First-hand information for everyone
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.
The paper interprets the bend as evidence for two distinct size-growth modes. Low-mass galaxies grow in step with their halos. Above the critical mass, many star-forming galaxies appear to “decouple” from halo-driven growth and become compact. The authors link that compaction to rapid central (bulge) growth and likely black hole growth in halos of roughly 10^12 M☉ or more. These compact, massive star-forming galaxies share masses, light profiles, shapes, and number densities with massive quiescent galaxies, so they are promising immediate progenitors of quenched systems. The authors suggest that this “compaction pathway,” not major mergers, dominates the formation of massive quiescent galaxies at z ≳ 2.
There are important caveats. Sizes, masses, and distances here depend on multi-band photometry and spectral-energy-distribution fitting, so systematic errors in those fits can affect the results. The authors note that including MIRI photometry improves mass and redshift estimates and that, when MIRI is missing for massive galaxies at z > 5, they apply corrections (reducing masses by ~0.04 dex at 5.0 < z < 5.5 and ~0.18 dex at 5.5 < z < 6.0). They also use a specific criterion based on recent specific star-formation rate to separate star-forming and quiescent galaxies, and they checked that their main conclusions hold under alternative color-based classifications. Finally, while the PRIMER JWST data greatly reduce previous observational limits, the authors acknowledge prior studies showed mixed results because of small samples and measurement uncertainties. These new JWST-based findings strengthen the case for a physical mass scale in galaxy evolution, but further work will be needed to test the detailed physical mechanisms behind the compaction pathway.