Transition from Outside-in to Inside-Out at $z\sim 2$: Evidence from Radial Profiles of Specific Star Formation Rate based on JWST/HST

TL;DR

This study uses JWST and HST data to analyze galaxy star formation profiles across redshifts, revealing a transition from outside-in to inside-out growth around z~2.

Contribution

It provides the first large-scale spatially resolved sSFR profiles across a broad redshift range, challenging existing models of galaxy size evolution.

Findings

At z>2.5, sSFR profiles are mildly negative, indicating limited size growth via in-situ star formation.

At z<2, sSFR profiles become positive, supporting inside-out galaxy growth.

Galaxies at z>2.5 cannot grow in size solely through in-situ star formation.

Abstract

By combining high-resolution observations from JWST and HST, we have measured the stellar masses, star formation rates (SFRs), and multi-wavelength morphologies of galaxies in the CANDELS fields. Furthermore, based on rest-frame 1 $\mu$m morphologies, we have derived spatially resolved stellar mass and SFR surface density ($\Sigma_*$ and $\Sigma_{\rm SFR}$) profiles for 46,313 galaxies with reliable structural measurements at $0<z<4$ and $\log(M_\ast /M_{\odot})>8$, and provide the corresponding catalogue. For star-forming galaxies (SFGs), our results show excellent consistency with previous studies in terms of the star formation main sequence and the size-mass relation, demonstrating the robustness of our stellar mass and SFR measurements. For spatially resolved profiles, we find that at higher redshifts ($z>2.5$), the median radial profile of $\Sigma_{\rm SFR}$ is nearly parallel to but slightly steeper than that of $\Sigma_*$. This results in mildly negative gradients in the specific SFR (sSFR) profiles across all stellar mass bins considered. These findings indicate that galaxies at $z>2.5$ cannot grow in size via only in-situ star formation, challenging the understanding of galaxy size evolution beyond the cosmic noon. In contrast, at $z<2.0$, the sSFR profiles transition to exhibit more and more positive gradients at lower redshifts, consistent with an inside-out growth scenario where star formation preferentially expands the galactic outskirts.