A 3D model for carbon monoxide molecular line emission as a potential cosmic microwave background polarization contaminant

We present a model for simulating carbon monoxide (CO) rotational line emission in molecular clouds, taking account of their 3D spatial distribution in galaxies with different geometrical properties. The model implemented is based on recent results in the literature and has been designed for performing Monte Carlo (MC) simulations of this emission. We compare the simulations produced with this model and calibrate them, both on the map and the power spectrum levels, using the second release of data from the Planck satellite for the Galactic plane, where the signal-to-noise ratio is highest. We use the calibrated model to extrapolate the CO power spectrum at low Galactic latitudes where no high sensitivity observations are available yet. We then forecast the level of unresolved polarized emission from CO molecular clouds which could contaminate the power spectrum of cosmic microwave background polarization B modes away from the Galactic plane. Assuming realistic levels of the polarization fraction, we show that the level of contamination is equivalent to a cosmological signal with r ≲ 0.02. The MC MOlecular Line Emission (MCMOLE3D) PYTHON package, which implements this model, is being made publicly available.