Department of Physics, Raleigh, NC 27695-8202, USA 9: School of Physics and Chemistry, Gwangju Institute of Science and Technology, Gwangju, 61005, Korea 10: Sydney Institute for Astronomy, School of Physics, University of Sydney, Sydney, NSW 2006, Australia 11: Instituto de F´ısica, Pontificia Universidad Catolica´ de Valpara´ıso, Casilla 4059, Valpara´ıso, Chile ∗To whom correspondence should be addressed; E-mail:
[email protected]. 1 Present-day galaxies are surrounded by cool and enriched halo gas extending to hundreds of kiloparsecs. This halo gas is thought to be the dominant reser- voir of material available to fuel future star formation, but direct constraints on its mass and physical properties have been difficult to obtain.