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

The Millimeter-wave Intensity Mapping Experiment (mmIME) recently reported a detection of excess spatial fluctuations at wavelength 3 mm, which can be attributed to unresolved emission several CO rotational transitions between $z\sim1-5$. We study the implications this data for high-redshift interstellar medium using suite state-of-the-art semianalytic simulations have successfully reproduced many other sub-millimeter line observations across relevant redshift range. find that predictions are mildly in tension with mmIME result, predicted power $\sim3.5\sigma$ below what was observed. explore some simple modifications models could resolve tension. Increasing molecular gas abundance redshifts $\sim10^8\ M_\odot\ \rm{Mpc}^{-3}$, value well above obtained from directly imaged sources, would discrepancy, as assuming CO-$H_2$ conversion factor $\alpha_{\rm{CO}}$ $\sim1.5\ M_{\odot}$ K$^{-1}$ $(\rm{km}/\rm{s})^{-1}$ pc$^{2}$, somewhat lower than is commonly assumed. go on demonstrate these conclusions quite sensitive detailed assumptions our simulations, highlighting need more careful modeling efforts intensity mapping become available.