Planck PSZ2 Cosmological Clusters
- Planck PSZ2 Cosmological Cluster Sample is an all-sky, mass-selected catalogue that identifies galaxy clusters via the Sunyaev–Zeldovich effect with minimal redshift bias.
- It employs three independent detection pipelines and extensive multi-wavelength validation to derive reliable scaling relations and calibrate cluster masses.
- The sample's high completeness and purity, along with detailed analyses of mass bias and scaling systematics, enable precise constraints on parameters like Ωₘ and σ₈.
The Planck PSZ2 Cosmological Cluster Sample is the largest all-sky galaxy cluster sample selected via the Sunyaev–Zeldovich (SZ) effect, constructed from the full Planck mission data and consisting of 439 robustly selected clusters (with S/N > 6 for cosmology) as part of an overall parent catalogue of 1,653 SZ sources. As a mass-selected sample with minimal redshift dependence and extensive multi-wavelength counterpart validation, this catalogue is foundational for contemporary cosmological analyses that utilize cluster abundances and scaling relations to constrain fundamental parameters such as the matter density (Ωₘ), amplitude of density fluctuations (σ₈), and extensions of the standard ΛCDM model.
1. Catalogue Definition, Selection, and Validation
The PSZ2 catalogue, derived from 29 months of Planck data, employs three independent detection pipelines—MMF1, MMF3 (matched multi-filters), and PowellSnakes (PwS, a Bayesian algorithm)—to robustly extract SZ sources across 83.6% of the sky (Collaboration et al., 2015). The cosmological subset, typically defined with S/N > 6 and additional validation cuts, spans z ≈ 0–1 and masses ≈(2–10)×10¹⁴ M_⊙ (Collaboration et al., 2015).
The selection function is quantified via extensive Monte Carlo injection of simulated clusters (using cosmo-OWLS-based pressure profiles) into real Planck maps and is provided as a multi-dimensional cube in (Y₅₀₀, θ₅₀₀, S/N) (Collaboration et al., 2015). Completeness is well-approximated by analytic error functions for unresolved sources but drops for large, resolved systems and in sky areas contaminated by Galactic dust. Catalogue purity is estimated to be above 83% (for the full sample), and above 90% for the intersection sample (detections by all three algorithms).
Cluster validation and redshift assignment rely on cross-matching with external X-ray (MCXC), optical (SDSS/redMaPPer), and infrared (WISE) catalogues, supplemented by targeted optical and spectroscopic follow-ups (Burenin et al., 2018, Banerjee et al., 2019, Bahk et al., 6 Mar 2024). Optical confirmation campaigns have focused on identifying galaxy overdensities, measuring velocity dispersions, and characterizing counterparts’ richness properties (Streblyanska et al., 2019, Aguado-Barahona et al., 2019). As of the latest updates, over 1,300 SZ sources have confirmed cluster status with accurate redshift information (Bahk et al., 6 Mar 2024).
2. SZ Signal Extraction, Scaling Relations, and Mass Calibration
Each detected cluster is assigned an SZ observable (Y₅₀₀ or Y₅R₅₀₀), which quantifies the integrated thermal pressure of the intracluster medium. This is derived by simultaneously fitting multi-frequency Planck data to generalized NFW (GNFW) pressure profiles convolved with the Planck beam. The measured SZ quantity is translated into a total cluster mass (M₅₀₀) via a scaling relation of the form (Collaboration et al., 2015, Collaboration et al., 2015): where (1–b) is the mass bias parameter, α and β are the mass and redshift exponents, and Dₐ(z) is the angular diameter distance.
The mass bias (1–b) parametrizes the offset between the true cluster mass and the hydrostatic/X-ray mass and is the dominant systematic uncertainty in cosmological inference. Calibration employs external weak-lensing studies (e.g., Weighing the Giants and CCCP), dynamical analyses (velocity dispersions), hydrostatic X-ray techniques from Chandra/XMM, and more recently, CMB lensing (Aguado-Barahona et al., 2021, Aymerich et al., 2 Sep 2025). Consensus values for the hydrostatic bias vary: weak-lensing and dynamical calibrations yield (1–b) ≈ 0.8 (stat.+sys. ≈ 6%) (Aguado-Barahona et al., 2021, Aymerich et al., 2 Sep 2025), but some analyses of CMB primary anisotropies in tension suggest required (1–b) ≲ 0.62 (Lesci et al., 2023).
