Constraining local non-Gaussianities with kinetic Sunyaev-Zel'dovich tomography

Kinetic Sunyaev Zel'dovich (kSZ) tomography provides a powerful probe of the radial velocity field of matter in the Universe. By cross-correlating a high resolution cosmic microwave background (CMB) experiment like CMB-S4 and a galaxy survey like Dark Energy Spectroscopic Instrument (DESI) or Large Synoptic Survey Telescope (LSST), one can measure the radial velocity field with a very high signal-to-noise ratio over a large volume of the universe. In this paper we show how this measurement can be used to improve constraints on primordial non-Gaussianities of the local type. The velocity field provides a measurement of the unbiased matter perturbations on large scales, which can be cross-correlated with the biased large-scale galaxy density field. This results in sample valiance cancellation for a measurement of scale-dependent bias due to a nonzero f(NL). Using this method we forecast that CMB-S4 and LSST combined reach a sensitivity sigma(fNL) similar to 0.5, which is a factor of 3 improvement over the sensitivity using LSST alone (without internal sample variance cancellation). We take into account critical systematics like photometric redshifts, the kSZ optical depth degeneracy, and systematics affecting the shape of the galaxy auto-power spectrum and find that these have negligible impact, thus making kSZ tomography a robust probe for primordial non-Gaussianities. We also forecast the impact of mass binning on our constraints. The techniques proposed in this paper could be an important component of achieving the theoretically important threshold of sigma(fNL) less than or similar to 1 with future surveys.