Excitation temperature of the warm neutral medium as a new probe of the Lyα radiation field

We use the Karl G. Jansky Very Large Array 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 component with excitation temperature (T_s) = 7200+1800-1200 K (68% confidence). This temperature lies above theoretical predictions based on collisional excitation alone, implying that Lya 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 Lya 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.