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CMB lensing tomography to z=2 with galaxies from the unWISE catalog

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CMB lensing tomography has the potential to map the amplitude and growth of structure over cosmic time, provide some of the most stringent tests of gravity, and break important degeneracies between cosmological parameters. I use the unWISE photometric galaxy catalog to create three samples at median redshifts z~0.6, 1.1, and 1.5, and cross-correlate them with the most recent Planck CMB lensing maps. The resulting significance of ~80 at 100 < ell < 1000 is the highest significance detection to date of CMB lensing cross-correlation.  The redshift distribution of the two-band unWISE galaxies is a major source of systematic error. I primarily use cross-correlations with BOSS galaxies and quasars and eBOSS quasars to measure the redshift distribution, supplemented with cross-matching to deep COSMOS photometric redshifts.  I demonstrate how to propagate the uncertainty in the redshift distribution to the modeling of the signal, and perform a number of null tests. Finally, I discuss the cosmological implications of this measurement and lessons learned for CMB lensing cross-correlations with future photometric surveys.