Cosmic metal density evolution in neutral gas: insights from observations and cosmological simulations

Abstract

We contrast the latest observations of the cosmic metal density in neutral gas ($\rho _{\rm {met,neu}}$) with three cosmological galaxy evolution simulations: L-Galaxies 2020, TNG100, and EAGLE. We find that the fraction of total metals that are in neutral gas is <40 per cent at 3 ≲ z ≲ 5 in these simulations, whereas observations of damped Lyman-α (DLA) systems suggest ≳ 85 per cent. In all three simulations, hot, low-density gas is also a major contributor to the cosmic metal budget, even at high redshift. By considering the evolution in cosmic SFR density ($\rho _{\rm {\small {SFR}}}$), neutral gas density ($\rho _{\rm {HI}}$), and mean gas-phase metallicity ($[\langle {}\rm {M/H}\rangle ]_{\rm {neu}}$), we determine two possible ways in which the absolute $\rho _{\rm {met,neu}}$ observed in DLAs at high redshift can be matched by simulations: (a) the $\rho _{\rm {\small {SFR}}}$ at z ≳ 3 is greater than inferred from current FUV observations, or (b) current high-redshift DLA metallicity samples have a higher mean host mass than the overall galaxy population. If the first is correct, TNG100 would match the ensemble data best, however there would be an outstanding tension between the currently observed $\rho _{\rm {\small {SFR}}}$ and $\rho _{\rm {met,neu}}$. If the second is correct, L-Galaxies 2020 would match the ensemble data best, but would require an increase in neutral gas mass inside subhaloes above z ∼ 2.5. If neither is correct, EAGLE would match the ensemble data best, although at the expense of over-estimating $[\langle {}\rm {M/H}\rangle ]_{\rm {neu}}$. Modulo details related to numerical resolution and Hi mass modelling in simulations, these incompatibilities highlight current tensions between key observed cosmic properties at high redshift.

The ALMA REBELS Survey: The Cosmic H i Gas Mass Density in Galaxies at z ≈ 7

“…We find that the evolution of ρ HI (z) predicted from the simulations and DLA measurements shows a significant excess over the inferred values of ρ HI from the [C II]-to-H I relation at z  5 from this work. We observe, however, a striking match between our measurements and the evolutionary trend predicted for ρ HI,ISM (z) from Equation (3) and the L-Galaxies simulations by Yates et al (2021). While both our measurements of ρ HI and the function described by Equation (3) rely on the [C II]-to-H I conversion factor, we emphasize that one approach adopts the independently derived [C II]-158 μm LF, whereas the other is inferred from literature measurements of ρ å (z) and the average redshift evolution of the H I gas mass excess.…”

“…from the available DLA measurements (Péroux & Howk 2020), the global ρ HI (z) predicted by hydrodynamical simulations using various prescriptions for the inherent UV background (Maio et al 2022), and the predictions for the evolution of the H I gas in galaxies from the "sub-res models" of the L-Galaxies simulations (Yates et al 2021). The simulations by Maio et al (2022) mainly track the amount of the cosmic gas mass density in neutral, atomic form, showing significant decreases at z ≈ 6-8 from the initial state due to the reionization phase transition.…”

“…The simulations by Maio et al (2022) mainly track the amount of the cosmic gas mass density in neutral, atomic form, showing significant decreases at z ≈ 6-8 from the initial state due to the reionization phase transition. The L-Galaxies simulations described by Yates et al (2021) on the other hand are by construction designed to only model the neutral gas inside galaxies. We further make a prediction for ρ HI,ISM (z) in galaxies by combining the redshift evolution of the stellar-mass density ρ å (z) from Walter et al…”

The Column Densities of Molecular Gas across Cosmic Time: Bridging Observations and Simulations

“…While properties of cold gas in these simulations show various levels of (dis)agreement with observations [e.g. a higher cosmic mass density of HI and H 2 in IllustrisTNG compared to observations at 𝑧 = 0 (Diemer et al 2019), tensions concerning the the cosmic metal density evolution in neutral gas in EAGLE, IllustrisTNG and L-GALAXIES 2020 (Yates et al 2021), the lower molecular mass as a function of stellar mass and number of H 2 rich galaxies in IllustrisTNG compared to the ASPECS survey (Popping et al 2019)], other observables, like the HI column density distribution function have been accurately reproduced (Rahmati et al 2013). Therefore, further studies and comparisons of these and similar observables, like the 𝑓 (𝑁 H 2 ), are needed to improve the models and to design future observations.…”

L-GALAXIES 2020: The formation and chemical evolution of stellar haloes in Milky Way Analogues and galaxy clusters

“…This is because in-situ components tend to be more metalrich than ex-situ components at radii < 30 kpc (e.g. Monachesi et al 2019), as these stars would either originate from the relatively metalrich stellar disc of the central galaxy or from CGM material which in L-G 2020-MM is highly enriched at early times (Yates et al 2021b). Overall, such increases in stellar halo mass and [Fe/H] could lead to an improvement in the agreement between the L-G 2020-MM MZ h R relation and that observed.…”