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

We present a detection of molecular gas emission at z similar to 1-5 using the technique of line intensity mapping. We make use of a pair of 3mm 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 zeropower hypothesis at 99.99% confidence, finding (I) over tilde (2)(s)(nu) = 770 +/- 210 mu K-2 Hz sr. After accounting for sample variance effects, the estimated spectral shot power is (I) over tilde (2)(s)(nu) = 1010(-390)(+550) mu K-2 Hz srs. 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)(+70) mu K-2 h(-3) Mpc(3), -210(-80)(+110) mu K-2 h(-3) Mpc(3), and 100(-40)(+50) mu K-2 h(-3) Mpc(3) 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 alpha(CO) = 3.6 M-circle dot (K km s(-1) pc(2))(-1), we estimate a cosmic molecular gas density of rho H-2 (z) similar to 10(8) M-circle dot Mpc(-3) between z = 1-3.