[Paper Review] Simulations and modelling of the ISM in galaxies

Claudia del P. Lagos, C. G. Lacey+1 more|arXiv (Cornell University)|Oct 17, 2012

Galaxies: Formation, Evolution, Phenomena6 references3 citations

TL;DR

This paper reviews hydrodynamical simulations and semi-analytic models of galaxy formation, emphasizing how improved modeling of the interstellar medium (ISM)—particularly molecular and atomic gas phases and star formation—enables better reproduction of observed scaling relations and redshift evolution. It demonstrates that including physical ISM treatments, such as CO emission and feedback-driven turbulence, significantly enhances model agreement with observations from ALMA and other radio/submillimeter telescopes.

ABSTRACT

The latest observations of molecular gas and the atomic hydrogen content of local and high-redshift galaxies, coupled with how these correlate with star formation activity, have revolutionized our ideas about how to model star formation in a galactic context. A successful theory of galaxy formation has to explain some key facts: (i) high-redshift galaxies have higher molecular gas fractions and star formation rates than local galaxies, (ii) scaling relations show that the atomic-to-stellar mass ratio decreases with stellar mass in the local Universe, and (iii) the global abundance of atomic hydrogen evolves very weakly with time. We review how modern cosmological simulations of galaxy formation attempt to put these pieces together and highlight how approaches simultaneously solving dark matter and gas physics, and approaches first solving the dark matter N-body problem and then dealing with gas physics using semi-analytic models, differ and complement each other. We review the observable predictions, what we think we have learned so far and what still needs to be done in the simulations to allow robust testing by the new observations expected from telescopes such as ALMA, PdBI, LMT, JVLA, ASKAP, MeerKAT, SKA.

Motivation & Objective

• To evaluate how modern cosmological simulations and semi-analytic models address the complex physics of the interstellar medium (ISM) in galaxies.
• To assess the impact of improved ISM modeling—especially on molecular and atomic gas, star formation, and feedback—on reproducing observed galaxy scaling relations.
• To identify key discrepancies between models and observations, particularly regarding the evolution of atomic hydrogen and molecular gas fractions with redshift.
• To evaluate the predictive power of current models in light of upcoming high-sensitivity radio and submillimeter surveys from ALMA, SKA, and related telescopes.
• To compare the strengths and limitations of hydrodynamical simulations (parallel approach) and semi-analytic models (serial approach) in modeling ISM physics and galaxy evolution.

Proposed method

• Utilizes hydrodynamical simulations that self-consistently model dark matter, gas dynamics, cooling, star formation, and supernova feedback on resolved ISM scales.
• Employs semi-analytic models that first solve the N-body problem for dark matter halos and then apply parameterized physical models for gas cooling, star formation, and feedback.
• Incorporates physical treatments of CO emission in ISM, including CO excitation and luminosity functions, to compare with observational data.
• Applies statistical modeling of gas mass functions (HI and H2) and scaling relations between gas content, stellar mass, and star formation rate.
• Uses simulated observations (e.g., synthetic ALMA maps of CO(3-2) and CO(6-5)) to test model predictions against expected telescope sensitivity and resolution.
• Compares model outputs with observational constraints such as the CO(1-0) luminosity function at z ≈ 0 and z ≈ 2, and the HI mass function in the local universe.

Experimental results

Research questions

• RQ1 How do hydrodynamical simulations and semi-analytic models differ in their treatment of ISM physics and their ability to reproduce observed gas scaling relations?
• RQ2 What role does supernova feedback play in sustaining ISM turbulence and regulating star formation in galaxies, and how is this captured in simulations?
• RQ3 To what extent can physical modeling of CO emission improve the agreement between simulations and observations of molecular gas at high redshift?
• RQ4 Why do current models struggle to reproduce the weak evolution of atomic hydrogen abundance with cosmic time, and what improvements are needed?
• RQ5 How will next-generation telescopes like ALMA, SKA, and MeerKAT constrain and refine ISM modeling in galaxy formation simulations?

Key findings

• Hydrodynamical simulations show that the observed relation between surface star formation rate and molecular gas surface density arises from a common dependence on gas surface density and the stabilizing effect of supernova-driven turbulence.
• Semi-analytic models that include hydrostatic, chemically equilibrated ISM and physical star formation prescriptions successfully reproduce the observed HI and H2 mass functions and the decreasing atomic-to-stellar mass ratio with increasing stellar mass.
• The inclusion of physical CO emission modeling in semi-analytic models (e.g., Lagos et al. 2012) improves agreement with observed CO(1-0) luminosity functions at z ≈ 0 and z ≈ 2, helping to rule out models with unphysical gas fractions.
• Simulated ALMA observations of CO(3-2) and CO(6-5) emission at z ≈ 2 show that full ALMA configurations can detect typical star-forming galaxies within 20 minutes and 5 hours of integration, respectively, validating model predictability.
• The models predict a strong evolution in the number density of bright CO(1-0) galaxies from z ≈ 0 to z ≈ 2, consistent with JVLA and ALMA follow-up campaigns in the COSMOS field.
• Despite progress, discrepancies remain in modeling morphological transformations and the detailed evolution of atomic gas, indicating a need for improved sub-grid physics in both simulation approaches.