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 ionized 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 the low density in these expansive regions. The most sensitive measurements come from detecting absorption due to 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. Extensive spectroscopic surveys of millions of objects provide an alternative approach to the problem. Here, we present evidence for a newly discovered, widely distributed, neutral, excited hydrogen component of the Galaxy’s halo. It is observed as the slight (0.779 ± 0.006%) absorption of flux near the rest wavelength of Hα 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. The stacking of nearly three-quarters of a million spectra has unearthed a previously unknown component of the Galactic halo: a widely distributed, neutral, excited hydrogen layer that could harbour a sizeable proportion of the Milky Way’s baryons.