Where Does the Disk Turn Into the Halo? Cool H I in the Outer Milky Way Disk

TL;DR

This study maps the distribution of cool atomic hydrogen in the outer Milky Way, revealing the structure and phase mixture of H I gas using absorption spectra, and finds a consistent cool-to-total H I ratio out to 25 kpc.

Contribution

It provides the first detailed mapping of cool H I in the outer Galactic disk using absorption data, highlighting the phase distribution and vertical structure.

Findings

Cool H I shows flaring with radius similar to warm H I.

The cool-to-total H I ratio remains nearly constant at 15-20% out to 25 kpc.

The warp of the Galactic midplane is evident in absorption data.

Abstract

Using H I absorption spectra taken from the recent surveys of 21-cm line and continuum emission in the Galactic plane, the distribution of cool atomic clouds in the outer disk of the Milky Way is revealed. The warp of the midplane is clearly seen in absorption, as it is in emission, and the cool, neutral medium also shows flaring or increase in scale height with radius similar to that of the warm atomic hydrogen. The mixture of phases, as measured by the fraction of H I in the cool clouds relative to the total atomic hydrogen, stays nearly constant from the solar circle out to about 25 kpc radius. Assuming cool phase temperature ~50 K this indicates a mixing ratio of 15% to 20% cool H I, with the rest warm.