Submillimeter flux as a probe of molecular ISM mass in high-$z$ galaxies

TL;DR

This study demonstrates through simulations that submillimeter flux reliably traces molecular ISM mass in high-redshift galaxies, supporting its use as an efficient observational proxy.

Contribution

It provides the first detailed simulation-based validation that sub-mm luminosity correlates with molecular gas mass in high-$z$ galaxies, including during starburst phases.

Findings

Sub-mm luminosity tightly correlates with molecular ISM mass.

The normalization of the sub-mm to ISM mass relation agrees with observations.

Sub-mm flux remains a reliable tracer during starburst episodes.

Abstract

Recent long wavelength observations on the thermal dust continuum suggest that the Rayleigh-Jeans (RJ) tail can be used as a time-efficient quantitative probe of the dust and ISM mass in high-$z$ galaxies. We use high-resolution cosmological simulations from the Feedback in Realistic Environment (FIRE) project to analyze the dust emission of $M_*>10^{10}\;M_{\odot}$ galaxies at $z=2-4$. Our simulations (MassiveFIRE) explicitly include various forms of stellar feedback, and they produce the stellar masses and star formation rates of high-$z$ galaxies in agreement with observations. Using radiative transfer modelling, we show that sub-millimeter (sub-mm) luminosity and molecular ISM mass are tightly correlated and that the overall normalization is in quantitative agreement with observations. Notably, sub-mm luminosity traces molecular ISM mass even during starburst episodes as dust mass and mass-weighted temperature evolve only moderately between $z=4$ and $z=2$, including during starbursts. Our finding supports the empirical approach of using broadband sub-mm flux as a proxy for molecular gas content in high-$z$ galaxies. We thus expect single-band sub-mm observations with ALMA to dramatically increase the sample size of high-$z$ galaxies with reliable ISM masses in the near future.