Johanna Miller’s Search and Discovery story “Ten billion years ago, galaxies were already running out of gas” (Physics Today, December 2021, page 20) describes evidence that some galaxies deplete their interstellar matter to fuel star formation. Particularly important are cold interstellar gas clouds, where the atomic-to-molecular transition is a key step in sustaining new star formation. The story highlights recent Atacama Large Millimeter/Submillimeter Array observations of molecular gas in carbon monoxide microwave emission and far-IR emission from dust grains.1 But these two sentences were misleading: “Measuring the galaxies’ H2 content directly isn’t an option because H2 molecules themselves are essentially invisible. They’re symmetric and lack electric dipole moments, so they don’t absorb or emit radiation when they rotate and vibrate.”
Rumors of H2 invisibility have been greatly exaggerated. Although it’s true that the H2 ground electronic state has no dipole moment, two low-lying electronic states (molecular orbitals from 1s–2s and 1s–2p) do have dipole moments. Electronic absorption lines into those states lie in the far-UV and are known as the Lyman and Werner bands.2 Widely observed in the interstellar medium by the Copernicus and Far Ultraviolet Spectroscopic Explorer (FUSE) satellites, they provide diagnostics of the molecular fraction, gas temperature, and UV radiation field in diffuse and translucent interstellar clouds. FUSE also surveyed H2 in two external galaxies, the Large and Small Magellanic Clouds orbiting the Milky Way.3
UV H2 lines have also been seen in strong quasar absorption systems redshifted into the visible band.4 For systems in the Milky Way and local galaxies, measuring those absorption lines requires finding a bright UV background source, such as a hot star or quasar, behind the absorbing gas. That makes dark molecular clouds hard to probe in the UV. But FUSE observed H2 in a number of translucent clouds5 with one to five magnitudes of visual extinction and molecular fractions up to 75%.
Thanks to cosmological redshifting of light, H2 has now been observed in distant intervening galaxies in spectra of background quasars. The H2 far-UV absorption lines are shifted into portions of the UV accessible to the Hubble Space Telescope for galaxy redshifts z > 0.05 (a distance of 680 million light-years). At redshifts z > 1.8, the H2 lines can be observed by optical spectrographs. The European Southern Observatory’s Very Large Telescope in Chile has detected H2 in more than 22 distant galaxies (damped Lyman-alpha absorbers) at redshifts z between 1.96 and 4.22, corresponding to cosmological distances of 10.3 to 12.2 billion light-years. The recent Decadal Survey on Astronomy and Astrophysics supported plans for a 6 m optical, UV, and IR space telescope. With greatly enhanced sensitivity in the far-UV (90–200 nm), that facility would be a powerful tool for probing the atomic and molecular gas that fuels star formation in galaxies.
