Correcting the z~8 galaxy luminosity function for gravitational lensing magnification bias

Charlotte A. Mason, Tommaso Treu, Kasper B. Schmidt, Thomas E. Collett, Michele Trenti, Philip J. Marshall, Robert Barone-Nugent, Larry D. Bradley, Massimo Stiavelli, Stuart Wyithe

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We present a Bayesian framework to account for the magnification bias from both strong and weak gravitational lensing in estimates of high-redshift galaxy luminosity functions (LFs). We illustrate our method by estimating the z ~ 8 UV LF using a sample of 97 Y-band dropouts (Lyman break galaxies) found in the Brightest of Reionizing Galaxies (BoRG) survey and from the literature. We find the LF is well described by a Schechter function with characteristic magnitude of , faint-end slope of , and number density of . These parameters are consistent within the uncertainties with those inferred from the same sample without accounting for the magnification bias, demonstrating that the effect is small for current surveys at z ~ 8, and cannot account for the apparent overdensity of bright galaxies compared to a Schechter function found recently by Bowler et al. and Finkelstein et al. We estimate that the probability of finding a strongly lensed z ~ 8 source in our sample is in the range ~3–15% depending on limiting magnitude. We identify one strongly lensed candidate and three cases of intermediate lensing in BoRG (estimated magnification μ > 1.4) in addition to the previously known candidate group-scale strong lens. Using a range of theoretical LFs we conclude that magnification bias will dominate wide field surveys—such as those planned for the Euclid and WFIRST missions—especially at z > 10. Magnification bias will need to be accounted for in order to derive accurate estimates of high-redshift LFs in these surveys and to distinguish between galaxy formation models.
Original languageEnglish
JournalThe Astrophysical Journal
Issue number1
Early online date20 May 2015
Publication statusPublished - 21 May 2015


  • astro-ph.CO
  • galaxies: high-redshift
  • galaxies: luminosity function
  • mass function
  • gravitational lensing: strong


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