Excitation temperature of the warm neutral medium as a new probe of the
  Lyman-{\alpha} radiation field
  We use the Karl G. Jansky Very Large Array (VLA) to conduct a
high-sensitivity survey of neutral hydrogen (HI) absorption in the Milky Way.
In combination with corresponding HI emission spectra obtained mostly with the
Arecibo Observatory, we detect a widespread warm neutral medium (WNM) component
with excitation temperature <Ts>= 7200 (+1800,-1200) K (68% confidence). This
temperature lies above theoretical predictions based on collisional excitation
alone, implying that Ly-{\alpha} scattering, the most probable additional
source of excitation, is more important in the interstellar medium (ISM) than
previously assumed. Our results demonstrate that HI absorption can be used to
constrain the Ly-{\alpha} radiation field, a critical quantity for studying the
energy balance in the ISM and intergalactic medium yet notoriously difficult to
model because of its complicated radiative transfer, in and around galaxies
nearby and at high redshift.
