2020ApJ...901..141K

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 \tildeI_s²(ν)=770±210µK²Hzsr. After accounting for sample variance effects, the estimated spectral shot power is \tildeI_s²(ν)=1010_–390⁺⁵⁵⁰µK²Hzsr. We derive a model for the various line species our measurement is expected to be sensitive to, and estimate the shot power to be 120_–50⁺⁷⁰µK²h^–3Mpc³, 210_–80⁺¹¹⁰µK²h^–3Mpc³, and 100_–40⁺⁵⁰µK²h^–3Mpc³ 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 pc²)^–1, we estimate a cosmic molecular gas density of ρ_H2_(z) ∼ 10⁸ M_☉ Mpc^–3 between z = 1-3.