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 imaging in the CANDELS fields to derive both integrated and spatially resolved stellar mass and star-formation rate properties for a mass-complete sample up to $z<4$. By constructing radial profiles of $\Sigma_{*}$, $\Sigma_{\rm SFR}$, and $\mathrm{sSFR}$ from rest-frame $1\mu$m morphologies, the authors find that high-redshift galaxies ($z>2.5$) have mildly negative $\mathrm{sSFR}$ gradients, implying limited size growth from in-situ star formation at cosmic noon. At lower redshift ($z<2$), $\mathrm{sSFR}$ gradients become positive, consistent with inside-out growth and outward disk building. The results indicate a transition in galaxy growth mode around $z\sim2$, with implications for understanding galaxy size evolution, and they provide a large, robust catalog of integrated and resolved properties for future modeling of progenitor–descendant connections.

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 $μ$m morphologies, we have derived spatially resolved stellar mass and SFR surface density ($Σ_*$ and $Σ_{\rm SFR}$) 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 $Σ_{\rm SFR}$ is nearly parallel to but slightly steeper than that of $Σ_*$. 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.

Figures (13)

• Figure 1: The redshift and stellar mass distribution of our total good sample. The region enclosed by the black dashed lines indicates the selected sample used in this study, defined by $0<z<4$ and $\log(M_{\ast}/M_{\odot}) > 8$. The cyan solid curve represents the 90% stellar mass completeness limit corresponding to a magnitude limit of $\rm F444W_{lim}$ = 28 mag. The gray (yellow) histograms in the top and right panels show the redshift and stellar mass distributions of the good sample (selected sample), respectively.
• Figure 2: The fraction of enclosed light as a function of radius for each filter relative to F444W in the JADES-GDS field. The upper and lower panels present the results before and after PSF matching, respectively.
• Figure 3: Example of our spatially resolved SED fitting for a randomly selected galaxy. The left panel shows the F444W-band image, with black dashed lines marking elliptical annuli spaced at intervals of $0.2 R_{\rm e}$. Four representative radial regions are highlighted in red, orange, cyan, and magenta, corresponding to $0<r<0.2R_{\rm e}$, $0.4R_{\rm e}<r<0.6R_{\rm e}$, $1R_{\rm e}<r<1.2R_{\rm e}$, and $2 R_{\rm e}<r<2.2R_{\rm e}$, respectively. The middle and right columns display the SED fitting results for these four radial bins.
• Figure 4: Distribution of galaxy SFR as a function of stellar mass across different redshift bins, with each panel corresponding to different redshift interval. The gray contours enclose 25%, 50%, 75%, and 99% of the galaxy population, respectively. The gray solid line in each panel indicates the sSFR threshold of $1/t_{\rm H}$, where $t_{\rm H}$ is the Hubble time at the median redshift of the bin; galaxies above this threshold are classified as SFGs. Black points with error bars represent the median SFR and the corresponding scatter for SFGs in different stellar mass bins. The black solid line shows the best-fit SFMS assuming the formulation in Equation (\ref{['eq:1']}), while the orange and red solid line corresponds to the best-fit results derived from Equations (\ref{['eq:2']}) and (\ref{['eq:3']}), respectively. For comparison, we include results from previous studies: the cyan dashed line denotes the SFMS from 2014ApJS..214...15S, while the orange and violet dashed lines represent results from 2023MNRAS.519.1526P and 2024AA...691A.164K, respectively. Overall, our derived SFMS is consistent with these previous studies, demonstrating the reliability of our measurements.
• Figure 5: Left panel: The distribution of SFR for SFGs mapped onto the stellar mass versus universe age plane, where the color scale indicates the median SFR within each square bin. Middle panel: The best-fit SFMS derived using a linear relation. Right panel: The residuals between the observed SFRs and the best-fit linear model. The small residuals demonstrate that a linear relation provides a good representation of the SFMS for our sample across the explored stellar mass and cosmic time ranges.