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

We explore how the fraction of red (quenched) galaxies varies in the dwarf galaxy regime (10^7 MSun < Mstar < 10^9.5 MSun), using a mass-complete sample of ~5900 dwarfs at z<0.15, constructed using deep multi-wavelength data in the COSMOS field. The red fraction decreases steadily until Mstar ~ 10^8.5 MSun 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 Mstar < 10^8 MSun, 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 Mstar > 10^9 MSun, the quenching is driven by a mix of SN feedback and 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 MSun < Mstar < 10^9 MSun, 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 vs stellar mass relation (particularly in the dwarf regime) is a powerful calibrator for the processes that regulate star formation in galaxy formation models.

💡 Research Summary

The authors investigate whether the well‑established “downsizing” trend—where more massive galaxies cease star formation earlier than less massive ones—continues into the dwarf galaxy regime (10⁷ M☉ < M★ < 10⁹·⁵ M☉). Using the deep, multi‑wavelength COSMOS2020 catalogue, they construct a mass‑complete sample of ~5 900 dwarf galaxies at z < 0.15, reaching a stellar‑mass limit of 10⁷ M☉. Stellar masses, photometric redshifts, rest‑frame colours and star‑formation rates are taken directly from the catalogue. For a subset (~1 000 galaxies) covered by JWST, HSC and HST imaging, an expert visually classifies each object into early‑type (ETG), late‑type (LTG) or compact categories, employing both original and unsharp‑masked images to enhance low‑contrast features.

The central observable is the red (quenched) fraction, defined as the proportion of galaxies whose rest‑frame (g – i) colour places them on the red sequence. When plotted as a function of stellar mass, the red fraction exhibits a pronounced “U‑shape”: it declines with increasing mass down to a minimum at M★ ≈ 10⁸·⁵ M☉ (≈ 15 % red), then rises both toward lower masses (≈ 45 % red at 10⁷ M☉) and toward higher masses (≈ 55 % red at 10⁹·⁵ M☉). This behaviour persists across all environmental metrics—high‑density groups, filaments, and low‑density field—demonstrating that external mechanisms (e.g., ram‑pressure stripping, tidal interactions) are not the primary drivers of quenching in this mass range.

The authors interpret the U‑shape in terms of internal feedback processes. In the lowest‑mass dwarfs (M★ ≲ 10⁸ M☉) the shallow gravitational potential allows supernova (SN) feedback to efficiently expel gas, suppressing star formation and producing a high red fraction. In the intermediate mass range (10⁸ – 10⁹ M☉) the potential well deepens enough that SN‑driven outflows become less effective, while central black holes are still too small to generate significant active‑galactic‑nucleus (AGN) feedback. Consequently, quenching is weakest and the red fraction reaches its minimum. At higher masses (M★ ≳ 10⁹ M☉) black‑hole growth and AGN activity become substantial; AGN feedback (thermal heating, kinetic winds) works together with residual SN feedback to again suppress star formation, raising the red fraction.

To test theoretical expectations, the observed red‑fraction curve is compared with predictions from three state‑of‑the‑art cosmological hydrodynamical simulations: Illustris‑TNG, EAGLE, and Horizon‑AGN. While all three reproduce the general trend of increasing red fraction with mass in the high‑mass regime, none capture the detailed U‑shape nor the precise transition mass (~10⁸·⁵ M☉). The mismatch points to uncertainties in how low‑mass SN feedback is implemented (energy coupling, mass loading) and how early black‑hole seeds and AGN feedback are modeled.

The paper therefore proposes that the red‑fraction versus stellar‑mass relation, especially within the dwarf regime, serves as a powerful calibration tool for galaxy‑formation models. By tuning SN feedback efficiencies, black‑hole seeding prescriptions, and AGN feedback scaling to reproduce the observed U‑shape, simulations can achieve a more realistic representation of star‑formation regulation across the full mass spectrum.

In summary, this work overturns the assumption that downsizing proceeds uninterrupted into dwarf galaxies. It demonstrates that internal feedback mechanisms dominate quenching, with a clear mass‑dependent transition from SN‑driven quenching at the lowest masses to a mixed SN+AGN regime at higher masses. The findings provide a stringent empirical benchmark for future theoretical work and underscore the importance of deep, wide‑area surveys (e.g., LSST, Euclid, Roman) for probing dwarf galaxy evolution beyond the local Universe.