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The Baryon Budget of Galaxies across the First Billion Years -- Theoretical Predictions for Gas Phases, Depletion Times, and Stellar Return Fractions

Umberto Maio, Céline Péroux

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.

The Baryon Budget of Galaxies across the First Billion Years -- Theoretical Predictions for Gas Phases, Depletion Times, and Stellar Return Fractions

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.
Paper Structure (24 sections, 6 equations, 29 figures, 6 tables)

This paper contains 24 sections, 6 equations, 29 figures, 6 tables.

Table of Contents

  1. Introduction
  2. Method
  3. Results
  4. Global trends
  5. Mass density parameters
  6. Mass functions
  7. Baryon budget, galaxies and the IGM
  8. Mass phases in bound structures and IGM
  9. Phase relations
  10. Baryon cycling: Connecting gas and stars
  11. Stellar return fraction
  12. Gas depletion and star formation
  13. Discussion
  14. Model assumptions
  15. Model variations and implications for the early SFRD
  16. ...and 9 more sections

Figures (29)

  • Figure 1: Mass-weighted temperature maps at $z \simeq 14$, 10 and 6 smoothed on a grid of 1024 pixels a side for the gas in the largest halo that has a physical virial radius (circles) of $\sim2.4$, 5.9, and 18.7 kpc and a total mass of $1.4 \times 10^{9}$, $7.3 \times 10^{9}$, and $6.9 \times 10^{10} \,\rm M_\odot$, respectively, from left to right.
  • Figure 2: Baryon density parameters as function of redshift for cold (solid line), warm (dotted line), hot (short-dashed line), stellar (dot-dashed line with asterisks), HI (long-dashed line), and H$_2$ (dot-dot-dot-dashed line) phases. The horizontal grey line marks the value of $\Omega_{\rm 0,b}$.
  • Figure 3: Cold (top), warm (centre), and hot (bottom) gas mass functions at $z =$5.25, 6.14, 8.09, 10.00, 13.01, and 16.01.
  • Figure 4: HI (top), H$_2$ (centre),and stellar (bottom) mass functions at different $z$ with a fit to $z=0$ ALFALFA HI data by Jones2018 (top panel, dotted grey line), $z=6$--10 HST/Spitzer masses by Stefanon2021 (bottom panel, coloured crosses) and expectations by Jaacks2019 (bottom panel, coloured diamonds).
  • Figure 5: Baryon density parameters in bound objects (left) and the IGM (right) as function of redshift for cold (solid lines), warm (dotted lines), hot (short-dashed lines), stellar (dot-dashed lines with asterisks), HI (long-dashed lines), and H$_2$ (dot-dot-dot-dashed lines) phases. The horizontal grey line marks the value of $\Omega_{\rm 0,b}$. Stars form out of cold gas that dominates the baryon budget at $z \gtrsim 6$ and is overtaken by warm gas at later times.
  • ...and 24 more figures