Transition from outside-in to inside-out at z  ∼  2: Evidence from radial profiles of the specific star formation rate based on JWST/HST

¹ Department of Astronomy, University of Science and Technology of China, Hefei 230026, China
² School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, China
³ Institute of Deep Space Sciences, Deep Space Exploration Laboratory, Hefei 230026, China
⁴ School of Physics and Astronomy, Anqing Normal University, Anqing 246133, China
⁵ Institute of Astronomy and Astrophysics, Anqing Normal University, Anqing 246133, China
⁶ Key Laboratory of Modern Astronomy and Astrophysics, Nanjing University, Ministry of Education, Nanjing, Jiangsu 210093, China
⁷ School of Astronomy and Space Science, Nanjing University, Nanjing, Jiangsu 210093, China
⁸ Department of Astronomy, Tsinghua University, Beijing 100084, China

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Received: 12 January 2026
Accepted: 30 March 2026

Abstract

By combining high-resolution observations from JWST and HST, we measured the stellar masses, star formation rates (SFRs), and multiwavelength morphologies of galaxies in the CANDELS fields. Furthermore, based on rest-frame 1 μm morphologies, we derived the spatially resolved stellar mass and SFR surface density (Σ_* and Σ_SFR) profiles for 46,313 galaxies with reliable structural measurements at 0 < z < 4 and log(M_*/M_⊙) > 8, and we provide the corresponding catalog. For star-forming galaxies, 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 Σ_SFR is nearly parallel to but slightly steeper than that of Σ_*. This results in mildly negative gradients in the specific SFR (sSFR) profiles in all stellar mass bins we considered. These findings indicate that galaxies at z > 2.5 cannot grow in size via in situ star formation alone, challenging the understanding of galaxy size evolution beyond the cosmic noon. In contrast, at z < 2.0, the sSFR profiles transition to exhibit increasingly more positive gradients at lower redshifts, consistent with an inside-out growth scenario in which star formation preferentially expands the galactic outskirts.