The Atacama Cosmology Telescope: DR4 Maps and Cosmological Parameters

TL;DR

This paper presents ACT DR4 arcminute-resolution maps of the CMB temperature and polarization at 98 and 150 GHz from 2013–2016, covering ~17,000 deg^2 with deep regions below 10 μK-arcmin. Using power spectra derived from these maps, the authors constrain ΛCDM cosmology and demonstrate consistency with Planck and WMAP, obtaining H0 ≈ 67.6–67.9 km/s/Mpc and a good fit for a flat universe with standard lensing. The DR4 results show no evidence for curvature or anomalous lensing, while mild tensions in parameter degeneracies (e.g., n_s, Ω_b h^2) can be alleviated by incorporating external large-scale information; extensions beyond ΛCDM are explored but do not significantly improve fits. The DR4 data products, including masks and maps, will be publicly released, and future ACT analyses are expected to further tighten cosmological constraints, particularly through polarization and additional frequency channels.

Abstract

We present new arcminute-resolution maps of the Cosmic Microwave Background temperature and polarization anisotropy from the Atacama Cosmology Telescope, using data taken from 2013-2016 at 98 and 150 GHz. The maps cover more than 17,000 deg$^2$, the deepest 600 deg$^2$ with noise levels below 10 $μ$K-arcmin. We use the power spectrum derived from almost 6,000 deg$^2$ of these maps to constrain cosmology. The ACT data enable a measurement of the angular scale of features in both the divergence-like polarization and the temperature anisotropy, tracing both the velocity and density at last-scattering. From these one can derive the distance to the last-scattering surface and thus infer the local expansion rate, $H_0$. By combining ACT data with large-scale information from WMAP we measure $H_0 = 67.6 \pm 1.1$ km/s/Mpc, at 68% confidence, in excellent agreement with the independently-measured Planck satellite estimate (from ACT alone we find $H_0 = 67.9 \pm 1.5$ km/s/Mpc). The $Λ$CDM model provides a good fit to the ACT data, and we find no evidence for deviations: both the spatial curvature, and the departure from the standard lensing signal in the spectrum, are zero to within 1$σ$; the number of relativistic species, the primordial Helium fraction, and the running of the spectral index are consistent with $Λ$CDM predictions to within $1.5 - 2.2σ$. We compare ACT, WMAP, and Planck at the parameter level and find good consistency; we investigate how the constraints on the correlated spectral index and baryon density parameters readjust when adding CMB large-scale information that ACT does not measure. The DR4 products presented here will be publicly released on the NASA Legacy Archive for Microwave Background Data Analysis.

Figures (24)

• Figure 1: Mollweide projection of the ACT DR4 footprints in Galactic coordinates. The D1, D5, and D6 regions were observed in season s13, D56 in seasons s14 and s15, D8 and BN in s15. The AA region was observed in s16 and covers roughly 40% of the sky. D1 overlaps with BN, D5 and D6 overlap with D56, and all regions overlap with AA. Two constant-declination lines are shown in black roughly indicating the observable sky from Chile. The grey region between ${\rm Dec}=[-60,+22]$ was not observed, although accessible, to maximize overlap with current optical surveys. The grid spacing in both Galactic longitude and latitude is $30^{\circ}$.
• Figure 2: Window functions for the mean instantaneous beam of each array and band in each season. The window functions used for interpretation of the survey maps are slightly modified to account for residual pointing variance in the observations contributing to each map. The window function errors shown in the bottom panel are strongly correlated between multipoles.
• Figure 3: Weak polarized sidelobes in the PA3 $150$ GHz detectors. (Top panel) Map of the polarized sidelobes, obtained by stacking 20 observations of Saturn. The main lobe of the beam is at the origin and is masked in this image. The grayscale is linear in units of 1/1000 of the main beam peak, with positive (negative) numbers indicating polarization parallel (perpendicular) to the radial direction. The sidelobes consist of four groups of compact features, at 30 to 40 arcminutes from the main beam. (Bottom panel) The effect of unmitigated sidelobes on CMB spectrum measurements, expressed as transfer beam functions $b_\ell^{T\to E}$ and $b_\ell^{T\to B}$. The sidelobes are strongly polarized in a direction perpendicular to the line connecting them to the main beam; this results in leakage that is primarily from temperature into E-mode polarization. Note that these sidelobes are projected out of the data prior to mapping the CMB, so the net leakage in the resulting spectra will be strongly suppressed relative to what is shown here.
• Figure 4: ACT DR4 survey depth and sky coverage maps for $98$ GHz (top panel) and $150$ GHz (bottom panel) in Equatorial coordinates. Lines at constant map-noise are plotted ranging from 7.5 $\mu$K--arcmin (red) to 101 $\mu$K--arcmin (blue). The Planck 353 GHz intensity map is shown in the background. The x-axis (y-axis) shows the RA (Dec) coordinates in degree.
• Figure 5: Map noise cumulative distribution function for $98$ and $150$ GHz and their combination. Two horizontal lines show the Planck full-sky averaged white-noise level for the 100 GHz and 143 GHz channels.