On Estimating the Cosmic Molecular Gas Density from CO Line Intensity Mapping Observations

Abstract

Abstract The Millimeter-wave Intensity Mapping Experiment (mmIME) recently reported a detection of excess spatial fluctuations at a wavelength of 3 mm, which can be attributed to unresolved emission of several CO rotational transitions between z ∼ 1 and 5. We study the implications of these data for the high-redshift interstellar medium using a suite of state-of-the-art semianalytic simulations that have successfully reproduced many other submillimeter line observations across the relevant redshift range. We find that the semianalytic predictions are mildly in tension with the mmIME result, with a predicted CO power ∼3.5σ below what was observed. We explore some simple modifications to the models that could resolve this tension. Increasing the molecular gas abundance at the relevant redshifts to ∼108 M ⊙ Mpc−3, a value well above that obtained from directly imaged sources, would resolve the discrepancy, as would assuming a CO–H2 conversion factor α CO of ∼1.5 M ⊙ K−1 (km s−1)−1 pc2, a value somewhat lower than is commonly assumed. We go on to demonstrate that these conclusions are quite sensitive to the detailed assumptions of our simulations, highlighting the need for more careful modeling efforts as more intensity mapping data become available.