On estimating the cosmic molecular gas density from CO Line Intensity Mapping observations

TL;DR

This paper investigates the cosmic molecular gas density at high redshift using CO line intensity mapping data, comparing it with simulations and exploring model adjustments to reconcile differences.

Contribution

It introduces a semianalytic modeling approach to interpret CO intensity mapping data and examines how model modifications can resolve discrepancies with observations.

Findings

Predicted CO power is mildly below observed values.

Increasing molecular gas abundance can reconcile models with data.

Assuming a lower CO-H2 conversion factor also resolves the tension.

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 $z\sim1-5$. 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.