Galaxy sizes and compactness at Cosmic Dawn

TL;DR

The paper investigates the origin of ultra-compact, high-mass galaxies at Cosmic Dawn by analyzing 7605 snapshots for 169 galaxies from the FirstLight simulation suite at $z \sim 5.25-9$ with ~10 pc resolution. It identifies an expansion–compaction–re-expansion (ECE) cycle with a stellar mass window $\log (M^\star_\mathrm{on}/M_\odot) \approx 8.53$ to $\log (M^\star_\mathrm{off}/M_\odot) \approx 9.57$, driven by a self-reinforced inflow of cold gas triggering a central starburst within $\lesssim 1$ kpc and producing an inverted size–mass relation in the intermediate mass range. The mechanism is linked to a transition from dark-matter to baryon-dominated central regions, described by a Bondi–Hoyle–like accretion model, and is argued to be a universal wet compaction at Cosmic Dawn, with an analytical infall expression provided for semi-analytic models. The results imply that high-redshift compaction does not quench star formation, contrasting with some low-redshift scenarios, and offer insights into bulge formation and early galaxy evolution, while noting limitations such as the absence of AGN feedback and the need to account for observational biases.

Abstract

The James Webb Space Telescope has found an unexpected population of high-mass galaxies ($\log (M^\star / {\rm M} _\odot) \gtrsim 10$) with extremely small effective radii ($\sim 100\,\rm pc$) at $z \gtrsim 6$. Also, the existence of an unusual size--mass relation has been claimed. These observations are only partially reproduced by current models, and the physics responsible for the observed relations is still under debate. We expect to unveil the formation channels of the observed compact galaxies. We analyse 7605 snapshots for 169 galaxies of the state-of-the-art cosmological simulation suite FirstLight. We find that galaxies undergo an expansion--compaction--re-expansion process. This process operates in a specific mass range; compaction starts at $\log (M^\star_ {\rm on} / {\rm M}_\odot) \sim 8.5$ and ends at $\log (M^\star_{\rm off} / \mathrm{M}_\odot) \sim 9.5$. In between these masses, the size--mass relation becomes inverted, with a negative slope. The physical mechanism driving this process in our simulations involves a self-reinforced inflow of gas from the outer regions, that triggers a strong, localised starburst at the centre (within $1\, {\rm kpc}$). This contraction continues until conditions favour star formation in a broader area, and the normal inside-out growth pattern resumes. We present evidence for the existence of a universal wet compaction operating at Cosmic Dawn. This mechanism is driven by spherical accretion triggered by the change of the state of the central matter of galaxies, from dark matter- to baryon-dominated. We also propose an analytical expression for the infall process, suitable for use in semi-analytic models. Contrary to low-redshift galaxies, in high-redshift systems compaction ends without gas depletion and star-formation quenching.

Figures (14)

• Figure 1: Distribution of the half-mass stellar radius (left panel) and the stellar-to-halo size ratio (right panel), for $5.25 < z < 9$ (colour coded). The dashed vertical line indicates the median value of $\xi^\star$ over the full sample and redshift range ($\left< \xi^\star \right> = 0.07$). A growing subpopulation of compact galaxies is seen; the dotted vertical line at $\log \xi^\star = -1.6$ roughly illustrates the separation.
• Figure 2: Relation between the stellar mass of galaxies and the virial mass of their haloes, compared to those predicted by semi-empirical models (top panel) and the Cosmos-Web survey subproducts (bottom panel). The shaded regions represent the dispersion of the models.
• Figure 3: Different behaviours observed in the evolution of the sizes of individual galaxies: always expanding objects (top row), those reverting expansion into compaction (second row), systems reverting compaction into expansion (third row), and galaxies showing the complete ECE process (bottom row). Dashed and dot-dashed lines mark the masses at which galaxies attain their maximum and minimum sizes, respectively. Each panel represents a galaxy, labelled by its simulation ID.
• Figure 4: Distribution of the masses at which galaxies attain their peak (left panel) and minimum (right panel) sizes. The dashed and dot-dashed lines represent the geometric mean values $M^\star_\mathrm{on}$ and $M^\star_\mathrm{off}$, respectively.
• Figure 5: Dependence of the stellar size on stellar mass for different redshifts and galaxy subsamples (indicated with different colours, SS1: red, SS2: yellow, SS3: green, SS4: blue). The dashed vertical line indicates the value of the turn-on stellar mass, $M^\star_\mathrm{on} = 10^{8.53}\,\mathrm{M}_\odot$.