The ISM in Distant Galaxies

The interstellar medium (ISM) is a key ingredient in galaxy formation and evolution as it provides the molecular gas reservoir which fuels star formation and supermassive black hole accretion. Yet the ISM is one of the least studied aspects of distant galaxies. Molecular and atomic transitions at (sub)millimetre wavelengths hold great promise in measuring macroscopic properties (e.g. masses, morphologies, star formation laws), as well as microscopic properties (e.g. gas densities, temperatures, cooling) of high-z galaxies. In this overview I summarize the growing number of high-z molecular line detections, highlighting some of the most intriguing results along the way. I end by discussing a few areas where future facilities (e.g. ALMA, EVLA, CCAT, LMT) will drastically improve on the current state of affairs.

💡 Research Summary

The paper provides a comprehensive overview of the current state of knowledge regarding the interstellar medium (ISM) in high‑redshift (high‑z) galaxies, emphasizing the pivotal role that (sub)millimetre molecular and atomic line observations play in probing both macroscopic and microscopic properties of these distant systems. The author begins by underscoring that the ISM supplies the molecular gas reservoir that fuels star formation and supermassive black‑hole accretion, yet it remains one of the least explored components of early‑universe galaxies due to observational challenges.

The review then outlines the principal (sub)mm tracers used to characterize the ISM. Carbon monoxide (CO) rotational transitions, ranging from the ground state CO(1‑0) up to higher‑J lines such as CO(6‑5) or CO(7‑6), are highlighted as the workhorse for estimating total molecular gas masses, dynamical masses, and excitation conditions. The author explains how line‑ratio diagnostics, combined with radiative‑transfer modelling (LVG, RADEX) and photon‑dominated region (PDR) or X‑ray dominated region (XDR) frameworks, allow researchers to infer gas temperature, density, and the relative importance of star‑formation‑driven versus AGN‑driven heating.

Atomic fine‑structure lines, especially