The Galaxy's Veil of Excited Hydrogen

Many of the baryons in our Galaxy probably lie outside the well known disk and bulge components. Despite a wealth of evidence for the presence of some gas in galactic halos, including absorption line systems in the spectra of quasars, high velocity neutral hydrogen clouds in our Galaxy halo, line emitting ionised hydrogen originating from galactic winds in nearby starburst galaxies, and the X-ray coronas surrounding the most massive galaxies, accounting for the gas in the halo of any galaxy has been observationally challenging primarily because of its low density in the expansive halo. The most sensitive measurements come from detecting absorption by the intervening gas in the spectra of distant objects such as quasars or distant halo stars, but these have typically been limited to a few lines of sight to sufficiently bright objects. Massive spectroscopic surveys of millions of objects provide an alternative approach to the problem. Here, we present the first evidence for a widely distributed, neutral, excited hydrogen component of the Galaxy's halo. It is observed as the slight, (0.779 $\pm$ 0.006)\%, absorption of flux near the rest wavelength of H$\alpha$ in the combined spectra of hundreds of thousands of galaxy spectra and is ubiquitous in high latitude lines of sight. This observation provides an avenue to tracing, both spatially and kinematically, the majority of the gas in the halo of our Galaxy.