Galaxy sizes and compactness at Cosmic Dawn

Context. The James Webb Space Telescope has found an unexpected population of high-mass galaxies (log( M ★ /M ⊙ )≳10) with extremely small effective radii (∼100 pc) at z ≳ 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. Aims. We aim to understand the physical mechanisms governing the size evolution of galaxies, and its dependence on their properties in the early Universe. We expect to unveil the formation channels of the observed compact galaxies. Methods. We analysed 7605 snapshots for 169 galaxies of the state-of-the-art cosmological simulation suite F IRST L IGHT , focusing on the high-redshift stellar size–mass relation and its evolution with a resolution of tens of parsecs. Results. We find that galaxies undergo an expansion–compaction–re-expansion process. The sizes attained by galaxies during compaction are comparable with those observed. This process operates in a specific mass range; compaction starts at log M ★ on /M ⊙ ∼ 8.5 and ends at log M ★ off /M ⊙ ∼ 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, which triggers a strong, localised starburst at the centre (within 1 kpc). This contraction continues until conditions favour star formation in a broader area, and the normal inside-out growth pattern resumes. Conclusions. 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.