The Effects of AGN Feedback on Galaxy Formation

Feedback from supermassive black holes is thought to play an important role in shaping why galaxies above the Milky Way’s size split into two main color groups. Many galaxy formation simulations include active galactic nuclei (AGN) feedback, but often in a simplified way or with limited detail about how gas and stars behave inside galaxies. In this work, we use the FIRE cosmological zoom-in simulations, which include AGN feedback and evolve galaxies down to the present day (z ∼ 0). These simulations combine detailed models of star formation and the interstellar medium with different AGN feedback channels: radiation, powerful outflows, and in some cases, cosmic rays.

We find that massive galaxies in these simulations reproduce several observed relationships, such as the stellar mass–halo mass trend and the black hole mass–velocity dispersion relation. In the stronger model that includes cosmic rays, galaxies also match observed size–mass and Faber–Jackson relations. Galaxies in the AGN feedback runs often stop forming stars and develop elliptical shapes, similar to observed massive galaxies. By contrast, runs without AGN feedback make galaxies that are far too massive, produce too many new stars, are too compact, and spin too fast compared to observations. However, the AGN feedback models are not perfect: the stronger model works best for massive galaxies but shuts down star formation too strongly in lower-mass systems. This shows that more work is needed to refine AGN feedback models.