The Atacama Cosmology Telescope: Multi-probe cosmology with unWISE galaxies and ACT DR6 CMB lensing

We present a joint analysis of the CMB lensing power spectra measured from the Data Release 6 of the Atacama Cosmology Telescope and Planck PR4, cross-correlations between the ACT and Planck lensing reconstruction and galaxy clustering from unWISE, and the unWISE clustering auto-spectrum. We obtain 1.5% constraints on the matter density fluctuations at late times parametrised by the best constrained parameter combination S₈³ˣ²ᵖᵗ≡σ₈ (Ωₘ/0.3)0.4=0.815±0.012. The commonly used S₈≡σ₈ (Ωₘ/0.3)0.5 parameter is constrained to S₈=0.816±0.015. In combination with baryon acoustic oscillation (BAO) measurements we find σ₈=0.815± 0.012. We also present sound-horizon-independent estimates of the present day Hubble rate of H₀=66.4+3.2-3.7 \,km\,s⁻¹Mpc⁻¹ from our large scale structure data alone and H₀=64.3+2.1-2.4\,km\,s⁻¹Mpc⁻¹ in combination with uncalibrated supernovae from Pantheon+. Using parametric estimates of the evolution of matter density fluctuations, we place constraints on cosmic structure in a range of high redshifts typically inaccessible with cross-correlation analyses. Combining lensing cross- and auto-correlations, we derive a 3.3% constraint on the integrated matter density fluctuations above $z=2.4$, one of the tightest constraints in this redshift range and fully consistent with a ΛCDM model fit to the primary CMB from Planck. Combining with primary CMB observations and using the extended low redshift coverage of these combined data sets we derive constraints on a variety of extensions to the ΛCDM model including massive neutrinos, spatial curvature, and dark energy. We find in flat ΛCDM ∑ m_ν<0.12 eV at 95% confidence using the LSS data, BAO measurements from SDSS and primary CMB observations.