Notes from March 8 telecon on science requirements for clusters/high-ell

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CMB-S4 Cluster/High-ell Science Requirements Telecon 8 March 2017

Topic 1: Sigma8(z) Agreed with the approach to specify science requirement in terms of sigma8(z). We could translate this into constraints on wo-wa, Mnu and gamma in the accompanying justification text, as desired. We should look at the possible constraints from 1, 1.5 and 2 arcmin beams, as requested by the CDT. Nick is looking into this. It was noted that there is value in comparing LSST and S4 lensing masses in their redshift overlap region. We will compare mass calibration sensitivity with polarization to the case with polarization + temperature. First step will be a comparison based on raw sensitivity (Nick, Mat). In a second step, we will estimate component separation residuals for temperature reconstruction (e.g., getting out the tSZ, dust, etc.); Colin is prepared to do this. The discussion focussed on two timescales: immediate response for the CDT and further modelling as the CDT process continues.

  • Proposed update to CDT on Friday March 10th: *
  • Science requirement: *Measure sigma8(z) to X% over the range 0<z<3 in bins of dz = Y.
  • Justification:* This is the measurement of structure growth directly probed by clusters and is model independent. This cane be converted into to A, B and C constraints, respectively, on the dark energy equation-of-state parameters wo-wa, the sum of the neutrino masses Mnu and the gravity growth parameter gamma, as desired.

Path forward: We are using simulations to guide choices for X and Y and should have these in hand for Friday March 17.

Topic 2: Feedback/environment

Nick points out that stacking and cross-correlation studies are essentially the same thing, the former at zero lag and the latter over a range of larger scales. Steve notes that feedback is top of list for justification of future X-ray missions. Thus it’s a good selling point, but we should also emphasize the uniqueness of S4 in this context. Colin: Y-M relation

Discussion led to idea of building the science requirement around two points: a) How well we need to know P(k) for cosmology and dark energy studies; as we move into the non-linear regime (important in cosmic shear surveys), baryonic can influence P(k) significantly.

  • Form of science requirement:* By measuring the relative distribution of baryons - gas through SZ and galaxies from surveys - and dark matter to X% over 0<z<3 in bins of dz = Y and dlnk = Z, we can adequately constrain these effects for stage-4 dark energy goals.

b) Observing the gas fraction and thermal energy of halos (i.e., the CGM).

  • Form of science requirement: *Measure gas mass fraction and gas thermal energy to A% and B%, respectively, relative to virial energy of halos over 0<z<3 in bins of dz = C and dlnM = D.

Path forward: Determine X, Y and Z by looking at accuracy required on P(k) for dark energy stage-4. Find justification for values of A, B, C and D.

Question: How best to link the two-halo and one-halo measurements represented by items a and b.

Topic 3: kSZ and re-ionisation Three types of models for reionization that kSZ can help distinguish. Essentially measure Tau x astrophysics; can we separate the two? Tau x astrophysics. Can we get at astrophysics. Power spectrum and 4-pt function mostly sensitive to duration of reionization.

No clear science requirement formulation yet. Simone will put a list of science topics on the logbook for further discussion to see if we can identify a key science driver.


Additional notes: It would also be straightforward for us to identify a `gold sample' of the N most massive/highest-z clusters for astrophysical follow-up studies with other observatories.