Non-monotonic relations of galaxy star formation, radius, and structure at fixed stellar mass

Abstract

We investigate the relation between galaxy structure and star formation rate (SFR) in a sample of ∼ 2.9 × 10⁴ central galaxieswith z < 0.0674 and axial ratios b/a > 0.5. The star-forming main sequence (SFMS) shows a bend around the stellar mass of M∗ ≤ M_c = 2 × 10¹⁰M. At M∗ ≤ M_c, the SFMS follows a power-law SFR ∝ M^0.85∗ , while at higher masses it flattens. M_ccorresponds to a dark matter halo mass of Mvir ∼ 10^11.8M where virial shocks occurs. Some galaxy structure (e.g. half-lightradius, R_e) exhibits a non-monotonic dependence across the SFMS at a fixed M∗. We find SFR ∝ R^−0.28 e at fixed M∗, consistentwith the global Kennicutt–Schmidt (KS) law. This finding suggests that galaxy sizes contribute to the scatter of the SFMS.However, at M∗ > M_c the relationship between SFR and R_e diminishes. Low-mass galaxies above the mean of the SFMS havesmaller radii, exhibit compact and centrally concentrated profiles resembling green valley (GV) and quiescent galaxies at thesame mass, and have higher M_H2 /M_{H I}. Conversely, those below the SFMS exhibit larger radii, lower densities, have no GV orquiescent counterparts at their mass and have lower M_H2 /M_{H I}. The above data suggest two pathways for quenching low-massgalaxies, M∗ ≤ M_c: a fast one that changes the morphology on the SFMS and a slow one that does not. Above M_c, galaxiesbelow the SFMS resemble GV and quiescent galaxies structurally, implying that they undergo a structural transformation alreadywithin the SFMS. For these massive galaxies, CG are strongly bimodal, with SFMS galaxies exhibiting negative colour gradients,suggesting most star formation occurs in their outskirts, maintaining them within the SFMS.

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