The Outer Disk of the Milky Way Seen in 21-cm Absorption

TL;DR

This study uses 21-cm absorption and emission spectra to analyze the distribution, temperature, and phase mixture of neutral hydrogen in the Milky Way's outer disk, revealing consistent phase ratios and structural features like the Galactic warp.

Contribution

It provides the first detailed analysis of H I phase distribution and temperature in the Milky Way's outer disk using combined emission-absorption data up to 25 kpc.

Findings

The cool and warm neutral hydrogen phases have similar distribution parameters.

The mean spin temperature remains nearly constant with radius, indicating a stable phase mixture.

The Galactic warp is clearly visible in absorption data, with structural features consistent across emission and absorption observations.

Abstract

Three recent surveys of 21-cm line emission in the Galactic plane, combining single dish and interferometer observations to achieve resolution of 1 arcmin to 2 arcmin, 1 km/s, and good brightness sensitivity, have provided some 650 absorption spectra with corresponding emission spectra for study of the distribution of warm and cool phase H I in the interstellar medium. These emission-absorption spectrum pairs are used to study the temperature of the interstellar neutral hydrogen in the outer disk of the Milky Way, outside the solar circle, to a radius of 25 kpc. The cool neutral medium is distributed in radius and height above the plane with very similar parameters to the warm neutral medium. In particular, the ratio of the emission to the absorption, which gives the mean spin temperature of the gas, stays nearly constant with radius to 25 kpc radius. This suggests that the mixture of cool and warm phases is a robust quantity, and that the changes in the interstellar environment do not force the H I into a regime where there is only one temperature allowed. The mixture of atomic gas phases in the outer disk is roughly 15% to 20% cool (40 K to 60 K), the rest warm, corresponding to mean spin temperature 250 to 400 K. The Galactic warp appears clearly in the absorption data, and other features on the familiar longitude-velocity diagram have analogs in absorption with even higher contrast than for 21-cm emission. In the third and fourth Galactic quadrants the plane is quite flat, in absorption as in emission, in contrast to the strong warp in the first and second quadrants. The scale height of the cool gas is similar to that of the warm gas, and both increase with Galactic radius in the outer disk.