The Atacama Cosmology Telescope: Cosmology from cross-correlations of unWISE galaxies and ACT DR6 CMB lensing

Abstract

We present tomographic measurements of structure growth using cross-correlations of Atacama Cosmology Telescope (ACT) DR6 and Planck CMB lensing maps with the unWISE Blue and Green galaxy samples, which span the redshift ranges $0.2\lesssim z\lesssim 1.1$ and $0.3\lesssim z\lesssim 1.8$, respectively. We improve on prior unWISE cross-correlations not just by making use of the new, high-precision ACT DR6 lensing maps, but also by including additional spectroscopic data for redshift calibration and by analysing our measurements with a more flexible theoretical model. An extensive suite of systematic and null tests within a blind analysis framework ensures that our results are robust. We determine the amplitude of matter fluctuations at low redshifts ($z\simeq 0.2-1.6$), finding $S_8\equiv \ddot{I}{ƒ}_8(\hat{I}{\text{c◯}}_m/0.3{)}^{0.5}=0.813\pm 0.021$ using the ACT cross-correlation alone and $S_8=0.810\pm 0.015$ with a combination of Planck and ACT cross-correlations; these measurements are fully consistent with the predictions from primary CMB measurements assuming standard structure growth. The addition of Baryon Acoustic Oscillation data breaks the degeneracy between $\ddot{I}{ƒ}_8$ and $\hat{I}{\text{c◯}}_m$, allowing us to measure $\ddot{I}{ƒ}_8=0.813\pm 0.020$ from the cross-correlation of unWISE with ACT and $\ddot{I}{ƒ}_8=0.813\pm 0.015$ from the combination of cross-correlations with ACT and Planck. These results also agree with the expectations from primary CMB extrapolations in $\hat{I}›$CDM cosmology; the consistency of $\ddot{I}{ƒ}_8$ derived from our two redshift samples at $z\sim 0.6$ and $1.1$ provides a further check of our cosmological model. Our results suggest that structure formation on linear scales is well described by $\hat{I}›$CDM even down to low redshifts $z\lesssim 1$.

73 pages (incl. 31 pages of appendices), 52 figures, 16 tables, published in ApJ. Watch G. S. Farren and A. Krolewski discuss the analysis and results under https://cosmologytalks.com/2023/09/11/act-unwise