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Clusters and High ell science

Chairing: David Spergel

General context

Cosmology focused part - mnu and w

  • 11.08 - 11.18 Nick Battaglia - mnu and w from lensing-calibrated clusters slides
  • 11.20 - 11.30 Mat Madhavacheril - mnu and w from CMB-calibrated clusters slides
  • 11.32 - 11.42 Colin Hill - mnu and w from tSZ power spectrum slides
  • 11.44 - 11.49 Lindsey Bleem - Cluster Simulation tools at ANL here
  • 11.50 - 12.00 Christian Reichardt - forecasting cosmology from cluster lensing, and kSZ with 4MOST File:20170227 cmbs4.pdf
  • 12.02 - 12.12 David Alonso - growth function from kSZ, comparison to DESI/Euclid File:KSZgrowth.pdf

Astrophysics focused part - reionization and cluster astrophysics

  • 12.14 - 12.24 Simone Ferraro - reionization case from kSZ slides
  • 12.26 - 12.36 Nick Battaglia - cluster astro case from tSZ+kSZ slides
  • 12.38 - 12.48 Jim Bartlett - Probing the Circumgalactic Medium File:Bartlett.pdf
  • David Spergel - few minute wrap-up

Notes from session

Notes from Steve Allen's talk:

  • 1 arcminute resolution provides an enormous increase in cluster discovery potential relative to 3 arcminutes
  • 1 arcminute would greatly increase galaxy science, and the size of the community served.

Here is the strawman conversion between size of telescope and FWHM as a function of channel that Mat and Nick were looking at (provided by Darcy Barron/Mike Niemack). Just a strawman. Ignore the noise column.

Tel size.png


  • Can do extraordinary things in cluster astro and cluster cosmology with sufficiently small beam.
  • Current leading catalogs are X-ray, optical, SZ. Need a well-defined selection function, and mass-observable relation. This used to be a problem. But, now we split into two parts: relative mass (x-rays) and absolute mass (lensing).
  • Current best constraints are Mantz et al from 220 ROSAT clusters, z<0.5, with Chandra follow-up and WtG WL masses. Similar results come from from Planck clusters. Currently have 15% on DE w from clusters.
  • Radical change coming. Optical: DES/HSC, Euclid, LSST. Mm: SPT-3G, ACT, S4. End of 2017 eROSITA for X-ray.
  • Strengths for optical: cluster finding, photo-z, WL mass calibration. Strengths for mm: high-z clusters, CMB-WL mass calibration. Strengths for X-rays: cluster-finding, low-scatter mass proxies.
  • Target: 100k clusters; far stronger in combination.
  • Optical clusters are limited to z=1.2. X-ray eROSITA find all clusters at low-z, some up to 1.5. Unique discovery space for CMB is at z=1.5 and above, up to z=3. A beam of 3’ loses much of the science.
  • On galaxy cluster astrophysics: we can find virialized haloes out to where they first formed. Things to study: Impact of environment on triggering and quenching of star formation and AGN activity. Evolution of feedback process. Integrated history of star formation.
  • The majority of SPT papers came from cluster astrophysics/cosmology and galaxy astrophysics. (Spergel - ACT too)
  • Conclusion: with 1’ we can be transformative.
  • Douglas Scott: dubious about cluster cosmology. Allen: results have held up.


  • Forecasting constraining power of mass-calibrated SZ cluster counts
  • Summary of code: non-white-noise, 1.5 uK’ at 90 and 150, includes extragalactic foregrounds, marginalizes over scaling relation. Planck tau prior 0.01.
  • Need to calibrate SZ masses, here use optical WL to get mass errors as function of mass. Assume HSC-like coverage and use optical WL out to z=1 or z=2.
  • Going from 5m to 6m to 7m does not affect counts and forecasts, but 3m is much worse
  • Main conclusion: neutrino mass is well constrained by clusters +Planck (as good as CMB lensing +DESI), using both z=1 and z=2
  • Completely independent and complementary to lensing and DESI. Forecast a better w than DESI BAO.
  • Going from 5m to 7m gets about 2x more clusters.
  • Optical surveys are required to obtain redshifts.
  • Requirements: couple percent on w, and 2.5-3 sigma on mnu minimal mass.
  • Allen: likely can do even better, and may want higher res, if look at w model that varies at high z.


