Galaxy size and mass build-up in the first 2 Gyr of cosmic history from multi-wavelength JWST NIRCam imaging

The evolution of galaxy sizes in different wavelengths provides unique insights on galaxy build-up across cosmic epochs. Such measurements can now finally be done at z > 3 thanks to the James Webb Space Telescope’s (JWST) exquisite spatial resolution and multi-wavelength capability. With the public data from the CEERS, PRIMER-UDS, and PRIMER-COSMOS surveys, we measure the sizes of ∼3500 star-forming galaxies at 3 ≤ z < 9, in seven NIRCam bands using the multi-wavelength model fitting code GalfitM. The size–mass relation is measured in four redshift bins, across all NIRCam bands. We find that the slope and intrinsic scatter of the rest-optical size–mass relation are constant across this redshift range and consistent with previous studies at low-z with the Hubble Space Telescope. When comparing the relations across different wavelengths, the average rest-optical and rest-UV relations are consistent with each other up to z = 6, but the intrinsic scatter is largest in rest-UV wavelengths compared to rest-optical and redder bands. This behaviour is independent of redshift and we speculate that it is driven by bursty star formation in z > 4 galaxies. Additionally, for 3 ≤ z < 4 star-forming galaxies at M_∗ > 10¹⁰ M_⊙, we find smaller rest-1 μm sizes in comparison to rest-optical (and rest-UV) sizes, suggestive of colour gradients. When comparing to simulations, we find agreement over M_∗ ≈ 10⁹ − 10¹⁰ M_⊙ but beyond this mass, the observed size–mass relation is significantly steeper. Our results show the power of JWST/NIRCam to provide new constraints on galaxy formation models.