3. Cosmological Constraints and Modeling
The primary scientific use of the PSZ2 cosmological sample is to constrain Ωₘ, σ₈, and the parameter combination S₈ ≡ σ₈√(Ωₘ/0.3), via likelihood analyses comparing observed cluster counts in (z, S/N) space against predictions from the halo mass function (Collaboration et al., 2015, Aymerich et al., 2 Sep 2025). The likelihood incorporates the survey selection function and scaling relation with its mass bias uncertainty.
Recent analyses jointly fit the cluster abundance and mass calibration (using DES shear profiles and Chandra X-ray data) and find (Aymerich et al., 2 Sep 2025):
- Ωₘ = 0.312{+0.018}_{-0.024}
- σ₈ = 0.777 ± 0.024
- S₈ = 0.791{+0.023}_{-0.021}
- (1–b) = 0.844{+0.055}_{-0.062} Assuming a mass-evolving bias, the constraints shift to Ωₘ = 0.353{+0.025}_{-0.031}, σ₈ = 0.751 ± 0.023, S₈ = 0.814{+0.019}_{-0.020}.
These results are broadly in agreement with late-time structure probes and SPT cluster analyses (which share calibration data), are mildly (∼1.6σ) lower in S₈ than Planck CMB results, and in significant (∼2.9σ) tension with eROSITA-based cluster cosmology (Aymerich et al., 2 Sep 2025). Analyses that allow for redshift or mass evolution in the scaling relation (e.g., with β = 0.86 ± 0.07 instead of the canonical 2/3) find evidence for either cluster physics evolution or non-standard cosmological evolution (w = –0.82 ± 0.07) (Lee et al., 28 Mar 2024).
4. Mass Bias, Scaling Systematics, and Tensions
The value and possible mass or redshift evolution of the mass bias parameter (1–b) is a central systematic (Collaboration et al., 2015, Lesci et al., 2023, Lee et al., 28 Mar 2024). Weak-lensing and dynamical calibrations cluster around (1–b) ≈ 0.8, but full consistency with the primary CMB-inferred σ₈ and Ωₘ (higher values) would require (1–b) ≈ 0.62 (Lesci et al., 2023). This discrepancy motivates extensive efforts to map biases from non-thermal pressure, projection effects, miscentring, and analysis-specific choices (e.g., halo center definition for shear extraction (Aymerich et al., 2 Sep 2025)). Testing mass bias evolution with mass or redshift can affect the derived cosmological constraints, shifting S₈, Ωₘ, and σ₈, and sometimes reducing or exacerbating tensions with other measurements (Aymerich et al., 2 Sep 2025, Lee et al., 28 Mar 2024).
Additionally, joint analyses with ACT, SPT, and eROSITA cluster catalogues, as well as multi-wavelength confirmation campaigns (e.g., LP15) and cross-identification with optical and X-ray cluster catalogues (AMF DR9, redMaPPer, WHL, MCXC), are used to enhance completeness and purity and further probe systematic errors (Aguado-Barahona et al., 2019, Banerjee et al., 2019, Bahk et al., 6 Mar 2024).
5. Cluster Validation, Optical Follow-up, and Purity
Extensive optical campaigns (e.g., LP15 using INT, TNG, GTC; follow-ups with SDSS, DES, WHT, and KPNO) have confirmed, characterized, or ruled out optical counterparts for hundreds of PSZ2 sources (Streblyanska et al., 2019, Burenin et al., 2018, Streblyanska et al., 2018, Boada et al., 2018). Confirmation is based on spatial and photometric coincidence, color-magnitude diagram red sequences, richness estimators, and spectroscopic velocity dispersions (500–650 km/s thresholds depending on redshift) (Streblyanska et al., 2019, Aguado-Barahona et al., 2019). These campaigns have increased catalogue purity in the northern sky from 76.7% to 86.2% (for S/N > 4.5) and demonstrated that regions of high dust emission (traced by Planck 857 GHz) are correlated with a higher rate of false or unconfirmed SZ sources (Aguado-Barahona et al., 2019).
