Downsizing does not extend to dwarf galaxies: identifying the stellar mass regimes shaped by supernova and AGN feedback

ABSTRACT We explore how the fraction of red (quenched) galaxies varies in the dwarf galaxy regime (10$^{7}$ M$_{\odot }$ < $M_{\star }$ < 10$^{9.5}$ M$_{\odot }$), using a mass-complete sample of $\sim$5900 dwarfs at $z\lt 0.15$, constructed using deep multi-wavelength data in the COSMOS field. The red fraction decreases steadily until $M_{\star }$$\sim$ 10$^{8.5}$ M$_{\odot }$ and then increases again towards lower stellar masses. This ‘U’ shape demonstrates that the traditional notion of ‘downsizing’ (i.e. that progressively lower-mass galaxies maintain star formation until later epochs) is incorrect-downsizing does not continue uninterrupted into the dwarf regime. The U shape persists regardless of environment, indicating that it is driven by internal processes rather than external environment-driven mechanisms. Our results suggest that, at $M_{\star }$$\lesssim$ 10$^8$ M$_{\odot }$, the quenching of star formation is dominated by supernova (SN) feedback and becomes more effective with decreasing stellar mass, as the potential well becomes shallower. At $M_{\star }$$\gtrsim$ 10$^9$ M$_{\odot }$, the quenching is driven by a mix of SN feedback and active galactic nucleus (AGN) feedback (which becomes more effective with increasing stellar mass, as central black holes become more massive). The processes that quench star formation are least effective in the range 10$^{8}$ M$_{\odot }$ < $M_{\star }$ < 10$^{9}$ M$_{\odot }$, likely because the potential well is deep enough to weaken the impact of SN feedback, while the effect of AGN feedback is still insignificant. The cosmological simulations tested here do not match the details of how the red fraction varies as a function of stellar mass. We propose that the red fraction versus stellar mass relation (particularly in the dwarf regime) is a powerful calibrator for the processes that regulate star formation in galaxy formation models.