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

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 10 5-10 6 M ⊙ containing cool H I gas with temperatures below 100 K. These clouds are only marginally stable against gravitational collapse. © 2007. The American Astronomical Society. All rights reserved.