Density-split analyses of the pairwise kinetic Sunyaev-Zeldovich effect
We consider the dependence of the kinetic Sunyaev-Zeldovich (kSZ) effect on the environments of foreground cosmological structures. Standard pairwise kSZ estimators exploit the fact that galaxies below separations of $\sim 200\,h^{-1}{\rm M}_\odot$ fall towards each other on average, yielding a negative mean pairwise kSZ signal. However, this signal is the residual from a near-cancellation between outflows from underdense regions and infall towards overdensities giving a large variance in the pairwise velocity distribution and hence a low signal-to-noise ratio for the mean kSZ signal. This effect is especially marked at the largest scales, where the pairwise velocity distribution becomes more symmetric about zero. We show that splitting galaxies according to their local overdensity allows the convergence and divergence flows to be separated, yielding pairwise kSZ signals of opposing sign, with a reduction in variance for each subset. We use simulations to perform a detailed analysis of this phenomenon, comparing the cosmological information content in the global pairwise kSZ signal with our new density-split pairwise kSZ estimator. We consider galaxy catalogues comparable to datasets from the Dark Energy Spectroscopic Instrument, together with cosmic microwave background instrumental noise characteristic of recent and planned ground-based experiments. We predict that the density-split pairwise kSZ will increase the significance of detection by only a modest factor; nevertheless, we expect improvements by factors of respectively 11 and 4 in constraining $σ_8$ and $Ω_{\rm m}$ when using the combination of Dark Energy Spectroscopic Instrument and Simons Observatory Large Aperture Telescope surveys.