ACT DR6 SZ Cluster Catalog

• The ACT DR6 Sunyaev–Zel’dovich Cluster Catalog is a comprehensive database of galaxy clusters detected via the SZ effect in ACT multi-frequency maps.
• It employs a multi-frequency matched filter and rigorous weak‐lensing calibration to achieve high completeness and purity across 16,293 deg².
• The catalog supports precision cosmology, cluster evolution studies, and cross-survey analyses with detailed redshift and mass estimates.

The ACT DR6 Sunyaev–Zel’dovich (SZ) Cluster Catalog is the most comprehensive compilation of galaxy clusters detected via the SZ effect in millimeter-wavelength maps from the Atacama Cosmology Telescope (ACT), covering 16,293 deg² across three frequency bands (90, 150, and 220 GHz), with cluster detections spanning from 2008 to 2022. The catalog delivers redshifts and mass estimates for 9,977 optically confirmed clusters with reference-filter S/N > 4, extending over a broad mass and redshift range, with robust calibration against weak-lensing measurements. Its data products and software support precision cosmological analyses and the study of cluster evolution on Gyr timescales (Collaboration et al., 29 Jul 2025, Hilton et al., 2020, Hilton et al., 2017).

1. Survey Scope, Sky Coverage, and Evolution

The DR6 cluster search encompasses 16,293 deg² of extragalactic sky, making it the deepest and largest ACT cluster sample to date. The footprint is designed to minimize Galactic contamination via masking (e.g., Planck 353 GHz dust, |b| < 20°). The dataset combines all ACT seasons from 2008–2022, leveraging data from Advanced ACTPol and Planck for multi-frequency coverage and large-scale uniformity. Earlier catalogs (e.g., ACTPol E-D56 with 987.5 deg² (Hilton et al., 2017) and DR5/DR6 preliminary releases with 13,211 deg² (Hilton et al., 2020)) are fully subsumed within DR6, which further increases redshift reach and mass completeness, offering statistical power for cosmological constraints, cluster astrophysics, and cross-survey analyses.

2. Cluster Detection and Filtering Pipeline

Cluster detection employs a multi-frequency matched filter (Melin et al. 2006 formalism) operating in Fourier space on combined ACT and Planck maps. The filter kernel is:

$$\psi(k_x,k_y,\nu_i) = A \sum_j \left[N^{-1}\right]_{ij}(k_x,k_y) f_{\rm SZ}(\nu_j) S(k_x,k_y,\nu_j),$$

where $N_{ij}$ is the noise covariance matrix, $f_{\rm SZ}(\nu)$ encapsulates the SZ spectral signature $x\,(e^x+1)/(e^x-1)-4$ ($x=h\nu/k_{\rm B}T_{\rm CMB}$), and $S$ is the beam-convolved Universal Pressure Profile (UPP, Arnaud et al. 2010). Sixteen filter scales correspond to UPP models with $M_{500c} \in \{1,2,4,8\}\times 10^{14} M_{\odot}$ and $z \in \{0.2,0.4,0.8,1.2\}$ for cluster angular size matching.

Two detection statistics are reported per candidate:

• $q$: Optimal S/N across all filter scales, for candidate selection.
• $q_{2.4'}$: S/N at fixed reference filter scale $\theta_{500c} = 2.4'$ (used for consistent mass and completeness analysis).

Candidates with $q_{2.4'}>4$ and quality flags $=0$ form the principal confirmed sample. The “Legacy” subset (flags$=0$, $q_{2.4'}>5.5$, 10,347 deg²) achieves ≥ 90% purity. Simulation-calibrated false-positive rates at $q_{2.4'}>5.5$ yield only ≈7 contaminants expected over the Legacy footprint (Collaboration et al., 29 Jul 2025).

3. Mass Calibration, Scaling Relations, and Completeness

The SZ-mass relation in DR6 is recalibrated to weak-lensing results, reducing systematic offsets relative to X-ray-based scaling:

$$\tilde{y}_0 = 10^{A_0}\,E(z)^2\left( \frac{M_{500c}}{M_{\rm pivot}} \right)^{1+B_0} Q(\theta_{500c})\,f_{\rm rel}(M_{500c},z)$$

• $10^{A_0} = 3.0 \times 10^{-5}$ (DR6 normalization; 60% of Arnaud et al. 2010)
• $B_0 = 0.08$, $M_{\rm pivot} = 3 \times 10^{14} M_\odot$
• $E(z)=\sqrt{\Omega_m(1+z)^3+\Omega_\Lambda}$
• $Q(\theta_{500c})$ corrects for filter mismatch, $f_{\rm rel}$ applies the relativistic correction.

Log-normal intrinsic scatter $\sigma_{\rm int}=0.20$ is included; masses are derived from the Bayesian posterior $$P(M_{500c}|\tilde{y}_0,z)\propto P(\tilde{y}_0|M_{500c},z)P(M_{500c}|z)$$ with $P(M|z)$ set by the Tinker et al. (2008) halo mass function. The mass normalization is set such that $M_{500c}^{\rm WL}$ and $M_{500c}$ agree at the $\sim$7% level when compared with DES and HSC weak-lensing scaling ($1-b=0.65\pm0.05$).

