This paper reports that star-forming galaxies show a clear change, or “bend,” in the relationship between their physical size and their stellar mass across the range 0.5 < z < 6.0 (where higher z means looking farther back in time). In simple terms, galaxies smaller than about 10^10 solar masses grow in size in one way, while more massive star-forming galaxies tend to stop following that trend and become comparatively compact. The result comes from deep imaging with the James Webb Space Telescope (JWST) and points to a common mass scale where galaxy growth changes behavior.

The team used imaging from the PRIMER survey taken with JWST’s Near Infrared Camera (NIRCam) and Mid-Infrared Instrument (MIRI). They combined those data with other public JWST, Hubble, and ground-based observations to improve distances and mass estimates. Photometric redshifts (estimates of distance from many-band brightness measurements) were calculated with EAzY, and stellar masses and rest-frame colors were fitted with BAGPIPES. Galaxy sizes were measured by fitting each galaxy’s light profile with a standard single-component model and reporting the effective radius, Re, which encloses half the model light. The dataset is deep enough to be nearly complete down to stellar masses of about 10^7.9 solar masses at z = 0.5 and 10^8.9 at z = 5.0.

Their main empirical finding is that star-forming galaxies (SFGs) are better described by a broken power-law relation between size and mass, with a nearly constant pivot mass Mp ≈ 10^10 M☉ (ten billion Suns). Below Mp, galaxy size increases steadily with mass in a way that appears tied to the growth of the surrounding dark matter halo. Above Mp, the slope flattens: an increasing fraction of massive SFGs are more compact than expected. For quiescent galaxies (QGs) — those with very low recent star formation — the authors model sizes with a mixture power-law and find the pivot mass for quiescent systems rises with redshift, from about 10^10 M☉ at z ≈ 0.75 to about 10^10.5 M☉ at z ≈ 2.6.