The Baryon Budget of Galaxies across the First Billion Years

TL;DR

This study uses advanced simulations to comprehensively analyze the distribution and evolution of baryonic matter in early galaxies across cosmic time, linking gas phases with galaxy properties.

Contribution

It provides a detailed, phase-resolved census of baryons in early galaxies, including new insights into gas phases, stellar return fractions, and their dependencies on galaxy characteristics.

Findings

Cosmic gas is mostly cold before reionisation and warm dominates later.

Stellar return fractions are about 0.15-0.20, lower than commonly assumed.

Gas-to-star fractions decline with galaxy mass, with star formation efficiencies of a few percent.

Abstract

We provide a complete census of the baryons in early galaxies to investigate the phases in which gas and stars reside, their corresponding budgets, depletion times, and stellar return fraction as a function of redshift and stellar age. We use the ColdSIM hydrodynamical time-dependent non-equilibrium chemistry simulations and perform a detailed analysis of the cold, warm, hot, and stellar phases for both bound structures (galaxies/CGM) and the diffuse IGM. We investigate in depth the cold HI and H2 components, explicitly computed in our simulations, and their relations with host mass, SFR, metallicity and depletion times. We also provide observational insights and discuss the implications for stellar mass functions, PopIII star formation and changes in the IMF. We find that cosmic gas prior to reionisation is mostly cold, while at later epochs the warm phase becomes dominant due to enhanced star formation activity and increasing UV reionising radiation. Stellar return fractions at these times are ~0.15-0.20, a factor of two lower than the values usually adopted. Cold, warm, and hot gas masses as well as HI and H2 components show increasing trends with mass and SFR, while depletion times decrease down to 0.01-0.1 Gyr with a weak metallicity dependence. The resulting star formation efficiency remains at the level of a few per cent and gas-to-star fractions decline with mass, influenced by local feedback and environment. Our findings are consistent with ALMA, VLA and IRAM surveys at later epochs, including ALFALFA, xCOLDGASS, GASS, xGASS, EDGE-CALIFA, PHIBBS, and ASPECS. Gas phases are quantitatively related to the underlying stellar populations and can be used to infer unknown quantities. In the appendix we provide fit functions describing the trends of the stellar return fraction, the main sequence, phase mass relations, gas-to-star fractions and depletion times.