The art of modeling CO, [CI], and [CII] in cosmological galaxy formation models

TL;DR

This paper systematically explores how different sub-grid modeling choices in cosmological galaxy formation simulations affect the predicted sub-millimeter line emissions, emphasizing the importance of certain physical assumptions for accurate modeling.

Contribution

It introduces a comprehensive analysis of sub-grid modeling impacts on CO, [CI], and [CII] emissions, identifying key physical assumptions that improve model accuracy.

Findings

A molecular cloud mass-size relation is crucial for accurate emission predictions.

Assumptions about gas clumping and cloud mass distribution slope have minimal impact.

The [CII]-SFR ratio inversely correlates with molecular cloud pressure.

Abstract

The advent of new sub-millimeter observational facilities has stimulated the desire to model the sub-mm line emission of galaxies within cosmological galaxy formation models. This is typically done in post-processing by applying sub-resolution recipes to describe the properties of the unresolved interstellar medium. At the same time, while there is freedom in how one implements these sorts of recipes, the impact of various choices has yet to be systematically explored in simulations. In this paper, we do just that. We combine a semi-analytic model of galaxy formation with chemical equilibrium networks and numerical radiative transfer models and explore how different choices for the sub-grid modeling affect the predicted CO, [CI], and [CII] emission of galaxies. We find that a key component for a successful model includes a molecular cloud mass-size relation and scaling for the ultraviolet and cosmic ray radiation field that varies based on local ISM properties. Our most successful model adopts a Plummer profile for the radial density distribution of gas within molecular clouds. On the other hand, different assumptions for the clumping of gas within molecular clouds and changes in the slope of the molecular cloud mass distribution function hardly affect the CO, [CI], and [CII] luminosities of galaxies. At fixed star-formation rate the [CII]-SFR ratio of galaxies scales inversely with the pressure acting on molecular clouds, increasing the molecular clouds density and hence decreasing the importance of [CII] line cooling. Overall we find that it is essential that a wide range of sub-mm emission lines arising in vastly different phases of the ISM are used as model constraints in order to limit the freedom in sub-grid choices and we present the details of the most successful model variant.