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

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 ${\tilde{I}}_s^2\left(\nu \right)=770\pm 210\mu K^{2}Hzsr$. After accounting for sample variance effects, the estimated spectral shot power is ${\tilde{I}}_s^2\left(\nu \right)={1010}_{-390}^{+550}\mu K^{2}Hzsr$. 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 α _CO = 3.6 M _⊙ (K km s⁻¹ pc²)⁻¹, we estimate a cosmic molecular gas density of ${\rho }_{H_2}\left(z\right)$ ∼ 10⁸ M _⊙ Mpc⁻³ between z = 1–3.