First Light And Reionisation Epoch Simulations (FLARES) - IV. The size evolution of galaxies at z ≥ 5

William J. Roper, Christopher Cummiskey Lovell, Aswin P. Vijayan, Madeline Marshall, Dimitrios Irodotou, Jussi Kuusisto, Peter A. Thomas, Stephen M. Wilkins

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We present the intrinsic and observed sizes of galaxies at z ≥ 5 in the First Light And Reionisation Epoch Simulations (FLARES). We employ the large effective volume of FLARES to produce a sizeable sample of high-redshift galaxies with intrinsic and observed luminosities and half-light radii in a range of rest-frame ultraviolet (UV) and visual photometric bands. This sample contains a significant number of intrinsically ultracompact galaxies in the far-UV (1500 Å), leading to a negative intrinsic far-UV size-luminosity relation. However, after the inclusion of the effects of dust these same compact galaxies exhibit observed sizes that are as much as 50 times larger than those measured from the intrinsic emission, and broadly agree with a range of observational samples. This increase in size is driven by the concentration of dust in the core of galaxies, heavily attenuating the intrinsically brightest regions. At fixed luminosity we find a galaxy size redshift evolution with a slope of m = 1.21-1.87 depending on the luminosity sample in question, and we demonstrate the wavelength dependence of the size-luminosity relation that will soon be probed by the James Webb Space Telescope.
Original languageEnglish
Pages (from-to)1921–1939
Number of pages19
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
Early online date19 May 2022
Publication statusPublished - 1 Aug 2022


  • galaxies: evolution
  • galaxies: high-redshift
  • galaxies: photometry
  • UKRI
  • STFC
  • ST/K00042X/1
  • ST/P002293/1
  • ST/R002371/1
  • ST/S002502/1
  • ST/R000832/1

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