Atomic carbon as a powerful tracer of molecular gas in the high-redshift Universe: perspectives for ALMA

TL;DR

This study demonstrates that atomic carbon emission lines, observed with ALMA, serve as effective tracers of molecular hydrogen in high-redshift galaxies, enabling detailed studies of galaxy evolution over cosmic time.

Contribution

The paper introduces a simulation-based method showing that atomic carbon lines can reliably trace molecular gas in high-redshift galaxies using ALMA, extending observational capabilities.

Findings

Atomic carbon lines effectively trace H2 in high-redshift galaxies.

ALMA can detect most H2 in such galaxies within hours.

Low-resolution imaging reveals galaxy dynamics and mass.

Abstract

We use a high-resolution hydrodynamic simulation that tracks the non-equilibrium abundance of molecular hydrogen within a massive high-redshift galaxy to produce mock Atacama Large Millimeter Array (ALMA) maps of the fine-structure lines of atomic carbon, CI 1-0 and CI 2-1. Inspired by recent observational and theoretical work, we assume that CI is thoroughly mixed within giant molecular clouds and demonstrate that its emission is an excellent proxy for H2. Nearly all of the H2 associated with the galaxy can be detected at redshifts z<4 using a compact interferometric configuration with a large synthesized beam (that does not resolve the target galaxy) in less than 4 h of integration time. Low-resolution imaging of the \CI lines (in which the target galaxy is resolved into three to four beams) will detect ~80 per cent of the H2 in less than 12 h of aperture synthesis. In this case, the resulting data cube also provides the crucial information necessary for determining the dynamical state of the galaxy. We conclude that ALMA observations of the CI 1-0 and 2-1 emission are well-suited for extending the interval of cosmic look-back time over which the H2 distributions, the dynamical masses, and the Tully-Fisher relation of galaxies can be robustly probed.