A bending in the size-mass relation of star-forming galaxies across $0.5 < z < 6.0$ at a critical stellar mass of $10^{10}M_\odot$ revealed by JWST

Abstract

Galaxy size provides key insights into the physical processes driving galaxy formation and evolution. Using deep JWST/NIRCam and MIRI imaging from the PRIMER survey, we investigate the rest-frame optical size-stellar mass relation of galaxies at $0.5 < z < 6.0$. We find that star-forming galaxies (SFGs) exhibit a broken power-law size-mass relation at all redshifts, with a nearly constant pivot mass ($M_{\rm p}$) of $\sim 10^{10} M_\odot$, and a slope flattening above $M_{\rm p}$. This highlights the prevalence of a population of compact, massive SFGs, likely underrepresented in previous studies. The size distribution of quiescent galaxies (QGs) is well described by a mixture power-law model, with the pivot mass increasing with redshift, from $M_{\rm p} \sim 10^{10.0} M_\odot$ at $z =0.75$ to $M_{\rm p} \sim 10^{10.5} M_\odot$ at $z = 2.6$, suggesting the minimum halo mass required to quench a high-mass QG increases with redshifts. The bending in the size-mass relation of SFGs supports two distinct size growth modes. At $M_{\star} < M_{\rm p}$, galaxy size growth is closely coupled to halo growth, while at $M_{\star} > M_{\rm p}$, an increasing fraction of SFGs decouple from halo growth and become more compact, likely associated with rapid bulge (and black hole) growth in $M_{\rm h} \gtrsim 10^{12} M_{\odot}$ halos. These compact SFGs are promising progenitors of massive QGs, as evidenced by their similar masses, surface brightness profiles, morphologies and number densities. These results suggests that the compaction pathway, rather than major mergers of extended SFGs, dominates the formation of massive QGs at $z \gtrsim 2$.

Figures (13)

• Figure 1: Rest-frame $U - V$ versus $V - J$ color--color diagrams for galaxies in the redshift range $0.5 < z < 6.0$, shown in six redshift bins. Galaxies are color-coded by their specific star formation rates (sSFRs). The red dashed lines denote the UVJ selection criteria from Williams2009ApJ (see Appendix \ref{['appendix:uvj']}).
• Figure 2: Examples of galight fitting results. In each row, the images show original data, model, normalized residuals, and 1D surface brightness profile from left to right. The upper panel shows one galaxy fitted by both a single Sérsic model and a point source model, with $z = 2.2$, $M_{\star} = 10^{10.5} M_{\odot}$ and $R_{e} = 2.7 \, \text{kpc}$. The lower panel shows another galaxy fitted by only a single Sérsic model, with $z = 3.5$, $M_{\star} = 10^{10.6} M_{\odot}$ and $R_{e} = 0.9 \, \text{kpc}$.
• Figure 3: Size--mass relations fitted by a single power-law model for SFGs and high-mass QGs ($M_{\star} > 10^{10.3}M_\odot$). Blue and red contours show the KDE-derived density distributions of SFGs and QGs, with five levels marking the 10th, 30th, 50th, 70th and 90th percentiles. Individual outliers are plotted as blue and red points. Green squares and yellow diamonds indicate median sizes in 0.5-dex mass bins for SFGs and QGs, respectively. Solid lines show the best-fit relations, and dotted lines reproduce the result at $0.5 < z < 1.0$ for reference. The vertical dashed line marks the mass completeness limit. The bands used for size measurements are labeled in each panel. Specifically, F277W band are adopted for $4.0 < z < 5.0$ galaxies and F356W band for $5.0 < z < 6.0$ galaxies. Blue and red numbers in the right-top shows the number of SFGs and QGs included in each redshift bin. Black stars show the medians of SFGs from 2014ApJ...788...28V. Insets show the histogram of $\Delta \log R$ (fraction) for SFGs ($M_{\star} > 10^{10.3}M_\odot$) with blue KDE overlaid. The black dashed curve shows a KDE mirrored from the right-hand side of the peak to the left, providing a visual reference for asymmetry. The last panel highlights an example galaxy to illustrate the definition of $\Delta \log R$, where $\Delta \log R$ is defined as the vertical offset from the best-fit size–mass relation fitted by single power-law model. The last panel highlights an example galaxy to illustrate the definition of $\Delta \log R$.
• Figure 4: Size--mass relations fitted by a broken power-law model for SFGs at $0.5 < z < 6.0$ and a mixture model for QGs at $0.5 < z < 4.0$ (same symbols as in Figure \ref{['fig:single']}). At $z > 4.0$, the lack of low-mass QGs makes the mixture model fit unconstrained and therefore not applicable. Grey dashed lines represent the relations fitted by single power-law shown in Figure \ref{['fig:single']} for comparison.
• Figure 5: Redshift evolution of the size-mass relations and associated parameters for SFGs at $1.0 < z < 6.0$. Left: Evolution of the size-mass relations for SFGs. The solid lines represent the best-fit relations using a broken power-law model, with pivots indicated by purple dots. Middle: Evolution of the parameters from power-law fits, including $k$ (red triangle), $\alpha$ (blue dots) and $\beta$ (green squares) from broken power-law fits. Right: Evolution of the parameters from power-law fits in log scale. The top panel shows the intercept (magenta dots) from single power-law model fits, while the bottom panel shows the pivot mass (purple dots) and pivot $R_e$ (orange dots) from broken power-law model fits. The best-fit single power-law parameters from 2014ApJ...788...28V (black stars) and 2024ApJ...962..176W (black inverted triangles) are also shown for comparison.