The Atacama Cosmology Telescope: high-redshift measurement of structure growth from the cross-correlation of Quaia quasars and CMB lensing from ACT DR6 and Planck PR4

We measure the amplitude of matter fluctuations over a wide range of redshifts by combining CMB lensing observations from ACT DR6 and Planck PR4 with the overdensity of quasars from Quaia, a Gaia and unWISE quasar catalog. Our analysis includes the CMB lensing power spectrum from ACT DR6, the auto-correlation of two Quaia quasar samples centered at z ≃ 1.0 and z ≃ 2.1, and their cross-correlations with CMB lensing from both ACT DR6 and Planck PR4. By performing a series of contamination and systematic null tests, we find no evidence for contamination in the lensing maps, contrary to what was suggested in previous Quaia cross-correlation analyses using Planck PR4 CMB lensing data. From the joint analysis of the quasar auto- and cross-correlations with CMB lensing, and including BOSS BAO data to break the degeneracy between Ω_m and σ ₈, we obtain σ ₈ = 0.802^+0.045_-0.057, consistent with ΛCDM predictions from Planck primary CMB measurements. We also find consistent results using DESI BAO data. Combining the CMB lensing auto-spectrum with the cross-correlation measurement improves the constraint on σ ₈ by 12% relative to the lensing auto-spectrum alone, yielding σ ₈ = 0.804 ± 0.013. This dataset combination also enables a reconstruction of structure growth across redshifts. We infer a 12% constraint on the amplitude of matter fluctuations at z > 3, with a measurement at the median redshift of the signal of σ ₈(z̃ = 5.1) = 0.146^+0.021_-0.014, consistent with Planck at the 1.4σ level. These results provide one of the highest redshift constraints on the growth of structure to date.