EXCITATION TEMPERATURE OF THE WARM NEUTRAL MEDIUM AS A NEW PROBE OF THE Lyα RADIATION FIELD

Abstract

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 withthe Arecibo Observatory, we detect a widespread warm neutral medium component with excitation temperature(<i>T<sub>s</sub></i>) = 7200<span style="position:relative;bottom:0.6em;font-size:0.8em;">+1800</span><span style="position:relative;top:0.3em;font-size:0.8em;margin-left:-2.9em;">−1200</span> K (68% confidence). This temperature lies above theoretical predictions based on collisionalexcitation alone, implying that Ly<i>α</i> scattering, the most probable additional source of excitation, is more importantin the interstellar medium (ISM) than previously assumed. Our results demonstrate that HI absorption can be usedto constrain the Ly<i>α</i> radiation field, a critical quantity for studying the energy balance in the ISM and intergalacticmedium yet notoriously difficult to model because of its complicated radiative transfer, in and around galaxiesnearby and at high redshift.