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

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 $z\geq5$.

Figures (11)

• Figure 1: Example of an extended, high-mass disk at $z=6$ (FL961). Top panels show a short-wavelength band (F115W) and bottom panels show a longer wavelength (F356W). Left panels represent the transparent case. Middle panels include absorption and scattering by dust. Right panels add instrumental effects. An effective radius of $R_{\rm e} \simeq1.5 \,{\rm kpc}$ (horizontal bar) is computed using the area that contains half of the luminosity of a region inside a given isophote (green contour). Each image shows a face-on view with a side length of 10 kpc. Color bars show surface brightness in ${\rm MJy \ sr}^{-1}$.
• Figure 2: Example of a compact, high-mass disk at $z=6$ (FL964) with an effective radius of $R_{\rm e} \simeq0.7 \,{\rm kpc}$ in the rest-frame UV (F115W). Each image shows an edge-on view with a side length of 10 kpc. Labels are defined as in Figure \ref{['fig:example1']}.
• Figure 3: Three examples of low-mass galaxies with $M_*\simeq10^9 \ {\rm M}_\odot$ at $z=6$ in the F356W band with a compact (left), average (center), and extended (right) morphology. Each image has a side length of 10 kpc. Labels are defined as in Figure \ref{['fig:example1']}.
• Figure 4: Two examples of low-mass galaxies with $M_*\simeq10^9 \ {\rm M}_\odot$ at $z=10$ in the F200W band. Top panels show a relatively extended galaxy with $R_{\rm e} \simeq 0.5 \,{\rm kpc}$, undergoing a merger. Bottom panels show a compact counterpart with $R_{\rm e} \simeq 0.16 \,{\rm kpc}$, similar to observed galaxies like JADES-GS-z14-0 Carniani24. Each image has a side length of 10 kpc. Labels are defined as in Figure \ref{['fig:example1']}.
• Figure 5: Size-Mass relation in the rest-frame UV at different redshifts. Points are coloured by the galaxy specific star-formation-rate (sSFR). Extended (compact) galaxies tend to have higher (lower) than average sSFR. The lower limit in effective radius is set by the pixel size or the PSF. Lines represent a fit to these points above $M_*>5 \times 10^8 \ {\rm M}_\odot$ (Table \ref{['tab:fit']}). Dash lines mark extrapolations above the sample. The slope of these lines is consistent with observations. Grey regions Morishita24 and points Morishita24Naidu25Donnan26 correspond to JWST results.