Cold molecular gas, traced by CO emission, is the primary fuel for star formation and key to understanding how galaxies grow, evolve, and eventually quench. Until now, uncertainties in these measurements have been dominated by cosmic variance, the field-to-field variation that dominates small or single-field surveys.
A major step forward has been achieved in a recent study by Bollo et al. (2025), which presents new results from the ALMACAL-22 survey, a deep and uniquely wide-area analysis using over 1100 ALMA data cubes spanning 401 independent calibrator fields. Archival calibration data present an essentially unbiased, CO-selected galaxy sample, reducing cosmic variance to below 5% (Figure 1).
The results reveal a smooth evolution of the cosmic molecular gas mass density (ρH2) from redshift z ~ 6, peaking around z ~ 1.5, before declining towards the present day (Figure 2). This trend mirrors the evolution of the cosmic star formation, highlighting the critical role of molecular gas in fuelling star formation throughout the Universe's history. The study also confirms that the ratio of molecular gas to stellar mass aligns with the so-called bathtub model of galaxy evolution, where gas is steadily replenished as it is consumed by star formation.
By explicitly quantifying cosmic variance, ALMACAL-22 explains past inconsistencies between different studies and sets a new benchmark for future cosmic gas measurements, emphasising the need to target multiple independent sightlines to complement deep fields.
Contributed by Victoria Bollo
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