A novel CMB polarization likelihood package for large angular scales built from combined WMAP and Planck LFI legacy maps

We present a cosmic microwave background (CMB) large-scale polarization dataset obtained by combining Wilkinson Microwave Anisotropy Probe (WMAP) in the K, Q, and V bands with the Planck 70 GHz maps. We employed the legacy frequency maps released by the WMAP and Planck collaborations and performed our own Galactic foreground mitigation technique, relying on Planck 353 GHz for polarized dust and on Planck 30 GHz and WMAP K for polarized synchrotron. We derived a single, optimally noise-weighted, low residual foreground map and the accompanying noise covariance matrix. These are shown through χ2 analysis to be robust over an ample collection of Galactic masks. We used this dataset, along with the Planck legacy Commander temperature solution, to build a pixel-based low-resolution CMB likelihood package, whose robustness we tested extensively with the aid of simulations, finding an excellent level of consistency. Using this likelihood package alone, we are able to constrain the optical depth to reionization, τ = 0.069−0.012+0.011 at 68% confidence level, on 54% of the sky. Adding the Planck high-ℓ temperature and polarization legacy likelihood, the Planck lensing likelihood, and BAO observations, we find τ = 0.0714−0.0096+0.0087 in a full ΛCDM exploration. The latter bounds are slightly less constraining than those obtained by employing the Planck High Frequency Instrument’s (HFI) CMB data for large-angle polarization, which only include EE correlations. Our bounds are based on a largely independent dataset that includes TE correlations. They are generally compatible with Planck HFI, but lean towards slightly higher values for τ. We have made the low-resolution Planck and WMAP joint dataset publicly available, along with the accompanying likelihood code.