CMB-S4 Galaxy Clusters

Srinivasan Raghunathan

CMB-S4 Clusters Analysis Working Group

CMB-S4 Summer Collaboration Meeting

18 August 2022

Outline

• CMB-S4 galaxy cluster target.
• CMB-S4 cluster forecasts.
• Preliminary Baseline Design Report (PBDR) setup.
• CMB-S4 cluster survey completeness.
• Cluster sensitivity and counts.
• Cosmological and astrophysical constraints.
• Analysis of alternatives.
• CMB-S4 SouthPole LAT (SPLAT) Crossed-Dragone (CD) vs Three-mirror anastigmat (TMA) designs.
• CMB-S4 + Advanced SO configurations.
• Breakout session (Beyond PBDR and AoA).

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CMB-S4 Cluster Science

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Thermal Sunyaev-Zeldovich (SZ) effect

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• Galaxy clusters contain hot gas (free electrons).
• CMB photons, that pass through clusters of galaxies, are inverse Compton scattered by free electrons in the intracluster medium (ICM).
• Used for blind detections of clusters in CMB surveys.
• Surface brightness of the SZ effect is redshift independent and hence allows us detect distant clusters.
• Cluster abundance as a fn(M,z) is also an excellent probe of structure formation with parameter degeneracies that are different compared to the primary CMB.

Frequency dependence of thermal SZ (Image: ESA)

CMB-S4 Galaxy Cluster Forecasts

• CMB-S4 cluster SZ surveys:
• 6-metre telescopes → 1.4 arcmin beam at 145 GHz.
• CMB-S4 Wide: Chilean survey: f_sky = 67%.
• CMB-S4 Ultra-deep: South Pole survey: f_sky = 3%.
• 6m Crossed-Dragone design V3R025.
• Also have estimates for 5m TMA.
• Forecasting inputs:
• Bands: 27, 39, 93, 145, 225 and 278 GHz
• Noise and Beams: PBDR values.
• Footprint: fsky = 0.67 using a minimum observing elevation=40 degrees.
• Split into clean (fsky = 0.5) and dirty (fsky = 0.17) regions.
• Note: Fiducial forecasts rely only on the clean (fsky = 0.5) patch.
• Extragalactic foregrounds: Radio, CIB, tSZ and kSZ power spectra from SPT measurements.
• Galactic foregrounds: Dust and Synchrotron power spectra obtained from pySM3 simulations.
• Cluster signal: Generalised NFW (Navarro Frenk and White) with Planck Y-M relation.

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CMB-S4 Galaxy Cluster Forecasts

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• CMB-S4 cluster SZ surveys:
• 6-metre telescopes → 1.4 arcmin beam at 145 GHz.
• CMB-S4 Wide: Chilean survey: fsky = 67% (or fsky=50% if we ignore regions significantly contaminated by galactic emission).
• CMB-S4 Ultra-deep: South Pole survey: fsky = 3%.

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• Signal-to-noise threshold: 5σ.

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• Observable:

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CMB-S4 shall detect (at 5σ) all galaxy clusters with an integrated Compton Y_SZ ≥ XX at z ≥ 1.5 over the large area survey footprint (f_sky = 67%). Furthermore, it shall detect (at 5 sigma) all galaxy clusters with an integrated Compton Y_SZ ≥ YY at z ≥ 1.5 over the de-lensing survey footprint (f_sky = 3%).

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Compton-y Noise Curves for Multiple Configurations

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We are CIB-limited but the current PBDR (in black) design is the most optimal given the number of detectors.

Effect of swapping detectors b/w medium (90/150 GHz) and high (220/278 GHz) frequency bands.

Also see work by Colin Hill on the CMB-S4 wiki.

Same is also true for CMB spectrum (i.e:) Neff measurements.

