Intertwined formation of H₂, dust, and stars in cosmological simulations

¹ IATE – Instituto de Astronomía Teórica y Experimental, Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina (CONICET), Universidad Nacional de Córdoba, Laprida 854, X5000BGR Córdoba, Argentina
² INAF, Osservatorio Astronomico di Trieste, via Tiepolo 11, I-34131 Trieste, Italy
³ IFPU, Institute for Fundamental Physics of the Universe, Via Beirut 2, 34014 Trieste, Italy
⁴ SISSA, Via Bonomea 265, I-34136 Trieste, Italy
⁵ Dipartimento di Fisica dell’Università di Trieste, Sez. di Astronomia, via Tiepolo 11, I-34131 Trieste, Italy
⁶ INFN, Instituto Nazionale di Fisica Nucleare, Via Valerio 2, I-34127 Trieste, Italy
⁷ ICSC – Italian Research Center on High Performance Computing, Big Data and Quantum Computing, via Magnanelli 2, 40033 Casalecchio di Reno, Italy

^⋆ Corresponding author; cinthia.ragone@unc.edu.ar

Received: 2 July 2024
Accepted: 25 September 2024

Abstract

Context. Molecular hydrogen (H₂) plays a crucial role in the formation and evolution of galaxies, serving as the primary fuel reservoir for star formation. In a metal-enriched Universe, H₂ forms mostly through catalysis on interstellar dust grain surfaces. However, due to the complexities of modelling this process, star formation in cosmological simulations often relies on empirical or theoretical frameworks that have only been validated in the local Universe to estimate the abundance of H₂.

Aims. The goal of this work is to model the connection between the processes of star, dust, and H₂ formation in our cosmological simulations.

Methods. Building upon our recent integration of a dust evolution model into the star formation and feedback model MUPPI, we included the formation of molecular hydrogen on the surfaces of dust grains. We also accounted for the destruction of molecules and their shielding from harmful radiation.

Results. The model reproduces, reasonably well, the main statistical properties of the observed galaxy population for the stellar, dust, and H₂ components. The evolution of the molecular hydrogen cosmic density (ρ_H2) in our simulated boxes peaks around redshift z = 1.5, consistent with observations. Following its peak, ρ_H2 decreases by a factor of two towards z = 0, which is a milder evolution than observed. Similarly, the evolution of the molecular hydrogen mass function since z = 2 displays a gentler evolution when compared to observations. Our model recovers satisfactorily the integrated molecular Kennicut-Schmidt (mKS) law between the surface star formation rate (Σ_SFR) and surface H₂ density (Σ_H2) at z = 0. This relationship is already evident at z = 2, albeit with a higher normalization. We find hints of a broken power law with a steeper slope at higher Σ_H2. We also study the H₂-to-dust mass ratio in galaxies as a function of their gas metallicity and stellar mass, observing a decreasing trend with respect to both quantities. The H₂-to-dust mass fraction for the global population of galaxies is higher at higher redshift. The analysis of the atomic-to-molecular transition on a particle-by-particle basis suggests that gas metallicity cannot reliably substitute the dust-to-gas ratio in models attempting to simulate dust-promoted H₂.