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

TL;DR

This study uses the FLARES simulations to analyze galaxy sizes at redshifts above 5, revealing how dust effects significantly enlarge observed galaxy sizes and how size evolution varies with redshift and wavelength.

Contribution

It provides the first detailed analysis of intrinsic and observed galaxy sizes at high redshift using large-volume simulations, highlighting dust effects and size evolution.

Findings

Dust increases observed galaxy sizes by up to 50 times compared to intrinsic sizes.

Intrinsic far-UV size-luminosity relation is negative, with compact galaxies prevalent.

Galaxy size evolution with redshift has a slope of 1.21 to 1.87 depending on luminosity.

Abstract

We present the intrinsic and observed sizes of galaxies at $z\geq5$ 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 angstrom), 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 which will soon be probed by the Webb Space Telescope.