Suppressing cooling flows in massive galaxies with cosmic ray injection and turbulent stirring

The quenching "maintenance" and related "cooling flow" problems are important in galaxies from Milky Way mass through clusters. We investigate this in halos with masses ~1e12-1e14 solar mass, using non-cosmological high-resolution hydrodynamic simulations with the FIRE-2 (Feedback In Realistic Environments) stellar feedback model. We first focus on physics present without AGN, and show that various proposed "non-AGN" solution mechanisms in the literature, including Type Ia supernovae, shocked AGB winds, other forms of stellar feedback (e.g. cosmic rays from supernovae), magnetic fields, Spitzer-Braginskii conduction, or "morphological quenching" do not halt or substantially reduce cooling flows nor maintain "quenched" galaxies in this mass range.  This all supports the idea that additional physics, e.g., AGN feedback, must be important in massive galaxies.

We then test various AGN feedback toy models with different forms of energy input and ranges of coupling, exploring what scenario can possibly quench galaxy and suppress the cooling flows without resulting in halo properties that contradict  observations. The test scenarios include momentum injection, turbulent stirring, thermal heating, and cosmic ray injection. We found that turbulent stirring confined within 100 kpc  and cosmic ray injection can both maintain a stable, low-SFR halo for extended periods of time, because they provide non-thermal pressure form which can stably lower the core density and cooling rate. This can be much more efficient  than heating up the gas thermally.  We conclude that the enhancements of turbulence and cosmic ray energy are very important aspects of AGN feedback, and can be the dominant processes that quench the massive ellipticals.

Speaker: Kung-Yi Su, Caltech