First Light And Reionisation Epoch Simulations (FLARES) IV: The size evolution of galaxies at $z\geq5$

Abstract

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 UV and visual photometric bands. This sample contains a significant number of intrinsically ultra-compact galaxies in the far-UV (1500 Å), leading to a negative intrinsic far-UV sizeluminosity 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 which will soon be probed by the Webb Space Telescope.

First Light And Reionisation Epoch Simulations (FLARES) V: The redshift frontier

“…The redshift range is limited by the number of galaxies in FLARES with a sufficient number of stellar particles to make robust size measurements (N ≥ 100). As in Roper et al (2022) we impose a 95 percent completeness limit in each redshift bin.…”

“…Compared to the (100 Mpc) 3 EAGLE reference simulation, FLARES contains 10-100× as many high-redshift galaxies. In this article we use FLARES to make predictions for the galaxy population at z > 10, building on earlier work focused at z = 5 − 10 ( Lovell et al 2021;Vijayan et al 2021Vijayan et al , 2022Roper et al 2022).…”