Robert Yates (Surrey)
The metal density evolution of the Universe: Insights from observations and cosmological simulations
I will present a recent study of the cosmic metal density (ZD) evolution in the Universe back to z ~ 5, with a particular focus on the dense neutral gas phase. Recent observations of the ZD from damped Lyman-alpha systems (DLAs) are compared to three cosmological galaxy evolution simulations: L-GALAXIES 2020, EAGLE, and TNG100.
Intriguingly, DLA observations suggest that >85% of all metals are found in dense neutral gas at z > 4, whereas cosmological simulations suggest only <40%. Metals in the low-density circumgalactic medium surrounding galaxies also have a major contribution in all three simulations, even at high z. This reservoir is particularly difficult to detect observationally.
By also considering the evolution in cosmic SFR density (SFRD) and mean gas-phase metallicity, we determine two possible ways in which the absolute ZD observed in DLAs at high z can be matched by simulations: (a) the true SFRD at z > 3 is greater than inferred by current FUV observations, or (b) current samples of DLA metallicities are biased towards larger mean host masses than the underlying galaxy population.
I will also present important differences between the three simulations, which can help us constrain the efficiency of star formation, supernova feedback, and gas accretion in galaxies, via an analysis of their metal production, retention, and re-cycling. For example, those simulations which allow a high degree of metal retention in the densest gas are found to reasonably match the cosmic neutral gas ZD, but fail to simultaneously match DLA metallicities.