An Intensity Mapping Constraint on the CO-Galaxy Cross Power Spectrum at Redshift ~ 3

The abundance of cold molecular gas plays a crucial role in models of galaxy evolution. While deep spectroscopic surveys of CO emission lines have been a primary tool for measuring this abundance, the difficulty of these observations has motivated alternative approaches to studying molecular gas content. One technique, line intensity mapping, seeks to constrain the average molecular gas properties of large samples of individually undetectable galaxies through the CO brightness power spectrum. Here we present constraints on the cross-power spectrum between CO intensity maps and optical galaxy catalogs. This cross-measurement allows us to check for systematic problems in CO intensity mapping data, and validate the data analysis used the auto-power spectrum measurement of the CO Power Spectrum Survey. We place a 2-sigma upper limit on the band-averaged CO-galaxy cross-power of $P_x<540$ uK Mpc$^3$/h$^3$. Our measurement favors a non-zero mean CO brightness temperature at around 90% confidence and gives an upper limit on the mean molecular gas density at z~2.6 of $7.7 \times 10^8$ Msun/Mpc$^3$. We forecast the expected cross-power spectrum by applying a number of literature prescriptions for the CO luminosity to halo mass relation to a suite of mock light cones. Under the most optimistic forecasts the cross-spectrum could be detected with only moderate extensions of the data used here, while more conservative models could be detected with a factor of 10 increase in sensitivity. Ongoing CO intensity mapping experiments will target fields allowing extensive cross correlation analysis and should reach the sensitivity required to detect the cross-spectrum signal.