Enhanced Subkiloparsec-scale Star Formation: Results from a JWST Size Analysis of 341 Galaxies at 5 < z < 14

Abstract We present a comprehensive search and analysis of high-redshift galaxies in a suite of nine public JWST extragalactic fields taken in Cycle 1, covering a total effective search area of ∼ 358 arcmin 2 . Through conservative (8 σ ) photometric selection, we identify 341 galaxies at 5 R eff ∝ M * 0.19 ± 0.03 , similar to that of star-forming galaxies at z ∼ 3, but scaled down in size by ∼0.7 dex. We find a much slower rate for the average size evolution over the redshift range, R eff ∝ (1 + z ) −0.4±0.2 , than that derived in the literature. A fraction (∼13%) of our sample galaxies are marginally resolved even in the NIRCam imaging (≲100 pc), located at ≳1.5 σ below the derived size–mass slope. These compact sources exhibit a high star formation surface density Σ SFR > 10 M ⊙ yr −1 kpc −2 , a range in which only <0.01% of the local star-forming galaxy sample is found. For those with available NIRSpec data, no evidence of ongoing supermassive black hole accretion is observed. A potential explanation for the observed high O iii -to-H β ratios could be high shock velocities, likely originating within intense star-forming regions characterized by high Σ SFR . Lastly, we find that the rest-frame UV and optical sizes of our sample are comparable. Our results are consistent with these early galaxies building up their structures inside out and being yet to exhibit the strong color gradient seen at lower redshift.