CMB-S4 Cluster Survey Completeness

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Planck collaboration 2014 XX, arXiv: 1303.5080

Alonso, Louis, Bull et al. 2016, arXiv: 1604.01382

CMB-S4 PBDR (in prep.)

Important to understand this for cosmological constraints.

CMB-S4 Cluster Survey Completeness

CMB-S4 shall detect (at 5σ) all galaxy clusters with an integrated Compton Y_SZ ≥ 2x10^-12 sr or 2.4 x10^-5 arcmin² at z ≥ 1.5 over the large area survey footprint (f_sky = 65% 50%). Furthermore, it shall detect (at 5σ) all galaxy clusters with an integrated Compton Y_SZ ≥ 10^-12 sr or 1.2 x10^-5 arcmin² at z ≥ 1.5 over the de-lensing survey footprint (f_sky = 3%).

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Science requirement:

CMB-S4 PBDR (in prep.)

CMB-S4 Cluster Sensitivity / Counts

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CMB-S4 PBDR (in prep.)

• S4-Wide: Contains clusters from low (f_sky = 0.5) + high (f_sky = 0.15) Galactic emission regions. Removing high Galactic emission region reduces ~20% objects.
• High-z (z>=2) clusters: S4-Wide → ~1000 clusters; S4-Ultra deep → ~350 clusters.
• Wee understood selection function even at high redshifts.

Optical / X-ray surveys

Unique region probed by future SZ surveys.

Cosmological Constraints

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CMB-S4 Wide: CMB (TT/EE/TE) with Cluster counts using CMB-cluster lensing mass calibration.

Look into arXiv:2107.10250 for more details.

Also see Louis & Alonso 2017, arXiv: 1609.03997; Madhavacheril, Battaglia & Miyatake 2017, arXiv: 1708.07502.

Including information from galaxy weak lensing will further strengthen the constraints and also offer an important systematic check.

See Madhavacheril, Battaglia & Miyatake 2017, arXiv: 1708.07502.

Constraining Astrophysics and Cosmology with Clusters

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Look into arXiv:2107.10250 for more details.

• Virialisation model 1:
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• 2-4 per cent on cluster virialisation parameter.
• Sub-percent constraint on (constant) HSE bias.
• Virialisation model 2:
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• <5 per cent on B_v and ~30 per cent on redshift evolution A_v.
• Swapping cluster virialisation model 1 to model 2 does not affect cosmological constraints significantly.

Analysis of alternatives

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Cluster Forecasts: SPLAT design alternative - CD vs TMA

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CMB-S4 PBDR (in prep.)

6m Crossed-Dragone (CD) design has 20 per cent better beam than the 5m Three-mirror anastigmat (TMA) design and this beam degradation affects cluster sensitivity.

Cluster Forecasts: SPLAT design alternative - CD vs TMA

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CMB-S4 PBDR (in prep.)

With TMA's 20 per cent larger beam compared to the current CD design, we will miss roughly x2.5 clusters at z>=1.5 and x3 at z>=2.

Cluster Forecasts: SPLAT design alternative - CD vs TMA

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CMB-S4 PBDR (in prep.)

With TMA's 20 per cent larger beam compared to the current CD design, we will miss roughly x2.5 clusters at z>=1.5 and x3 at z>=2.

South Pole-only option:

• We did not consider this for clusters.
• Mapping f_sky~25 per cent with the same map depth as the S4-Ultra deep (f_sky = 3 per cent) will help us detect a lot of low mass clusters.
• However, that option is not viable for N_eff science.

Cluster Forecasts: CMB-S4 + Advanced SO

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• The other analysis alternative is to replace one of the CMB-S4 Chilean LAT by the Advanced Simons Observatory (ASO) LAT.
• In the following slides we will compare cumulative cluster counts at the end of observation period from the following configurations:
• SO-Baseline (4 years of observation).
• Advanced SO (5 years of observation).
• CMB-S4 Single CHLAT + Advanced SO (+ SO-Baseline).
• Nominal CMB-S4 PBDR or PLR configuration (2 CMB-S4 CHLATs).
• Note:
• The SO noise levels are not exactly the same as in SO overview paper but a scaled version to include differences in sensitivities.
• SO forecasts assume the same sky fraction as CMB-S4 (f_sky = 0.67).

