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 $S_8^{3\mathrm{x}2\mathrm{p}\mathrm{t}}\equiv {\sigma }_8({\mathrm{\Omega }}_m\mathrm{/}0.3{)}^{0.4}=0.815\pm 0.012$. The commonly used $S_8\equiv {\sigma }_8({\mathrm{\Omega }}_m\mathrm{/}0.3{)}^{0.5}$ parameter is constrained to $S_8=0.816\pm 0.015$. In combination with baryon acoustic oscillation (BAO) measurements we find ${\sigma }_8=0.815\pm 0.012$. We also present sound-horizon-independent estimates of the present day Hubble rate of $H_0=66.4_{-3.7}^{+3.2}\mathrm{k}\mathrm{m}{\mathrm{s}}^{-1}{\mathrm{M}\mathrm{p}\mathrm{c}}^{-1}$ from our large scale structure data alone and $H_0=64.3_{-2.4}^{+2.1}\mathrm{k}\mathrm{m}{\mathrm{s}}^{-1}{\mathrm{M}\mathrm{p}\mathrm{c}}^{-1}$ 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 $\mathrm{\Lambda }$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 $\mathrm{\Lambda }$CDM model including massive neutrinos, spatial curvature, and dark energy. We find in flat $\mathrm{\Lambda }$CDM $\sum m_{\nu }<0.12$ eV at 95% confidence using the LSS data, BAO measurements from SDSS and primary CMB observations.