Completeness is computed semi-analytically as a function of $M_{500c}$ and $z$, accounting for spatially variable noise. For the Legacy sample, 90% completeness is achieved for $M_{500c} \gtrsim5.0\times 10^{14} M_\odot$ at $z=0.5$, with the deepest 20% of the area reaching $M_{500c} \simeq(2.9-3.5)\times 10^{14}M_\odot$.

4. Catalog Structure, Data Products, and Access

The DR6 cluster catalog comprises:

• 9,977 confirmed clusters ($q_{2.4'}>4$, flags$=0$), with 1,166 at $z>1$, 121 at $z>1.5$.
• Median redshift $z_{\text{med}} = 0.58$ (range $0.03 < z < 2.00$).
• Mass range $M_{500c} = (1.3–23.1)\times 10^{14}\,M_\odot$, median $M_{500c} = 2.8\times 10^{14}\,M_\odot$.

FITS tables include extensive metadata:

• Basic: ACT-CL name, RA, Dec, $q$, $q_{2.4'}$, $\tilde{y}_0$, $\theta_{500c}$, noise properties, tile identification.
• Redshifts: value, type (spec/phot), origin (redMaPPer, CAMIRA, eRASS1CL, WaZP, etc.)
• Masses: $M_{500c}$ (DR6 and Arnaud10 scales), $M_{200c}$, $M_{200m}$; uncorrected and correction-applied; with errors.
• Flags: photometric/spectroscopic completeness, masking, optical cross-matches, survey overlaps (DES, HSC, KiDS).
• Warnings and notes: projected systems, AGN, BCG activity, known strong lenses, alternative positions.

Data products, including candidate lists (full, clean, Legacy), filtered maps, noise/patched maps, masks, selection functions, point-source catalogs, and corresponding software (Nemo SZ finder) are publicly available via NASA LAMBDA and collaborative repositories (Collaboration et al., 29 Jul 2025).

5. Redshift Assignment and Optical Confirmation

Optical and infrared confirmation leverages deep multi-band surveys: SDSS DR13/Stripe 82, DES Y3, HSC-SSP S19A, KiDS DR4, DECaLS DR8, and ancillary spectroscopic campaigns. Automated cluster finders (redMaPPer, CAMIRA, WaZP) provide photometric redshifts; additional template-fitting (e.g., BPZ/EAZY) refines $p(z)$ and yields ensemble $n(z)$ distributions for robust cluster $z$ assignment. For the E-D56 field, pipeline performance achieves $\sigma(\Delta z/(1+z))=0.015$ tested against SDSS spec-$z$ (Hilton et al., 2017). In DR6, 41.5% (4,138/9,977) of cluster redshifts are spectroscopic.

6. Scientific Applications, Notable Systems, and Systematic Uncertainties

The DR6 sample facilitates:

• Cosmological analyses: Cluster number counts and mass functions inform $\Omega_m$, $\sigma_8$, and tests of $\Lambda$CDM+Gaussianity. DR6 finds no extreme clusters challenging standard cosmology—El Gordo (ACT-CL J0102−4915; $z=0.87$, $M_{200m}\sim3\times10^{15}M_\odot$) and ACT-CL J0329.2−2330 ($z=1.23$, giant radio halo) fall below the LCDM 95% exclusion curves after weak-lensing recalibration (Collaboration et al., 29 Jul 2025).
• Cluster astrophysics: Stacked SZ/X-ray analysis enables exploration of ICM pressure profiles, baryon feedback, and cluster scaling relations.
• Cross-survey studies: The ACT DR6 overlaps substantially with DES, HSC, and KiDS, supporting joint weak-lensing, CMB lensing, and multi-wavelength analyses.
• Catalog systematics are rigorously quantified: false-positive rates, mass scatter ($\sim0.2$ dex), filter-noise biases ($\sim7\%$ at low SNR), and mass normalization offsets to X-ray and weak-lensing calibrations. For external studies, $M_{500c}$ measures strictly normalized to DR6 weak-lensing stacking results are recommended.

7. Historical Context and Catalog Evolution

The DR6 catalog represents an order-of-magnitude increase over early ACT SZ catalogs (e.g., E-D56: 182 clusters in 987.5 deg², $z<1.4$ (Hilton et al., 2017); DR5: 4,195 in 13,211 deg², median $z=0.52$ (Hilton et al., 2020)), reflecting advances in detector technology (TES bolometers, dichroic arrays), mapping speed, and data processing methodology. Strategic synergy with Planck (for large-angular-scale recovery) and optical/IR surveys (for redshift assignment and lensing mass calibration) has enabled well-controlled selection functions and cosmologically useful mass-observable relations. DR6 sits at the current state-of-the-art for high-purity, high-completeness SZ-selected cluster catalogs.

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References

(Collaboration et al., 29 Jul 2025) The Atacama Cosmology Telescope: DR6 Sunyaev-Zel'dovich Selected Galaxy Clusters Catalog (Hilton et al., 2020) The Atacama Cosmology Telescope: A Catalog of > 4000 Sunyaev-Zel'dovich Galaxy Clusters (Hilton et al., 2017) The Atacama Cosmology Telescope: The Two-Season ACTPol Sunyaev-Zel'dovich Effect Selected Cluster Catalog