Cluster Forecasts: CMB-S4 + Advanced SO

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CMB-S4 PBDR (in prep.)

• Roughly 30 (22) per cent lower clusters at z>=2 (z>=1.5) when one S4 CHLAT is replaced by ASO LAT.
• Excluding SO-baseline reduces the number counts by 5 - 8 per cent.

Summary

• Given the number of detectors, the configuration we have currently for CMB-S4 CHLATs is close to the nominal for cluster survey (and also for N_eff).
• Science Requirement:
• S4-Wide: Y_SZ ≥ 2.4 x10^-5 arcmin² and S4-Ultra deep: Y_SZ ≥ 1.2 x10^-5 arcmin².
• Counts:
• S4-Wide - Total: 75k and z >= 2: 1000 clusters.
• S4-Ultra deep - Total: 13k and z>=2: 300 clusters.
• Parameter constraints: CMB (TT/EE/TE) + cluster counts with CMB-lensing mass calibration.
• σ(w_DE) ~ 1 per cent and 2.5-3σ detection of sum of neutrino masses.
• σ(bias_HSE) < 1 per cent.
• Design alternatives:
• 6m CD vs 5m TMA SPLAT: Degrades counts by x2.5 at z>=1.5 and x3 at z>=2.
• Combining CMB-S4 with ASO: >20 per cent lower clusters at z>=1.5.

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References: CMB-S4 PBDR; arXiv: 2112.07656; arXiv: 2112.07656;

Madhavacheril, Battaglia & Miyatake 2017, arXiv: 1708.07502; Louis & Alonso 2017, arXiv: 1609.03997.

CMB-S4 Cluster Calls and Breakout Session

• CMB-S4 Cluster telecon:
• Fortnightly on Thursdays at 10 a.m. pacific (but will likely change soon).
• Email list: clusters@cmb-s4.org
• Upcoming project proposals from Clusters AWG:
• CMB-S4 Pairwise kSZ forecasts: Led by Eduardo Schiappucci (PhD student at the University of Melbourne).
• CMB-S4 Cluster Breakout Session:
• Led by Lindsey Bleem.
• Focus: Update CMB-S4 science book v2 and science task list.
• Talks by:
• Lindsey Bleem, Sebastian Bocquet, Scott Chapman, Joe Hollowed, Florian Keruzore, Tony Mroczkowski, Daisuke Nagai and Srinivasan Raghunathan.
• Topics:
• Baryon pasting, Lensing, Halo mass function emulator, High redshift clusters / Protoclusters, Selection function, ++

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Backup slides

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CMB-cluster lensing Mass Estimates

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CMB lensing mass calibration: Average mass of the sample

• Foregrounds (tSZ/kSZ) in temperature-based lensing reconstruction are not an issue because we have multiple fancy estimators now.
• S4-Ultra deep lensing is dominated by polarisation.
• Combing T and P, we can measure the average mass of z≥2 clusters at 18-20 per cent level with S4-Wide or S4-Ultra deep.

CMB-S4 Cluster Forecasts: Expected Counts

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Look into arXiv:2107.10250 for more details.

Virialisation Mechanism for Distant Clusters

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Model 1:

Model 2:

Planck Y_SZ- M scaling relation with a constant HSE bias.

Simple linear scaling.

Analytic model tested using simulations.

What about the virialisation process of high-z clusters?

• Observations: Only one cluster at z~2. Mantz et al. 2014, 2018 (arXiv: 1401.2087, 1703.08221) find the properties of this cluster to be consistent with low-z clusters.
• CMB-S4 will make a giant leap in the field of cluster science.