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 S₈^ 3x2pt₈ (ₘ/0. 3) ^0. 4=0. 8150. 012. The commonly used S₈₈ (ₘ/0. 3) ^0. 5 parameter is constrained to S₈=0. 8160. 015. In combination with baryon acoustic oscillation (BAO) measurements we find ₈=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 CDM 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 CDM model including massive neutrinos, spatial curvature, and dark energy. We find in flat CDM m_<0. 12 eV at 95% confidence using the LSS data, BAO measurements from SDSS and primary CMB observations.