# Enhanced Sub-kpc Scale Star-formation: Results From A JWST Size Analysis of 339 Galaxies At 5<z<14

**Authors:** Takahiro Morishita, Massimo Stiavelli, Ranga-Ram Chary, Michele Trenti, Pietro Bergamini, Marco Chiaberge, Nicha Leethochawalit, Guido Roberts-Borsani, Xuejian Shen, Tommaso Treu  
**arXiv:** [2308.05018](https://arxiv.org/abs/2308.05018)  
**Date:** 2023-08-09  
**Summary page:** https://www.summarizepaper.com/en/paper/2308.05018v1/

## Key points

- Comprehensive search and analysis of high-redshift galaxies using JWST data
- 339 galaxies identified at redshifts ranging from 5 to 14
- Regression analysis reveals a power-law scaling relation between UV size and stellar mass
- Slower rate of average size evolution compared to previous studies, suggesting limited growth in physical sizes over time
- Approximately 13% of the sample consists of marginally resolved sources with sizes smaller than 100 parsecs, exhibiting high star formation surface densities
- No evidence of ongoing supermassive black hole accretion found, but high shock velocities within intense star forming regions could explain observed ratios
- Rest frame UV and optical sizes are comparable, indicating early galaxies building up their structures inside out.

## Summary

In this study, the authors present a comprehensive search and analysis of high-redshift galaxies using data from the James Webb Space Telescope (JWST). They focus on nine public JWST extragalactic fields taken in Cycle 1, covering a total effective search area of approximately 358 square arcminutes. Through conservative photometric selection, they identify 339 galaxies at redshifts ranging from 5 to 14. Of these identified galaxies, 109 have spectroscopic redshift measurements from the literature, including recent observations with JWST's Near-Infrared Spectrograph (NIRSpec). The authors perform a regression analysis and find that the rest-frame UV size-stellar mass relation follows a power-law scaling relation of $R_{\rm eff}\propto M_*^{0.20\pm0.03}$. This is similar to the size-stellar mass relation observed for star-forming galaxies at around redshift 3 but scaled down in size by approximately 0.7 dex. The study also investigates the evolution of galaxy sizes over the redshift range studied. The authors find that there is a much slower rate of average size evolution compared to previous studies, with an evolution rate of $R_{\rm eff}\propto(1+z)^{-0.4\pm0.2}$. This result suggests that high-redshift galaxies may not undergo significant growth in their physical sizes over time. Interestingly, a fraction (approximately 13%) of the sample consists of marginally resolved sources even in the NIRCam imaging, with sizes smaller than 100 parsecs. These compact sources deviate from the derived size-mass slope and exhibit high star formation surface densities ($\Sigma_{\rm SFR}>10\ M_\odot \ {\rm yr^{−1}\ kpc^{−2}}$). Such high density star forming regions are extremely rare in local star forming galaxies, accounting for less than 0.01% of the sample. The authors also investigate the presence of ongoing supermassive black hole accretion in the observed galaxies. For those with available NIRSpec data, no evidence of such accretion is found; however, they suggest that high shock velocities within intense star forming regions characterized by high $\Sigma_{\rm SFR}$ could explain the observed high [OIII]-to-Hbeta ratios. Additionally, they find that rest frame UV and optical sizes are comparable for their sample which is consistent with early galaxies building up their structures inside out and not yet exhibiting strong color gradients seen at lower redshifts.