Special attention is given to cases of line-of-sight projection (multiple optical counterparts contributing to the SZ signal), high-redshift or optically poor systems, and “X-ray-underluminous” clusters that highlight selection and multi-wavelength completeness effects (Collaboration et al., 2015, Burenin et al., 2018, Streblyanska et al., 2018).
6. Impact, Cross-survey Synergies, and Future Prospects
The PSZ2 sample is a foundational data set for precision cosmology and cluster astrophysics. Joint catalogues combining Planck with SPT and ACT data have demonstrated enhanced completeness and efficiency by leveraging the complementarity of Planck’s low-redshift, high-mass sensitivity and the high-redshift reach of ground-based millimeter surveys (Melin et al., 2020, Voskresenskaia et al., 2023). The application of machine learning to composite ACT+Planck maps (as in ComPACT) has further improved completeness and purity below Planck’s conventional detection threshold (Voskresenskaia et al., 2023).
Continued refinement of the mass–observable relation, its bias factors, and their possible mass and redshift evolutions are a central focus, with projections showing that 1% precision in mass calibration (attainable with next-generation lensing, X-ray, and CMB data) will be critical for fully resolving cluster–CMB parameter tensions and constraining extensions to ΛCDM such as massive neutrinos or evolving dark energy equations of state (Collaboration et al., 2015, Aymerich et al., 2 Sep 2025, Lee et al., 28 Mar 2024).
7. Scientific Discoveries and Notable Clusters
The PSZ2 sample has yielded individual systems of special significance, such as high-redshift clusters confirmed via optical spectroscopy and gravitational lensing (including with background galaxies at z > 4.2) (Burenin et al., 2018), bullet-like mergers analogous to the famous Bullet Cluster with offsets suitable for testing dark matter physics (Bartalucci et al., 11 Sep 2024), and low-mass merging clusters with exceptionally distant radio relics, which inform shock formation and structure-formation paradigms (Stroe et al., 13 Jan 2025).
Summary Table: Key Metrics for the PSZ2 Cosmological Sample
| Metric | Value / Range | Reference |
|---|---|---|
| Total SZ sources (PSZ2) | 1,653 | (Collaboration et al., 2015) |
| Confirmed clusters (latest compilations) | >1,300 | (Bahk et al., 6 Mar 2024) |
| Cosmological sample (S/N > 6, validated) | 439 | (Collaboration et al., 2015) |
| Typical mass range (M₅₀₀) | ~2–10 × 10¹⁴ M_⊙ | (Collaboration et al., 2015) |
| Redshift range | 0 ≲ z ≲ 1 | (Collaboration et al., 2015, Burenin et al., 2018) |
| Mass bias (1–b), weak-lensing/dynamical | 0.80 ± 0.04 (stat.) ± 0.05 (sys.) | (Aguado-Barahona et al., 2021, Aymerich et al., 2 Sep 2025) |
| Mass bias to reconcile with Planck CMB | ≈0.62{+0.14}_{-0.11} | (Lesci et al., 2023) |
| Cosmological constraints (baseline, +BAO) | Ωₘ = 0.312{+0.018}_{-0.024}; σ₈ = 0.777 ± 0.024; S₈ = 0.791{+0.023}_{-0.021} | (Aymerich et al., 2 Sep 2025) |
| Purity (northern sky, S/N>4.5, after validation) | 86.2% | (Aguado-Barahona et al., 2019) |
| Completeness (control region, S/N>6) | ≳95% | (Collaboration et al., 2015) |
The Planck PSZ2 Cosmological Cluster Sample represents the most statistically powerful all-sky SZ-selected sample for constraining cosmology with cluster number counts, exemplifying the essential role of multi-wavelength validation and robust mass calibration in the era of precision large-scale structure cosmology.