  • Showing forecasts with new code using CMB-lensing-calibrated masses. CMB lensing takes over at high z.
  • You can do better on w if you use internal CMB lensing mass calibration, compared to WL-calibration.
  • Find same scaling with size as for WL-calibration: want to be >= 5m.
  • Science targets: roughly 35 meV for mnu, 2.5% on w. This is without DESI, just with Planck.
  • Atmosphere and tau-limit means the improvement going from 5-7m is only 5%. CVL on tau gives more improvement going from 5m to 7m (40%).
  • If you use 5m with internal P only, do nearly as well as with T+P, which mitigates worries about temperature contamination.
  • Two calibration methods to get competitive neutrino mass, and interesting w limits.
  • Request from Charles to talk about resolution not size. Jo/others: the resolutions are now posted.
  • Allen – looking at w(z) could push to larger telescope.
  • Bond – what do we need to get ‘gold sample’? Might be higher resolution. Nick agrees.



  • Describes simulations from HACC code at Argonne: N-body simulations. Building emulator to get 1% halo mass function. Getting ready to do realistic cluster work.
  • Bartlett– really need to calibrate mass function. This is a big challenge. Bleem – yes, these sims will be important for that.


  • Polarization wins for cluster lensing below 2 uK/amin, good as foregrounds are simpler. Most biases considered, e.g. offset centres, are sub-percent.
  • S/N increases by 1.7 going from 3’ to 2’. This is consistent with findings of Mat/Nick.
  • CMB lensing systematics much easier to deal with than optical WL calibration.
  • Promise of kSZ with southern spectro survey 4MOST, first light in 2021.


  • Forecasting measurement of growth of structure from kSZ. There are hints of problems in current data so this could be useful.
  • Constraints depend on the quality of the overlapping spectro survey (e.g. DESI better than BOSS).
  • Strength of kSZ compared to other data: you can measure growth at low-z where RSD is limited. Beats DESI and at low-z Euclid.
  • But need to know details about how you make the measurement, as there are many uncertainties about extracting the signal and e.g. measuring tau (v and tau are degenerate).
  • Pros: This is an alternative measurement with different systematics. Cons: Drawbacks are systematics in y-tau relation and wrong kSZ profile model.
  • Conclusion: growth from kSZ may not be main science driver of S4 but adds important information that has different systematics.
  • Spergel – can get more information at higher-z if you include extra statistics




Spergel wrap-up

  • cluster science is so rich. We can’t lose it so need to determine what size telescope we need. It is unique science that other data cannot be do. Majority of papers from S4 will be from this IF we have high enough resolution.
  • Need to explain our case to our astrophysics colleagues, to convey what we can tell them about galaxy formation. Need to articulate it more broadly to astrophysics community.
  • Keating – is this really what we should be doing with polarimeter? Spergel - Remember CMB lensing relies on polarimetry. We are limited by calibration, and CMB lensing will let us calibrate the maps.
  • Comment: Having neutrino mass from two probes is huge benefit and cross-check is significant value (in particular to persuade particle physicists).
  • Bond – can we split into delta nu

(Jo is also taking notes, will paste after session!)

Action items/Next steps

Summarize action items here

High level:

  • Define neutrino mass requirement from cluster counts
  • Define sigma(w(z)) requirement from counts, and growth function requirement from kSZ
  • Define reionization requirement
  • Define astrophysics requirements (e.g. on feedback parameters)
  • Then define measurement requirements for these things

Details that came up in talks include:

  • Extend cluster forecasts to extended w models, check resolution requirements
  • Look at 4m-5m range
  • Include scatter in scaling relation
  • Combine cluster counts and SZ pdf together
  • Compare patchy reionization constraints clearly to 21cm