An Intensity Mapping Detection of Aggregate CO Line Emission at 3 mm

Abstract We present a detection of molecular gas emission at z ∼ 1–5 using the technique of line intensity mapping. We make use of a pair of 3 mm interferometric data sets, the first from the Atacama Large Millimeter/submillimeter Array (ALMA) Spectroscopic Survey in the Hubble Ultra Deep Field, and the second from a series of Atacama Compact Array (ACA) observations conducted between 2016 and 2018, targeting the COSMOS field. At 100 GHz, we measure nonzero power at 97.8% and 99.9% confidence in the ACA and ALMA data sets, respectively. In the joint result, we reject the zero-power hypothesis at 99.99% confidence, finding . After accounting for sample variance effects, the estimated spectral shot power is . We derive a model for the various line species our measurement is expected to be sensitive to, and estimate the shot power to be , , and for CO(2–1) at z = 1.3, CO(3–2) at z = 2.5, and CO(4–3) at z = 3.6, respectively. Using line ratios appropriate for high-redshift galaxies, we find these results to be in good agreement with those from the CO Power Spectrum Survey. Adopting α CO = 3.6 M ⊙ (K km s −1 pc 2 ) −1 , we estimate a cosmic molecular gas density of ∼ 10 8 M ⊙ Mpc −3 between z = 1–3.