Constraining local non-Gaussianities with kSZ 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 CMB experiment like CMB S4 and a galaxy survey like DESI or LSST,
one can measure the radial velocity field with very high signal to noise 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 variance cancellation for a
measurement of scale-dependent bias due to a non-zero $f_{NL}$. Using this
method we forecast that CMB S4 and LSST combined reach a sensitivity
$\sigma_{f_{NL}} \sim 0.5$, which is a factor of three 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 $\sigma_{f_{NL}} \lesssim 1$ with future surveys.
