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A distortion of very-high-redshift galaxy number counts by gravitational lensing

Abstract

The observed number counts of high-redshift galaxy candidates1,2,3,4,5,6,7,8 have been used to build up a statistical description of star-forming activity at redshift z 7, when galaxies reionized the Universe1,2,9,10. Standard models11 predict that a high incidence of gravitational lensing will probably distort measurements of flux and number of these earliest galaxies. The raw probability of this happening has been estimated to be 0.5 per cent (refs 11, 12), but can be larger owing to observational biases. Here we report that gravitational lensing is likely to dominate the observed properties of galaxies with redshifts of z 12, when the instrumental limiting magnitude is expected to be brighter than the characteristic magnitude of the galaxy sample. The number counts could be modified by an order of magnitude, with most galaxies being part of multiply imaged systems, located less than 1 arcsec from brighter foreground galaxies at z ≈ 2. This lens-induced association of high-redshift and foreground galaxies has perhaps already been observed among a sample of galaxy candidates identified at z ≈ 10.6. Future surveys will need to be designed to account for a significant gravitational lensing bias in high-redshift galaxy samples.

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Figure 1: Gravitational lens fractions among candidate high-redshift HUDF galaxies.
Figure 2: Probabilities of multiple imaging of high-redshift galaxies with JWST.
Figure 3: Gravitational-lens-induced modification of the bright end of the high-redshift galaxy luminosity function to be observed with JWST.

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Acknowledgements

We thank K.-H. Chae for discussing results of his lensing calculations. J.S.B.W. was supported in part by a QE-II fellowship and grants from the Australian Research Council. H.Y. acknowledges support from the long-term fellow-ship programme of the Center for Cosmology and AstroParticle Physics (CCAPP) at The Ohio State University. H.Y. and R.A.W. were supported by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. R.A.W. was supported by a NASA JWST Interdisciplinary Scientist grant.

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J.S.B.W. performed the calculations of lensing probabilities. H.Y. measured the distributions of galaxy properties from the HUDF. All authors were involved in the conception of the work, discussing the results, and writing the manuscript.

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Correspondence to J. Stuart B. Wyithe.

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Wyithe, J., Yan, H., Windhorst, R. et al. A distortion of very-high-redshift galaxy number counts by gravitational lensing. Nature 469, 181–184 (2011). https://doi.org/10.1038/nature09619

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