Plenary Summary

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Foregrounds for lensing: challenges, solutions and open questions

Foreground contamination

A new regime:

  • lensing power spectrum is obtained from a measurement of the four-point function of the CMB, e.g. <EBEB>
  • for most applications, most weight comes from polarization, which is probably cleaner (though should also use temperature if we have multifrequency data)
  • non-Gaussian galactic or extragalactic foreground can induce biases
  • precision requirements are extremely high - need biases below roughly 0.2%

Most significant polarization lensing foregrounds

1. Radio Sources

  • contamination was estimated to be moderate


  • can further reduce contamination by roughly an order of magnitude by "source hardening" - estimate source statistical anisotropy and project out of lensing


  • multifrequency data will help further - sources are probably a solved issue

2. Galactic Dust

  • though falls off on smaller scales, not obviously negligible compared to B, perhaps important for EB estimator


  • don't know high-ell polarized dust structure. To start, make EXTREMELY rough estimate using Planck 545 GHz maps. This definitely needs to be improved!!!


  • not huge (if estimate correct), but not negligible - may need multifrequency data even at high ell.

Open Questions and Tasks

  • Simulate high-L polarized dust (how?) and run through lensing pipelines
  • How do these foregrounds affect delensing? Do they set a limit on delensing?
  • Simulate full delensing procedure with foreground simulations
  • How many high-ell bands do we need for S4 lensing?
  • Can temperature foregrounds be removed to sufficient accuracy?
  • Include bias hardening in forecasts?