Impact of foregrounds on HI intensity mapping cross-correlations with optical surveys

Steve Cunnington, Laura Wolz, Alkistis Pourtsidou, David Bacon

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The future of precision cosmology could benefit from cross-correlations between intensity maps of unresolved neutral hydrogen (HI) and more conventional optical galaxy surveys. A major challenge that needs to be overcome is removing the 21cm foreground emission that contaminates the cosmological HI signal. Using N-body simulations we simulate HI intensity maps and optical catalogues which share the same underlying cosmology. Adding simulated foreground contamination and using state-of-the-art reconstruction techniques we investigate the impacts that 21cm foregrounds and other systematics have on these cross-correlations. We find that the impact a FASTICA 21cm foreground clean has on the cross-correlations with spectroscopic optical surveys with well-constrained redshifts is minimal. However, problems arise when photometric surveys are considered: we find that a redshift uncertainty σ z ≥ 0.04 causes significant degradation in the cross power spectrum signal. We diagnose the main root of these problems, which relates to arbitrary amplitude changes along the line-of-sight in the intensity maps caused by the foreground clean and suggest solutions which should be applicable to real data. These solutions involve a reconstruction of the line-of-sight temperature means using the available overlapping optical data along with an artificial extension to the HI data through redshift to address edge effects. We then put these solutions through a further test in a mock experiment that uses a clustering-based redshift estimation technique to constrain the photometric redshifts of the optical sample. We find that with our suggested reconstruction, cross-correlations can be utilized to make an accurate prediction of the optical redshift distribution.
Original languageEnglish
Number of pages23
JournalMonthly Notices of the Royal Astronomical Society
Early online date11 Jul 2019
Publication statusEarly online - 11 Jul 2019


  • RCUK
  • ST/S000437/1
  • ST/N000668/1
  • large-scale structure of Universe
  • distances and redshifts
  • cosmology: observations
  • techniques: spectroscopic
  • photometric
  • radio lines: galaxies


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