On the contributions of extragalactic CO emission lines to ground-based CMB observations

TL;DR

This paper assesses how extragalactic CO emission lines at various redshifts could significantly impact ground-based CMB observations, especially as foregrounds, and explores their potential detectability and implications for cosmological studies.

Contribution

It introduces an empirical model linking galaxy infrared luminosity to CO line emission, analyzing its impact on CMB foregrounds and potential for constraining CO luminosity functions.

Findings

CO-CIB cross-correlation could be detectable at 3000 in ACT-like experiments.

CO CIB correlation may be the second-largest extragalactic foreground at certain wavelengths.

Current observations can constrain the bright end of CO luminosity functions.

Abstract

We investigate the potential of CO rotational lines at redshifts $z\sim 0-6$ being an appreciable source of extragalactic foreground anisotropies in the cosmic microwave background. Motivated by previous investigations, we specifically focus on the frequency bands and small scales probed by ground-based surveys. Using an empirical parameterization for the relation between the infrared luminosity of galaxies and their CO line luminosity, conditioned on sub-mm observations of CO luminosity functions from $J=1$ to $J=7$ at $\nu = \{100,250\}$ GHz, we explore how uncertainty in the CO luminosity function translates into uncertainty in the signature of CO emission in the CMB. We find that at $\ell = 3000$ the amplitude of the CO cross-correlation with the CIB could be detectable in an ACT-like experiment with 90, 150 and 220 GHz bands, even in the scenarios with the lowest amplitude consistent with sub-mm data. We also investigate, for the first time, the amplitude of the CO$\times$CIB correlation between different frequency bands and find that our model predicts that this signal could be the second-largest extragalactic foreground at certain wavelengths, behind the CIB cross-frequency spectrum. This implies current observations can potentially be used to constrain the bright end of CO luminosity functions, which are difficult to probe with current sub-mm telescopes due to the small volumes they survey. Our findings corroborate past results and have significant implications in template-based searches for CMB secondaries, such as the kinetic Sunyaev Zel'dovich effect, using the frequency-dependent high-$\ell$ TT power spectrum.