Accelerated size evolution in the FirstLight simulations from z=14 to z=5

TL;DR

This study uses cosmological simulations to analyze the rapid evolution of galaxy sizes from redshift 14 to 5, revealing that size growth accelerates during cosmic dawn due to increased galaxy efficiency and complex dust interactions.

Contribution

It provides new insights into the size evolution of early galaxies using the FirstLight simulations, highlighting the rapid size increase and the role of dust and star formation.

Findings

Galaxy size at fixed mass increases by 0.5 dex from z=14 to 6.

Size-mass relation shows large diversity and little evolution in slope.

Size evolution accelerates at z>5 due to higher galaxy efficiency.

Abstract

Galaxies grow very rapidly during the first Gyr of the Universe, mostly driven by high galaxy efficiencies, particularly relevant at $z>5$. This efficiency is related to high gas densities and/or compact gas distributions within these early galaxies. We want to understand the evolution of the size of galaxies at cosmic dawn, from $z=14$ to $z=5$ and its main drivers. We use the FirstLight database of 430 zoom-in cosmological simulations and radiative transfer calculations to generate synthetic images in seven JWST bands. We add observational effects, inspired by recent JWST deep extragalactic surveys. The size-mass relation is already in place at $z\simeq14$ and it shows a large diversity of galaxy sizes at a fixed mass. Extended (compact) galaxies tend to have higher (lower) specific star-formation rate (sSFR). The mass-dependent slope does not evolve significantly. This is driven by a complex interaction between stellar light and dust. Differential dust attenuation dims galaxy centers and it makes larger sizes, modifying the mass-size slope even in the rest-frame optical. At a fixed mass, galaxy size evolves very fast, as the normalization of the size-mass relation increases by 0.5 dex between $z\simeq14$ and $z\simeq6$, in 600 Myr. The SFR surface density increases with redshift, driven by higher sSFRs and smaller sizes at higher redshifts. Size evolution at a fixed stellar mass accelerates at cosmic dawn, driven by an increasing galaxy efficiency at $z\geq5$.