Feeding cosmic star formation: Exploring high-redshift molecular gas with CO intensity mapping

TL;DR

This paper introduces a novel intensity mapping technique using $^{13}$CO to study molecular gas in high-redshift galaxies, providing insights into gas density and isotopologue ratios beyond traditional methods.

Contribution

It extends CO intensity mapping to include $^{13}$CO, enabling new measurements of gas properties and optical depth in distant galaxies.

Findings

Future surveys can constrain $^{13}$CO/$^{12}$CO ratios to ~30%.

Method offers unique insights into high-redshift molecular gas physics.

Provides a new tool for studying star formation and feedback in early galaxies.

Abstract

The study of molecular gas is crucial for understanding star formation, feedback, and the broader ecosystem of a galaxy as a whole. However, we have limited understanding of its physics and distribution in all but the nearest galaxies. We present a new technique for studying the composition and distribution of molecular gas in high-redshift galaxies inaccessible to existing methods. Our proposed approach is an extension of carbon monoxide intensity mapping methods, which have garnered significant experimental interest in recent years. These intensity mapping surveys target the 115 GHz $^{12}$CO (1-0) line, but also contain emission from the substantially fainter 110 GHz $^{13}$CO (1-0) transition. The method leverages the information contained in the $^{13}$CO line by cross-correlating pairs of frequency channels in an intensity mapping survey. Since $^{13}$CO is emitted from the same medium as the $^{12}$CO, but saturates at a much higher column density, this cross-correlation provides valuable information about both the gas density distribution and isotopologue ratio, inaccessible from the $^{12}$CO alone. Using a simple model of these molecular emission lines, we show that a future intensity mapping survey can constrain the abundance ratio of these two species and the fraction of emission from optically thick regions to order $\sim30\%$. These measurements cannot be made by traditional CO observations, and consequently the proposed method will provide unique insight into the physics of star formation, feedback, and galactic ecology at high redshifts.