HYACINTH: HYdrogen And Carbon chemistry in the INTerstellar medium in Hydro simulations

¹ Argelander Institute für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
e-mail: pkhatri@astro.uni-bonn.de
² SISSA, International School for Advanced Studies, Via Bonomea 265, 34136 Trieste, TS, Italy
³ Dipartimento di Fisica – Sezione di Astronomia, Università di Trieste, Via Tiepolo 11, 34131 Trieste, Italy
⁴ IFPU, Institute for Fundamental Physics of the Universe, via Beirut 2, 34151 Trieste, Italy
⁵ Universität zu Köln, I. Physikalisches Institut, Zülpicher Str 77, 50937 Köln, Germany
⁶ TÜV NORD EnSys GmbH & Co. KG, Am TÜV 1, 30519 Hannover, Germany

Received: 16 February 2024
Accepted: 13 June 2024

Abstract

Aims. We present a new sub-grid model, HYACINTH – HYdrogen And Carbon chemistry in the INTerstellar medium in Hydro simulations – for computing the non-equilibrium abundances of H₂ and its carbon-based tracers, namely CO, C, and C⁺, in cosmological simulations of galaxy formation.

Methods. The model accounts for the unresolved density structure in simulations using a variable probability distribution function of sub-grid densities and a temperature-density relation. Included is a simplified chemical network that has been tailored for hydrogen and carbon chemistry within molecular clouds and easily integrated into large-scale simulations with minimal computational overhead. As an example, we applied HYACINTH to a simulated galaxy at redshift z ~ 2.5 in post-processing and compared the resulting abundances with observations.

Results. The chemical predictions from HYACINTH are in reasonable agreement with high-resolution molecular-cloud simulations at different metallicities. By post-processing a galaxy simulation with HYACINTH, we reproduced the H I − H₂ transition as a function of the hydrogen column density N_H for both Milky-Way-like and Large-Magellanic-Cloud-like conditions. We also matched the N_CO versus N_H2 relation inferred from absorption measurements towards Milky-Way molecular clouds, although most of our post-processed regions occupy the same region as (optically) dark molecular clouds in the N_CO – N_H2 plane. Column density maps reveal that CO is concentrated in the peaks of the H₂ distribution, while atomic carbon more broadly traces the bulk of H₂ in our post-processed galaxy. Based on both the column density maps and the surface density profiles oŕ the different gas species in the post-processed galaxy, we find that C⁺ maintains a substantially high surŕace density out to ~10 kpc as opposed to other components that exhibit a higher central concentration. This is similar to the extended [C II] emission ŕound in some recent observations at high redshifts.