The Atacama Cosmology Telescope: Constraints on Local Non-Gaussianity from the ACT Cluster Catalog

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 10,347deg$^2$ 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 $Λ$CDM with well-motivated external priors, we obtain $f_{\rm NL} = 55 \pm 125$ (68% CL), consistent with Gaussian initial conditions. These constraints probe comoving scales of $5$–$10{\rm Mpc}~h^{-1}$, 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 $f_{\rm NL}$ 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.

💡 Research Summary

This paper presents a new constraint on local‑type primordial non‑Gaussianity (PNG) using the Atacama Cosmology Telescope (ACT) Data Release 6 (DR6) Sunyaev–Zel’dovich (SZ) cluster catalog. The authors model the redshift‑ and mass‑dependent number counts of 1,201 clusters detected over the 10,347 deg² “Legacy” region, incorporating the survey’s completeness function, the intrinsic SZ‑mass scatter, and a weak‑lensing calibrated mass bias.

The theoretical prediction for the cluster abundance is built on the Tinker08 Gaussian halo mass function, multiplied by a non‑Gaussian correction factor derived from a Log‑Edgeworth expansion (a modern implementation of the MVJ/LoVerde formalism). This factor, $R_{\rm NG}(M,z;f_{\rm NL})$, encodes the dependence on the local PNG amplitude $f_{\rm NL}$ and is calibrated against N‑body simulations by rescaling the collapse threshold with $q\simeq0.75$.

Observationally, each cluster’s SZ signal is converted to a mass $M_{500c}$ using the universal pressure profile (UPP) scaling relation, which is already calibrated to weak‑lensing measurements (fiducial mass bias $1-b_{\rm fid}=0.65$). The authors also allow for an additional residual bias $b$ as a free nuisance parameter. The intrinsic log‑normal scatter in the observable–mass relation is taken to be $\sigma_{\ln M}=0.185$. The completeness $C(M,z)$, derived from end‑to‑end simulations of the cluster finder, is ~90 % for $S/N\ge5.5$ above $M_{500c}\simeq5\times10^{14}M_\odot$ at $z\simeq0.2$, and declines smoothly at lower masses and higher redshifts.

A Bayesian likelihood is constructed by comparing the predicted counts in bins of mass and redshift to the observed catalog, marginalizing over the standard ΛCDM parameters ($\Omega_m$, $\sigma_8$, $H_0$, etc.) with external priors from Planck CMB, BAO, and Type‑Ia supernovae. The analysis yields
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