The 2-degree Field Lensing Survey: Photometric redshifts from a large new training sample to r < 19.5

C. Wolf, A. S. Johnson, M. Bilicki, C. Blake, A. Amon, T. Erben, K. Glazebrook, C. Heymans, H. Hildebrandt, S. Joudaki, D. Klaes, K. Kuijken, C. Lidman, F. Marin, D. Parkinson, G. Poole

Research output: Contribution to journalArticlepeer-review

11 Scopus citations


We present a new training set for estimating empirical photometric redshifts of galaxies, which was created as part of the 2-degree Field Lensing Survey project. This training set is located in a ~700 deg2 area of the Kilo-Degree-Survey South field and is randomly selected and nearly complete at r < 19.5. We investigate the photometric redshift performance obtained with ugriz photometry from VST-ATLAS and W1/W2 fromWISE, based on several empirical and template methods. The best redshift errors are obtained with kernel-density estimation (KDE), as are the lowest biases, which are consistent with zero within statistical noise. The 68th percentiles of the redshift scatter for magnitude-limited samples at r < (15.5, 17.5, 19.5) are (0.014, 0.017, 0.028). In this magnitude range, there are no known ambiguities in the colour-redshift map, consistent with a small rate of redshift outliers. In the fainter regime, the KDE method produces p(z) estimates per galaxy that represent unbiased and accurate redshift frequency expectations. The p(z) sum over any subsample is consistent with the true redshift frequency plus Poisson noise. Further improvements in redshift precision at r < 20 would mostly be expected from filter sets with narrower passbands to increase the sensitivity of colours to small changes in redshift.

Original languageEnglish (US)
Pages (from-to)1582-1159
Number of pages424
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
StatePublished - Apr 11 2017
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • Methods: statistical
  • Surveys
  • galaxies: distances and redshifts


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