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