We use data from the DESI Legacy Survey imaging to probe the galaxy density field in tomographic slices covering the redshift range $0<z<0.8$. After careful consideration of completeness corrections and galactic cuts, we obtain a sample of $4.9times 10^7$ galaxies covering 17 739 deg$^2$. We derive photometric redshifts with precision $sigma_z/(1+z)=0.012 - 0.015$, and compare with alternative estimates. Cross-correlation of the tomographic galaxy maps with Planck maps of CMB temperature and lensing convergence probe the growth of structure since $z=0.8$. The signals are compared with a fiducial Planck $Lambda$CDM model, and require an overall scaling in amplitude of $A_kappa=0.901pm 0.026$ for the lensing cross-correlation and $A_{rm ISW} = 0.984 pm 0.349$ for the temperature cross-correlation, interpreted as the Integrated Sachs-Wolfe effect. The ISW amplitude is consistent with the fiducial $Lambda$CDM prediction, but lies significantly below the prediction of the AvERA model of Racz et al. (2017), which has been proposed as an alternative explanation for cosmic acceleration. Within $Lambda$CDM, our low amplitude for the lensing cross-correlation requires a reduction either in fluctuation normalization or in matter density compared to the Planck results, so that $Omega_m^{0.78}sigma_8=0.297pm 0.009$. In combination with the total amplitude of CMB lensing, this favours a shift mainly in density: $Omega_m=0.274pm0.024$. We discuss the consistency of this figure with alternative evidence. A conservative compromise between lensing and primary CMB constraints would require $Omega_m=0.296pm0.006$, where the 95% confidence regions of both probes overlap.
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