Testing the Breathing Mode in Intermediate-mass Galaxies and Its Predicted Star Formation Rate-size Anti-correlation

Recent hydrodynamical simulations predict that stellar feedback in intermediate-mass galaxies (IMGs) can drive strong fluctuations in structure (e.g., half-light radius, Re). This process operates on timescales of only a few hundred Myr and persists even at late cosmic times. One prediction of this quasi-periodic, galactic-scale "breathing" is an anti-correlation between star formation rate (SFR) and Re as central gas overdensities lead to starbursts whose feedback drags stars to larger radii while star formation dwindles. We test this prediction with a sample of 284 isolated IMGs with stellar masses of ${10}^{9.0}\leqslant M/{M}_{\odot }\leqslant {10}^{9.5}$ at $0.3\lt z\lt 0.4$ in the Hubble Space Telescope (HST) I814 Cosmological Evolution Survey (COSMOS) footprint. We find that IMGs with higher specific SFRs (SSFR > 10−10 yr−1) are the most extended with median sizes of Re ∼ 2.8–3.4 kpc and are mostly disk-dominated systems. In contrast, IMGs with lower SSFRs are a factor of ∼2–3 more compact with median sizes of Re ∼ 0.9–1.3 kpc and have more significant bulge contributions to their light. These observed trends are opposite to the predictions for stellar feedback that operate via the "breathing" process described above. We discuss various paths to reconcile the observations and simulations, all of which likely require a different implementation of stellar feedback in IMGs that drastically changes their predicted formation history.