The Atacama Cosmology Telescope: DR6 Sunyaev-Zel'dovich Selected Galaxy Clusters Catalog

TL;DR

The paper presents the ACT DR6 Sunyaev-Zel'dovich (SZ)–selected galaxy cluster catalog, derived from three-band microwave maps over 16293 deg$^2$ using the Nemo cluster-detection pipeline. It reports 10040 clusters with ${\tilde{q}}>4$, provides redshifts (including 1180 at $z>1$ and 124 at $z>1.5$) and SZ-based mass estimates, and characterizes completeness with a 90% mass limit near $M_{500c}\approx5\times10^{14}M_\odot$ for $z\sim0.5$. A substantial optical/IR follow-up program (DES, HSC, and other data) yields robust redshifts, with $>99\%$ follow-up completeness for the Legacy sample at ${\tilde{q}}>5.5$ and $>90\%$ for the full flags=0 footprint at ${\tilde{q}}>5$, and the catalog includes a Legacy subsample of 3747 clusters suitable for cosmological analyses. The study finds no LCDM-excluding outliers and discusses potential mergers that could bias individual mass estimates, emphasizing the need for cross-validation with weak-lensing and X-ray masses. The DR6 data products and Nemo tooling are publicly released to enable reproducibility and cross-survey science, establishing ACT DR6 as the largest SZ-selected cluster catalog to date and a cornerstone for upcoming surveys like the Simons Observatory.

Abstract

We present the results of a search for galaxy clusters in the Atacama Cosmology Telescope (ACT) Data Release 6 (DR6) microwave sky maps covering 16293 square degrees in three frequency bands, using data obtained over the lifetime of the project (2008-2022). We report redshifts and mass estimates for 10040 clusters detected via their Sunyaev-Zel'dovich (SZ) effect with signal-to-noise greater than 4 at a 2.4 arcminute filter scale. The catalog includes 1180 clusters at redshifts greater than 1, and 124 clusters at redshifts greater than 1.5. Using a relation between cluster SZ signal and mass that is consistent with recent weak-lensing measurements, we estimate that clusters detected with signal-to-noise greater than 5 form a sample which is 90% complete for clusters with masses greater than $5 \times 10^{14}$ MSun (measured within a spherical volume with mean density 500 times the critical density). El Gordo, a cluster found in an initial ACT survey of 755 square degrees, remains the most extreme cluster in mass and redshift; we find no cluster with a mass and redshift combination high enough to falsify the standard LCDM cosmology with Gaussian initial perturbations. We make public a variety of data products, including the full cluster candidate list, noise maps, and sky masks, along with our software for cluster detection and instructions for reproducing our cluster catalogs from the public ACT maps.

Figures (21)

• Figure 1: The number of ACT-detected SZ-selected clusters reported over the lifetime of the project. References for the blue points: First detections Hincks_2010; ACT DR1 Marriage_2011Menanteau_2010; ACT DR2 Hasselfield_2013Menanteau_2013; ACT DR3 Hilton_2018; ACT DR5 Hilton_2021. The orange square marks the ACT-DR5 MCMF catalog Klein_2024, which is based on an independent re-run of the Nemo software on the ACT DR5 maps, followed by optical confirmation using the DESI Legacy Imaging Surveys DR10 Dey_2019. The green diamond marks the 10040 clusters reported in the current work, based on the ACT DR6 maps, which is $>1.5$ times larger than catalogs produced using ACT DR5 data.
• Figure 2: Top panel: The full ACT DR6 cluster search region (black outline), covering 16293 deg$^2$ before masking. The orange outline shows the 10317 deg$^2$ Legacy footprint (after masking and flagging) used to define the 'Legacy sample' (see Section \ref{['sec:catalog']}). The footprints of deep and wide optical surveys are highlighted, with the overlapping area with ACT DR6 (after masking and flagging) given in square degrees: DES (blue; DESDR2_2021; $4391~\deg^2$); HSC (magenta; Aihara_2022; $1078~\deg^2$); and KiDS (green; Wright_2024; $802~\deg^2$). The red shaded region indicates the region of sky covered by the Russian half of the eROSITA survey; the rest of the ACT search area has $7044~\deg^2$ of overlap with the German half of the eROSITA sky Meloni_2024. Bottom panel: Blue points mark the locations of 10040 optically confirmed SZ-detected clusters reported in this paper. The 'Legacy sample' of 3747 clusters detected at signal-to-noise ${\tilde{q}} > 5.5$ are marked with orange points. The full ACT search area is shown by the dark gray shaded region. In both panels, the Planck 353 GHz map is shown in the background, to highlight regions of sky that have more thermal emission from dust.
• Figure 3: A map illustrating the tiling geometry chosen for the ACT DR6 cluster search. The red boxes indicate the locations of tiles, of nominal size 25 deg $\times 2$ deg, that are processed independently by the cluster finder (see Section \ref{['sec:tiling']}). The full 16293 deg$^2$ cluster search area (before masking and flagging) is shown as the black outline. The map in the background shows the variation in the white noise level as a function of position in the ACT+Planck day+night f090 map.
• Figure 4: Distribution of signal-to-noise (${\tilde{q}}$) values close to the detection limit for the full cluster candidate list.
• Figure 5: Flag values across the cluster search region, used to identify objects located in potentially problematic regions (see Section \ref{['sec:flagging']}). Regions where flags = 0 are taken to be 'clean'; most of the flags = 1 region corresponds to the dust mask; and regions with larger flag values contain point sources (e.g. AGNs and dusty galaxies) detected by ACT. The full 16293 deg$^2$ cluster search area (before masking and flagging) is shown as the black outline. The total 'clean' area within this covers 12590 deg$^2$.