# A 3D model for CO molecular line emission as a potential CMB polarization contaminant

## TL;DR

This paper develops a 3D simulation model for CO molecular line emission in galaxies, calibrates it with Planck data, and forecasts its potential to contaminate CMB polarization measurements, especially B-modes.

## Contribution

The paper introduces the MCMole3D Python package for simulating CO emission and assesses its impact on CMB polarization studies, a novel approach for contamination forecasting.

## Key findings

- Calibrated the model using Planck data on the Galactic plane.
- Forecasted CO contamination level equivalent to r ≈ 0.02 in CMB B-mode polarization.
- Provided a publicly available simulation tool for future CMB analysis.

## Abstract

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 simulations of this emission. We compare the simulations produced with this model and calibrate them, both on the map level and on the power spectrum level, 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 (CMB) 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 $r \lesssim 0.02$. The Monte-Carlo MOlecular Line Emission (MCMole3D) Python package, which implements this model, is being made publicly available.