From South to North: Leveraging Ground-Based LATs for Full-Sky CMB Delensing and Constraints on $r$

TL;DR

Delensing is essential to uncover primordial B-m modes and constrain $r$; this work investigates full-sky delensing by adding a Northern Hemisphere LAT (LATN) to a baseline SAT+LATS setup, using internal reconstruction augmented by external LSS tracers. It develops and compares gradient-order template and inverse-lensing delensing methods, analyzes their biases, and builds a full data-driven pipeline with NILC cleaning, map coaddition, internal/external lensing reconstruction, LT construction, and HL/Cobaya-based likelihood analysis. The key result is that LATN enables full-sky internal delensing, reducing the $r$ uncertainty by about $18{-}19\%$ (comparable to the $\sim13{-}10\%$ gains from LSS tracers once LATN is included), while LSS tracers provide diminishing returns in the LATN era. The study demonstrates the practical impact of full-sky internal delensing for future CMB missions and highlights the balance between sky coverage and external data in achieving tight PGW constraints.

Abstract

Delensing--the process of mitigating the lensing-induced B-mode contamination in cosmic microwave background (CMB) observations--will be a pivotal challenge for next-generation CMB experiments seeking to detect primordial gravitational waves (PGWs) through B-mode polarization. This process requires an accurate lensing tracer, which can be obtained either through internal reconstruction from high-resolution CMB observations or from external large-scale structure (LSS) surveys. Ground-based large-aperture telescopes (LATs) are crucial for internal reconstruction, yet existing and planned facilities are confined to the southern hemisphere, limiting effective delensing to that region. In this work, we assess the impact of introducing a northern hemisphere LAT, assumed to be situated near AliCPT (hence termed Ali-like LAT, or LATN), on delensing performance and PGW detection, using simulations. Our baseline setup includes a space-based small-aperture mission (LiteBIRD-like, SAT) and a southern LAT (SO-like, LATS). External LSS tracers, which have been shown to play an important role in delensing before the availability of ultra-sensitive polarization data, are also considered. We find that southern-hemisphere internal delensing reduces the uncertainty in r by approximately 21% compared to the no-delensing case. Adding LATN enables full-sky internal delensing, achieving a further ~19% reduction--comparable to that from including LSS tracers (~17%). Once LATN is included, the marginal benefit of LSS tracers drops to ~10%. These results highlight the significant role of LATN in advancing delensing capabilities and improving PGW constraints.

Figures (21)

• Figure 1: The flowchart of the entire analysis pipeline is presented for clarity. The workflow is broadly divided into five main components: component separation using NILC, map combination, lensing reconstruction, construction of the lensing $B$-mode template, and parameter constraint estimation. While the aim is not to detail every operational step, the flowchart highlights the logical structure and data dependencies throughout the process. For reference, the resulting reduction of uncertainties in $r$ are also summarized alongside the corresponding configurations. Note that the arrow from $\kappa$ to the lensing template (LT) construction indicates a computational pathway only, and does not imply that the lensing B-modes are physically induced by the convergence field.
• Figure 2: Survey windows for each observation in Celestial coordinates used in the analysis. For the coadded case, we combine two ground-based LATs with the satellite SAT. The yellow region represents the overlap between SAT and one of the LATs, while the brown region indicates the common region covered by all three observations.
• Figure 3: The total noise power spectra (beam-corrected) for the ground-based LATs.
• Figure 4: Illustration of the sky coverage and multipole range for two example experiments used in the map combination. Panel (a) shows the sky regions observed by each experiment, while panel (b) represents the corresponding multipole range coverage.
• Figure 5: The power spectra of the selected polarization maps, both before and after map coaddition, are shown. The black solid lines represent the theoretical predictions. The 'Noise' labels indicate contributions from both instrumental noise residuals and foreground residuals after NILC cleaning, while the 'Noisy' labels refer to the NILC-cleaned maps. Labels with multiple experiments in parentheses correspond to the coadded maps, as described in Section \ref{['sec: map_coadd']}. Note that for cases involving SAT, data are shown only for $\ell < 280$, where the SAT contribution dominates. Additionally, the LATN and LATS do not fully overlap due to differences in their sky coverage, which is relevant for the Galactic foregrounds.