From Dawn till Dusk

About 90% of visible (baryonic) matter in the Universe is made of cosmic gas. This is the main driver of star formation and galaxy growth through cooling and condensation, from the early cosmic phases, the cosmic dawn, till reionization and later cosmological epochs. Gas fragments in low-temperature sites having a chemical composition dominated by neutral hydrogen, HI, and molecular hydrogen, H2, the most abundant molecule in the Cosmos. Therefore, investigating cold-gas evolution and the origin of its (atomic and molecular) components is one of the most challenging goals from both a theoretical and an observational point of view.

Lately, a joint study conducted by scientists at the Trieste Astronomical Observatory, the Max Planck Institute for Astrophysics (MPA) in Germany and the European Southern Observatory (ESO) has shed light on the origin and evolution of these chemical species. The work, which is to appear on Astronomy & Astrophysics, leading author Umberto Maio, from Trieste Observatory, has employed advanced three-dimensional numerical simulations and some ten million high-performance-computing hours to follow the physical processes governing the thermal state of atoms and molecules from primordial to more recent times. Besides the several H2 formation channels, the models have implemented for the first time very detailed mechanisms taking place in cold cosmic gas, such as molecule formation on dust grain surface, photoelectric or cosmic-ray heating, as well as gas self-shielding from UV radiation.

The authors have compared their theoretical predictions against ALMA, VLA, NOEMA and UKIRT observational HI and H2 determinations and discovered that H2 abundances can be explained via electron/H- catalysis at low metallicities and via dust grain catalysis at high metallicities in UV-shielded cold gas. Notwithstanding the non-trivial impacts of the various processes, the timing in which stars and galaxies produce UV photons and reionize cosmic gas is crucial to reach an agreement between theory and observations. The results of this study show that the build-up of a UV background by the first half billion years is in tension with subsequent molecule formation. A slightly delayed reionization epoch, during which UV radiation is produced at milder rates, is instead favoured. Furthermore, gas depletion timescales highlight the possibility to form large reservoirs of H2, locally, and explain its origin even in the very early epochs.

In light of such findings, future discoveries of primordial molecular-rich star forming galaxies are expected in the next years and new data from upcoming international facilities will be decisive to understand the whole baryon cycle.

Simulated matter density field with corresponding neutral (left) and molecular-hydrogen (right) maps.