Extragalactic CO in the CMB

Extragalactic CO emission has until recently been expected to represent a negligible foreground in CMB analyses. However, recent work has shown it to potentially have a significant imprint on the small scales where current and upcoming, next-generation CMB surveys are designed to measure and constrain cosmology. The amplitude of this component and its cross correlation with the broadband CIB, both of which trace star formation, are both highly uncertain. To quantify these possible signatures, we calculate CO and CIB power spectra from a variety of models in the literature, which span roughly 2 orders of magnitude, across a common set of simulations. We find that while the autopower of CO is small relative to other foregrounds, its cross correlation with the CIB may be comparable to other CMB foregrounds of interest, like the kSZ signal, since both the CIB and CO are tracers of star formation. However, there remains a great deal of uncertainty on the expected power due to the range of proposed theoretical models. Understanding extragalactic CO is therefore essential to extracting small scale CMB science, such as detecting the kSZ effect. Data from current and upcoming broadband CMB surveys also then provide a unique opportunity to measure CO with a method complementary to future line intensity mapping surveys.