The Atacama Cosmology Telescope: $B$-mode delensing with DR6 data and external tracers of large-scale structure

Abstract

Large-scale $B$-mode polarization of the cosmic microwave background (CMB) is a prime target for current and future experiments in search of primordial gravitational waves (PGW). With increasingly sensitive instruments being deployed, secondary $B$-modes induced by the weak gravitational lensing of CMB photons are becoming an important limitation and need to be removed, a process known as delensing. In this work, we combine internally reconstructed CMB lensing maps from the Atacama Cosmology Telescope (ACT) DR6 with galaxy samples from unWISE and a map of the cosmic infrared background (CIB) fluctuations from Planck to produce a well-correlated tracer of the CMB lensing field. Our co-added tracer, shown to be 55-85% correlated with the true lensing convergence at multipoles $L \leq 2000$, is then convolved with ACT DR6 $E$-mode polarization to yield a template of the lensing $B$-modes. We assess its performance on a wide range of scales by using it to delens ACT DR6 and Planck $B$-modes over 23% of the sky, removing around 39% of the lensing power at $100\leq l \leq 1500$ and 47% at $30 \leq l \leq 300$, respectively. Our template achieves the highest delensing efficiency to date and will be useful for the analysis of early polarization maps from the Simons Observatory (SO). We finally outline prospects for further improvements by including additional large-scale structure tracers from upcoming cosmological surveys.