Delensing the CMB with the Cosmic Infrared Background

As confusion with lensing B-modes begins to limit experiments that search for primordial B-mode polarization, robust methods for delensing the CMB polarization sky are becoming increasingly important. We investigate in detail the possibility of delensing the CMB with the cosmic infrared background (CIB), emission from dusty star-forming galaxies that is an excellent tracer of the CMB lensing signal, in order to improve constraints on the tensor-to-scalar ratio $r$. We find that the maps of the CIB, such as current Planck satellite maps at 545 GHz, can be used to remove more than half of the lensing B-mode power. Calculating optimal combinations of different large-scale-structure tracers for delensing, we find that co-adding CIB data and external arcminute-resolution CMB lensing reconstruction can lead to significant additional improvements in delensing performance. We investigate whether measurement uncertainty in the CIB spectra will degrade the delensing performance if no model of the CIB spectra is assumed, and instead the CIB spectra are marginalized over, when constraining $r$. We find that such uncertainty does not significantly affect B-mode surveys smaller than a few thousand degrees. Even for larger surveys it causes only a moderate reduction in CIB delensing performance, especially if the surveys have high (arcminute) resolution, which allows self-calibration of the delensing procedure. Though further work on the impact of foreground residuals is required, our overall conclusions for delensing with current CIB data are optimistic: this delensing method can tighten constraints on $r$ by a factor up to $\approx2.2$, and by a factor up to $\approx4$ when combined with external $\approx 3 \mu$K-arcmin lensing reconstruction, without requiring the modeling of CIB properties. CIB delensing is thus a promising method for the upcoming generation of CMB polarization surveys.

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