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The Astronomical Journal Volume 134 Number 6 Create an alert RSS this journal

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

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

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S. T. Strasser1,2, J. M. Dickey3, A. R. Taylor4, A. I. Boothroyd5, B. M. Gaensler6, A. J. Green6, D. W. Kavars7, Felix J. Lockman8, P. G. Martin4,9, N. M. McClure-Griffiths10, T. A. Rothwell9 and J. M. Stil4

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Abstract References Cited By

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 Modot containing cool H I gas with temperatures below 100 K. These clouds are only marginally stable against gravitational collapse.


Keywords

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


Dates

Issue 6 (2007 December)

Received 2007 March 7, accepted for publication 2007 August 24

Published 2007 October 23



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