Cosmic CO and [CII] backgrounds and the fueling of star formation over 12 Gyr

TL;DR

This study detects the cosmic CO and [CII] backgrounds over redshifts 0 to 4.2, revealing a larger molecular gas reservoir fueling star formation and establishing empirical benchmarks for future line-intensity mapping experiments.

Contribution

First empirical detection of the cosmic CO and [CII] backgrounds across a broad redshift range, providing direct measurements of molecular gas density and excitation in the universe.

Findings

Total molecular gas density is about twice that in galaxy surveys.

Global depletion time of molecular gas is approximately 1 Gyr.

Super-linear Kennicutt-Schmidt law links star formation to molecular gas surface density.

Abstract

Molecular gas, modest in mass yet pivotal within the cosmic inventory, regulates baryon cycling as the immediate fuel for star formation. Across most of cosmic history, its reservoir has remained elusive, with only the tip of the iceberg revealed by luminous carbon monoxide (CO) emitting galaxies. Here we report the first detections of the mean cosmic CO background across its rotational ladder at 7$\sigma$, together with ionized carbon ([CII]) at 3$\sigma$, over $0<z<4.2$. This uses tomographic clustering of diffuse broadband intensities with reference galaxies, directly probing aggregate emission in the cosmic web. From CO(1-0) we infer the total molecular gas density, $\Omega_{\rm H_2}$, finding it about twice that resolved in galaxy surveys. The global depletion time is $\sim$1 Gyr, shorter than the Hubble time, requiring sustained inflow. CO excitation links to star-formation surface density and, with depletion time, yields a super-linear Kennicutt-Schmidt law that appears universal. Together these results establish a global picture of galaxy growth fueled by a larger, short-lived molecular reservoir. The CO and [CII] detections mark a turning point for line-intensity mapping, replacing forecasts with empirical line strengths and defining sensitivity requirements for upcoming 3D experiments poised to open new windows on galaxy formation and cosmology.