The Galaxy Stellar Mass-SFR-Size Relation in EAGLE, TNG100, and Observations

Abstract

Stellar mass, size, and star formation rate (SFR) are fundamental properties that encode the structural and evolutionary states of galaxies. Observations reveal a mass-SFR-size relation whereby galaxies become more compact both above and below the ridge of the star-forming main sequence (SFMS), linking galaxy structure to star formation activity. We investigate this relation by comparing galaxies from two cosmological hydrodynamical simulations, EAGLE and TNG100, with observational samples from SDSS and CANDELS over three redshift intervals (0 < z < 0.2, 0.5 < z < 1.5, and 1.5 < z < 2.5). Both simulations reproduce the observed trend that galaxy sizes decrease with increasing offset away from the SFMS. This trend, however, weakens and is not detected in the observational sample at 1.5 < z < 2.5, likely due to increased measurement uncertainties. In contrast, the trend persists in both simulations up to z = 2.5. Across all redshifts, EAGLE predicts a stronger size dependence on SFMS offset than observed, whereas TNG100 exhibits a weaker dependence. We discuss how this mass-SFR-size relation can be understood in terms of different time variability in star formation rate across the SFMS.

Figures (4)

• Figure 1: Mass–size–SFR relation of SDSS galaxies. The left panel shows central galaxies (296,557), and the right panel shows the full sample (416,550). Contours correspond to the 10th, 30th, 50th, 70th, and 90th percentiles of the joint distribution of stellar mass and star formation rate (SFR). The black line denotes the star-forming main sequence. The color scale represents the median galaxy size of galaxies within each pixel. For clarity, the color field is smoothed using the gaussian_filter function from the Scipy package 2020SciPy-NMeth. The central galaxy sample is defined following YangXiaohu2007.
• Figure 2: Mass–size–SFR relation of CANDELS galaxies at $0.5<z<1.5$ (left, 40,209 galaxies) and $1.5<z<2.5$ (right, 37,842 galaxies), following the same plotting style as Fig. \ref{['fig:SDSS']}.
• Figure 3: Relative galaxy size, $\Delta \log{\rm SFR}$, as a function of galaxy SFR and stellar mass: panels are arranged from top to bottom in order of increasing redshift, and from left to right for observations, EAGLE, and TNG100, as indicated in the legend, same plotting style as in Fig.,\ref{['fig:SDSS']}. Note that only galaxies with stellar masses greater than $10^{9.7},\rm M_{\odot}$ are included in this comparison.
• Figure 4: Relative galaxy size, $\Delta \log R_{\rm e}$, as a function of star formation rate offset, $\Delta \log \mathrm{SFR}$ (i.e., deviation from the ridge of SFMS). Panels are arranged from top to bottom in order of increasing redshift, and from left to right for the observational sample, EAGLE, and TNG100, as indicated in the legend. In each panel, the black solid line shows the median $\Delta \log R_{\rm e}$ as a function of $\Delta \log \mathrm{SFR}$, while the gray shaded region encloses the 16th--84th percentiles. The blue, red, and green lines represent the median relations for low-mass, intermediate-mass, and massive galaxies, respectively. Histograms above each panel show the distribution of $\Delta \log \mathrm{SFR}$.