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

Emilie Hertig, Antón Baleato Lizancos, Frank J. Qu, J. Richard Bond, Erminia Calabrese, Anthony Challinor, Mark J. Devlin, Jo Dunkley, Thibaut Louis, Mathew S. Madhavacheril, Toshiya Namikawa, Lyman A. Page, Neelima Sehgal, Blake Sherwin, Cristóbal Sifón, Suzanne T. Staggs, Edward J. Wollack

TL;DR

The paper addresses the challenge of primordial B-mode detection by mitigating lensing-induced B-modes through delensing. It develops a gradient-order lensing B-mode template by optimally combining ACT DR6 internal CMB lensing with external large-scale-structure tracers (Planck CIB GNILC and unWISE galaxies) via a Wiener-filtered multi-tracer approach, achieving 55-85% correlation with the true κ. The methodology yields up to 52% delensing efficiency over 23% of the sky, with high-significance detections on ACT and Planck B-modes (ACT: ~19.7σ; Planck: ~10σ overall, ~4.3σ for 30 ≤ l ≤ 300), marking a substantial advancement over previous results. The work demonstrates robust consistency with theory, accounts for survey systematics, and outlines clear paths to further gains with upcoming surveys (SO LAT, DESI, Euclid, Rubin LSST), ultimately enhancing the precision of constraints on the tensor-to-scalar ratio r.

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.

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

TL;DR

The paper addresses the challenge of primordial B-mode detection by mitigating lensing-induced B-modes through delensing. It develops a gradient-order lensing B-mode template by optimally combining ACT DR6 internal CMB lensing with external large-scale-structure tracers (Planck CIB GNILC and unWISE galaxies) via a Wiener-filtered multi-tracer approach, achieving 55-85% correlation with the true κ. The methodology yields up to 52% delensing efficiency over 23% of the sky, with high-significance detections on ACT and Planck B-modes (ACT: ~19.7σ; Planck: ~10σ overall, ~4.3σ for 30 ≤ l ≤ 300), marking a substantial advancement over previous results. The work demonstrates robust consistency with theory, accounts for survey systematics, and outlines clear paths to further gains with upcoming surveys (SO LAT, DESI, Euclid, Rubin LSST), ultimately enhancing the precision of constraints on the tensor-to-scalar ratio r.

Abstract

Large-scale -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 -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 , is then convolved with ACT DR6 -mode polarization to yield a template of the lensing -modes. We assess its performance on a wide range of scales by using it to delens ACT DR6 and Planck -modes over 23% of the sky, removing around 39% of the lensing power at and 47% at , 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.

Paper Structure

This paper contains 22 sections, 62 equations, 16 figures.

Table of Contents

  1. Introduction
  2. Theoretical framework
  3. Weak lensing of the CMB
  4. Internal lensing reconstruction
  5. Large-scale structure tracers
  6. Delensing performance
  7. Data
  8. ACT DR6 maps
  9. unWISE galaxies
  10. Planck CIB
  11. Simulations
  12. ACT polarization and lensing
  13. External tracers
  14. Methodology
  15. Results and discussion
  16. ...and 7 more sections

Figures (16)

  • Figure 1: Normalized projection kernel as a function of redshift for each LSS tracer used in our analysis: the CMB lensing convergence (Eq. \ref{['eq:lens_kernel']}; black), the unWISE Blue and Green galaxy samples (Eq. \ref{['eq:galaxy_kernel']}, Sec. \ref{['section:unWISE']}, Ref. schlafly_unwise_2019; blue and green lines, respectively) and the CIB (Eq. \ref{['eq:cib_kernel']}, Sec. \ref{['section:CIB']}; purple).
  • Figure 2: ACT DR6 lensing convergence map (dimensionless) in Equatorial coordinates. This map was produced with the MV quadratic estimator described in Sec. \ref{['section:internal_rec']}, using cross-correlations between data splits to mitigate noise biases madhavacheril_atacama_2024. Masked pixels are shown in grey, leaving an unmasked sky fraction $f_{\rm{sky}}= 0.23$.
  • Figure 3: Dimensionless overdensity field as defined in Eq. \ref{['eq:unWISE_density']} for the unWISE Blue galaxy sample. Masked pixels are represented in grey. A similar result, not shown here, is obtained for the Green sample. Both maps are generated in Equatorial coordinates, using the HEALPix equal-area pixelization gorski_healpix_2005 with $n_{\rm{side}}=2048$.
  • Figure 4: CIB intensity map extracted from Planck 353 GHz data with the GNILC algorithm. The map is generated at $n_{\rm{side}}=2048$ in Equatorial coordinates, with masked pixels shown in grey.
  • Figure 5: Left: auto-spectrum of the ACT DR6 MV lensing map and its cross-spectra with all external LSS tracers. Right: auto- and cross-spectra between the unWISE Blue and Green samples and the Planck GNILC CIB map. Circles represent mode-decoupled bandpowers obtained with NaMaster using a bin width $\Delta L=100$, and dotted lines correspond to best-fit theoretical models from Eq. \ref{['eq:limber']}. The latter are a reasonable match to the data except for a slight deviation in the unWISE Green $\times$ CIB cross-spectrum, which was shown not to affect delensing results significantly. The CIB has units of $\text{MJy\,sr}^{-1}$; all other fields are dimensionless. Vertical lines denote the multipole ranges which enter our combined lensing estimator: $2\leq L\leq2000$ for CMB lensing (solid) and $102\leq L\leq2000$ for external tracers (dashed). Error bars, reflecting sample variance, are computed with the analytic approximation from Ref. knox_1995.
  • ...and 11 more figures