A New Framework for ISM Emission Line Models: Connecting Multi-Scale Simulations Across Cosmological Volumes

TL;DR

This paper introduces a multi-scale simulation framework combining high-resolution FIRE simulations, semi-analytic line emission modeling, and machine learning to predict ISM emission lines in early galaxies, aligning well with observations and aiding future surveys.

Contribution

The novel framework connects detailed sub-grid ISM models with large cosmological simulations using machine learning, enabling accurate emission line predictions at cosmic scales.

Findings

Predicted line luminosities match current observations.

Framework highlights the importance of detailed sub-grid ISM models.

Forecasts for JWST line luminosity measurements and cosmic variance.

Abstract

The JWST and ALMA have detected emission lines from the ionized interstellar medium (ISM) in some of the first galaxies at $z \gtrsim 6$. These measurements present an opportunity to better understand galaxy assembly histories and may allow important tests of state-of-the-art galaxy formation simulations. It is challenging, however, to model these lines in their proper cosmological context. In order to meet this challenge, we introduce a novel sub-grid line emission modeling framework. The framework uses the high-$z$ zoom-in simulation suite from the Feedback in Realistic Environments (FIRE) collaboration. The line emission signals from HII regions within each simulated FIRE galaxy are modeled using the semi-analytic HIILines code. A machine learning approach is then used to determine the conditional probability distribution for the line luminosity to stellar-mass ratio from the HII regions around each simulated stellar particle. This conditional probability distribution can then be applied to predict the line luminosities around stellar particles in lower resolution, yet larger volume cosmological simulations. As an example, we apply this approach to the IllustrisTNG simulations at $z=6$. The resulting predictions for the [OII], [OIII], and Balmer line luminosities as a function of star-formation rate (SFR) agree well with current observations. Our predictions differ, however, from related work in the literature which lack detailed sub-grid ISM models. This highlights the importance of our multi-scale simulation modeling framework. Finally, we provide forecasts for future line luminosity function measurements from the JWST and quantify the cosmic variance in such surveys.