The Atacama Cosmology Telescope: map-based noise simulations for DR6

Abstract

The increasing statistical power of cosmic microwave background (CMB) datasets requires a commensurate effort in understanding their noise properties. The noise in maps from ground-based instruments is dominated by large-scale correlations, which poses a modeling challenge. This paper develops novel models of the complex noise covariance structure in the Atacama Cosmology Telescope Data Release 6 (ACT DR6) maps. We first enumerate the noise properties that arise from the combination of the atmosphere and the ACT scan strategy. We then prescribe a class of Gaussian, map-based noise models, including a new wavelet-based approach that uses directional wavelet kernels for modeling correlated instrumental noise. The models are empirical, whose only inputs are a small number of independent realizations of the same region of sky. We evaluate the performance of these models against the ACT DR6 data by drawing ensembles of noise realizations. Applying these simulations to the ACT DR6 power spectrum pipeline reveals a ∼ 20% excess in the covariance matrix diagonal when compared to an analytic expression that assumes noise properties are uniquely described by their power spectrum. Along with our public code, mnms, this work establishes a necessary element in the science pipelines of both ACT DR6 and future ground-based CMB experiments such as the Simons Observatory (SO).

The Atacama Cosmology Telescope: A Measurement of the DR6 CMB Lensing Power Spectrum and Its Implications for Structure Growth

“…It is designed to target the spatially varying noise power spectra, which makes it an attractive choice for our lensing reconstruction pipeline, which, in the large-lens limit, approximates a measurement of the spatially varying CMB power spectrum Prince et al 2018). In Appendix F we show that our cross-correlation-based lensing estimator (Section 5.8.1) is robust to the choice of noise model, producing consistent results when the isotropic wavelet model is replaced with one of the other noise models from Atkins et al (2023; the "tiled" or "directional wavelet" models); unlike the isotropic wavelet model, these additionally model the stripy correlated noise features present in the ACT noise maps. We also emphasize that since the cross-correlation-based estimator is immune to noise bias and hence insensitive to assumptions of the noise modeling, accurate noise simulations are only required in our pipeline for estimation of the lensing power spectrum's covariance matrix; in contrast, for bias calculation steps, accurate noise simulations are not needed.…”

“…Simulating the complicated ACT DR6 noise necessitated the development of novel map-based noise models, as described in Atkins et al (2023). In our main analysis, we utilize noise simulations drawn from that work's "isotropic wavelet" noise model.…”

“…The gradient null bandpowers in the right panel of Figure 42 are marginally consistent with zero with a PTE of 0.05; the curl in Figure 47 is also consistent with zero with a PTE of 0.48. To obtain accurate PTEs and debiasing in the bandpower test described in Appendix I.4 below, we produce special noise simulations capturing the characteristics of the two time splits using mnms (Atkins et al 2023).…”

“…When combined with the ACT scanning strategy, the atmospheric noise produces several nontrivial noise properties in the ACT DR6 maps. These include steep, red, and spatially varying noise power spectra, spatially varying stripy noise patterns, and correlations between frequency bands (Atkins et al 2023).…”

“…The additional 1/k averages over eight independent realizations. Refer toAtkins et al (2023) for a detailed discussion.83 We checked that the resulting coadded map is stable to different choices of binning Δℓ as long as the resultant N ℓ are smooth.…”