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Foregrounds and simulations

Post talks here:

  • r -- Lloyd Knox slides
  • TT/TE/EE -- Erminia Calabrese slides
  • Lensing -- Blake Sherwin slides
  • Clusters -- Colin Hill slides

Notes from session

r: Lloyd Knox

  • From Science Book, expect two distinct surveys for r: one at low resolution with many frequencies (for tensors), one with high resolution but fewer frequencies (for delensing)
  • Defined a staged series of data challenges. Simulated maps exist at NERSC, analysis of first data challenge is underway. Start simple, update survey and signals in later rounds.
  • Science Book used Fisher forecast to allocate sensitivity between eight frequency bands for low-res survey plus high-res survey.
  • Important question: Do we need additional frequencies below 30 GHz?
  • Survey definitions:
    • X=1: Science Book case with fsky=3% (check codes to confirm that we can reproduce Science Book results)
    • X=2: Updated set of bandpasses, multiple values of fsky.
    • X=3: Include both low-res and high-res surveys
  • Signal definitions:
    • Y=0: Lensed and partially-delensed CMB, Gaussian dust and synchrotron, no extragalactic foregrounds
    • Y=1: PySM 'standard' model = Galactic foregrounds (non-Gaussian, spatially varying indices for sync/dust), no extragalactic foregrounds
    • Y=2: PySM 'alternate' model
    • Y>2: Ben Thorne and Brandon Hensley working to add Hensley / Drain dust model, add extragalactic foregrounds consistent with CMB lensing (Alvarez/Battaglia/Bond)
  • Currently working on data challenge 1.0 -- recover the answer from Science Book. Check for biases and statistical uncertainty.
  • Need to connect with instrument working groups, start to include systematic effects in maps.

TT/TE/EE: Erminia Calabrese

  • Study impact of CMB secondaries, Galactic and extragalactic foregrounds on science from small-scale T/E. Build a covariance matrix including foreground uncertainty.
  • Use templates and nuisance parameters for tSZ, kSZ, tSZxCIB, CIB-P, CIB-C, Radio-P, Cirrus in TT, Radio-P in TE/EE.
  • Make simulations and multi-frequency covariance matrix. Repeat for different experiment configurations. Then analyze to solve for nuisance parameters, CMB, and eventually cosmological parameters.
  • Some constraints on secondary / Galactic / extragalactic nuisance parameters are orders of magnitude better than current limits. Will these models hold up at that precision?
  • Five frequencies vs three improves secondary/foreground constraints, but doesn't impact damping tail science (Neff). TE is most constraining spectrum.
  • No systematics currently included.

Lensing: Blake Sherwin

  • CMB-S4 is a new regime for lensing science: 0.2% precision, dominated by EB estimator. Stringent requirements for biases.
  • Biases from polarized Galactic foregrounds -- don't know what small-scale dust (or sync) polarization looks like.
    • Try scaling Planck dust intensity but assume constant angle.
    • Use orientation of HI filaments to trace dust polarization angles (in progress).
    • Simulations with magnetic turbulence -- yields large spread of bias in lensing estimator
  • Looks like foreground cleaning will be necessary for lensing science -- need multiple frequency channels at high resolution.
    • Try cleaning E-modes using multiple frequencies on a 5-meter telescope. Seems to work well with little degradation of lensing measurement.
    • Still need to try analogous process for delensing.
  • Extragalactic foregrounds only important for temperature, but can still be important.
    • Alvarez et al have developed simulations that contain tSZ, kSZ, CIB, and lensing, with appropriate correlations.
    • Temperature biases from extragalactic foregrounds can cause bias for cross-correlation estimators as well.

Clusters: Colin Hill

  • Use simulations to understand foreground biases for tSZ, kSZ. End to end test with set of simulations (that are also useful for high-ell science, lensing, delensing).
  • Galactic foregrounds at low ell. At high ell, mixture of CMB, tSZ, CIB, radio sources -- need multifrequency data.
  • For simulations, important that extragalactic foregrounds have the right correlations. Non-gaussian structure is important.
    • Need more information about non-white atmospheric noise dependence on frequency.
  • Extragalactic foreground sims from Alvarez et al (CITA)
  • Products are available at NERSC.
  • Working on component separation using ILC methods.

Action items/Next steps

Summarize action items here