Evolution of molecular gas over cosmic time: a new approach using 4MOST and Gaia – CI-CNM

The molecular gas phase in galaxies is an important tracer of the gas out of which stars are formed. However, observing this molecular gas is extremely difficult. Furthermore, state-of-the-art simulations provide different predictions for the amount of such gas in galaxies depending on the numerical methods applied. This project aims at providing new and complete observations to constrain the amount of molecular gas over cosmic time using intervening H2 and CI absorption lines in quasar spectra observed with the upcoming 4MOST instrument. The direct observable lines of H2 are only available at high redshift, z > 2.5. We therefore use CI as a tracer at lower redshifts, z > 1.5. We will furthermore constrain dust properties of the gas in order to better model the selection effects in our data. Lastly, we will compare our results on H2 to 3 numerical simulations (FIREbox, SPHINX, NewHorizon) using different codes to test the effect of numerical methods. The simulations also include different physical mechanisms (or feedback) which allows us to compare results with and without the impact of super-massive black holes in galaxy evolution. The coordinator of this project is the PI of a large survey on 4MOST whose aim is to select quasars in a novel way using only Gaia astrometry. This selection removes previous color selection effects that have been imposed in all previous surveys. We will therefore for the first time be able to constrain the molecular gas absorption in a statistical sense. Our robust constraints on the molecular gas phase around cosmic noon will therefore be perfectly suited for comparisons to numerical simulations. Members of all 3 simulation teams are part of this project to ensure a fair comparison of the codes.