Galactic Outer Arms in H I Absorption

doi:10.1086/522794

Tracking the Outer Spiral Arms of the Galaxy in H I Absorption

S. T. Strasser et al 2007 The Astronomical Journal 134 2252-2271 doi: 10.1086/522794

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S. T. Strasser^1,2, J. M. Dickey³, A. R. Taylor⁴, A. I. Boothroyd⁵, B. M. Gaensler⁶, A. J. Green⁶, D. W. Kavars⁷, Felix J. Lockman⁸, P. G. Martin^4,9, N. M. McClure-Griffiths¹⁰, T. A. Rothwell⁹ and J. M. Stil⁴
¹ Department of Astronomy, University of Minnesota, Minneapolis, MN 55455, USA
² Dominion Radio Astrophysical Observatory, Penticton, BC V2A 6J9, Canada
³ School of Mathematics and Physics, University of Tasmania, Hobart, TAS 77001, Australia
⁴ Department of Physics and Astronomy, University of Calgary, Calgary, AB T2N 1N4, Canada
⁵ Canadian Institute for Theoretical Astrophysics, University of Toronto, Toronto, ON M5S 3H8, Canada
⁶ School of Physics, University of Sydney, Sydney, NSW 2006, Australia
⁷ Indiana Academy for Science, Mathematics, and Humanities, Ball State University, Muncie, IN 47306, USA
⁸ National Radio Astronomy Observatory, Green Bank, WV 24944, USA
⁹ Department of Astronomy and Astrophysics, University of Toronto, Toronto, ON M5S 3H8, Canada
¹⁰ Australia Telescope National Facility, CSIRO, Marsfield, NSW 2122, Australia
E-mail: strasser@astro.umn.edu

ABSTRACT. We have analyzed 793 atomic hydrogen emission and absorption spectrum pairs toward continuum background sources in the northern and southern Galactic plane. In this paper we focus on absorption features from cold gas in the outermost arms of the Galaxy. We find H I absorption associated on a global scale with the outer spiral arm (first and second Galactic quadrants). To a lesser extent we also see absorption associated with the most distant spiral arm in the third and fourth Galactic quadrants. In total, 236 spectra contain clear absorption features associated with the outermost spiral arms. Cool-phase gas therefore exists throughout these spiral arms. The mean distances between absorbing clouds are on the order of 90-220 pc. We identify a number of striking H I structures with masses on the order of 10⁵-10⁶ M containing cool H I gas with temperatures below 100 K. These clouds are only marginally stable against gravitational collapse.

Key words: Galaxy: disk; ISM: atoms; ISM: clouds; radio lines: ISM

Print publication: Issue 6 (2007 December)
Received 2007 March 7, accepted for publication 2007 August 24
Published 2007 October 23

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Tracking the Outer Spiral Arms of the Galaxy in H I Absorption

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