The Atacama Cosmology Telescope: DR6 Power Spectrum Foreground Model and Validation

We discuss the model of astrophysical emission at millimeter wavelengths used to characterize foregrounds in the multi-frequency power spectra of the Atacama Cosmology Telescope (ACT) Data Release 6 (DR6), expanding on Louis et al. (2025). We detail a number of tests to validate the capability of the DR6 parametric foreground model to describe both current observations and complex sky simulations, and show that cosmological parameter constraints are robust against model extensions and variations. We demonstrate consistency of the model with pre-DR6 ACT data and observations from Planck and the South Pole Telescope. We evaluate the implications of several model variations, including using different foreground templates and extending the model with new components and/or free parameters. In all scenarios, the ΛCDM and ΛCDM+Neff cosmological parameters from the DR6 data shift by less than 0.5σ relative to the baseline constraints. Foreground parameters may shift more; we estimate their systematic uncertainties associated with modeling choices. [AVE: From our constraint on the kinematic Sunyaev-Zel’dovich power, we obtain a conservative limit on the duration of reionization of Δzrei < 4.4, assuming a reionization midpoint consistent with optical depth measurements and assuming a minimal low-redshift contribution, with varying assumptions for this component leading to tighter limits.] Finally, we construct realistic non-Gaussian, correlated microwave sky simulations containing all non-negligible Galactic and extragalactic foreground fields. The simulations are built independently of the DR6 parametric foreground model. [ks: We process these simulations through the DR6 power spectrum and likelihood pipeline and show that we recover the input cosmological parameters of the underlying cosmic microwave background field, demonstrating such parameter recovery from realistically correlated, non-Gaussian simulations for the first time in such a CMB analysis.] These tests validate the robustness of the ACT DR6 foreground model and cosmological parameter constraints.