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 a wavelength of 3 mm, which can be
attributed to unresolved emission of several CO rotational transitions between
$z\sim1-5$. We study the implications of this data for the high-redshift
interstellar medium using a suite of state-of-the-art semianalytic simulations
which have successfully reproduced many other sub-millimeter 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
$\sim3.5\sigma$ below what was observed. We explore some simple modifications
to the models which could resolve this tension. Increasing the molecular gas
abundance at the relevant redshifts to $\sim10^8\ M_\odot\ \rm{Mpc}^{-3}$, a
value well above that obtained from directly imaged sources, would resolve the
discrepancy, as would assuming a CO-$H_2$ conversion factor $\alpha_{\rm{CO}}$
of $\sim1.5\ M_{\odot}$ K$^{-1}$ $(\rm{km}/\rm{s})^{-1}$ pc$^{2}$, 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.
