The Atacama Cosmology Telescope: Constraints on local non-Gaussianity from the ACT cluster catalog

Abstract We derive constraints on local-type primordial non-Gaussianity using the ACT DR6 Sunyaev-Zel'dovich cluster catalog. Modeling the redshift- and mass-dependent number counts of 1,201 clusters in the 10,347 deg 2 Legacy region, and accounting for survey completeness, intrinsic SZ scatter, and a weak-lensing-calibrated mass bias, we compute theoretical abundances using the Log-Edgeworth halo mass function. Assuming ΛCDM with well-motivated external priors, we obtain f NL = 55 ± 125 (68% CL), consistent with Gaussian initial conditions. These constraints probe comoving scales of 5–10 Mpc h -1 , complementing CMB bispectrum and scale-dependent bias measurements, which do not reach such small scales. We also find evidence for a 16.4% residual mass bias, which, although heavily informed by our adopted priors, plays a key role in matching observed and predicted counts but has negligible effect on f NL constraints. We briefly discuss robustness of the results under relaxed priors and the prospects for next-generation SZ and lensing surveys to strengthen cluster-based tests of primordial non-Gaussianity.