Constraints on primordial non-Gaussianity from halo bias measured through CMB lensing cross-correlations

Abstract

Local non-Gaussianities in the initial conditions of the Universe, parametrized by ${f}_{\mathrm{NL}}$, induce a scale dependence in the large-scale bias of halos in the late Universe. This effect is a promising path to constrain multifield inflation theories that predict nonzero ${f}_{\mathrm{NL}}$. While most existing constraints from the halo bias involve autocorrelations of the galaxy distribution, cross-correlations with probes of the matter density provide an alternative channel with fewer systematics. We present the strongest large-scale structure constraint on local primordial non-Gaussianity that utilizes cross-correlations alone. We use the cosmic infrared background (CIB) consisting of dusty galaxies as a halo tracer and cosmic microwave background (CMB) lensing as a probe of the underlying matter distribution, both from Planck data. Milky Way dust is one of the key challenges in using the large-scale modes of the CIB. Importantly, the cross-correlation of the CIB with CMB lensing is far less affected by Galactic dust compared to the autospectrum of the CIB, since the latter picks up an additive bias from Galactic dust. We find no evidence for primordial non-Gaussianity and obtain $\ensuremath{-}87<{f}_{\mathrm{NL}}<19$, with a Gaussian $\ensuremath{\sigma}({f}_{\mathrm{NL}})\ensuremath{\approx}41$, assuming universality of the halo mass function, or $\ensuremath{-}179<{f}_{NL}<39$ for a more aggressively cleaned patch of sky, on which we find a much better fit. We find that future CMB lensing data from Simons Observatory and CMB-S4 could achieve $\ensuremath{\sigma}({f}_{\mathrm{NL}})$ of 23 and 20 respectively. The constraining power of such an analysis is limited by current Galactic dust cleaning techniques which introduce a multiplicative bias on very large scales, requiring us to choose a minimum multipole of $\ensuremath{\ell}=70$. If this challenge is overcome with improved analysis techniques or external data, constraints as tight as $\ensuremath{\sigma}({f}_{\mathrm{NL}})=4$ can be achieved through the cross-correlation technique. More optimistically, constraints better than $\ensuremath{\sigma}({f}_{\mathrm{NL}})=2$ could be achieved if the CIB autospectrum is dust-free down to the largest scales.

Topics & Concepts