Thermostats, Not Engines: A New Picture of Halo Gas Regulation

TL;DR

This paper presents a new model where black hole feedback regulates halo gas via an entropy ceiling, influencing galaxy evolution and star formation activity.

Contribution

It introduces a novel entropy ceiling mechanism for gas regulation in halos, supported by simulations and observational evidence.

Findings

Entropy ceiling regulates gas in halos, preventing further cooling.

Above a critical halo mass, virial shocks override the entropy ceiling.

Rejuvenation of star formation occurs in the most massive galaxies.

Abstract

We propose that black hole feedback regulates gas in massive halos by establishing an entropy ceiling; the resulting buoyant gas migrates to the virial radius with no additional energy input required. The FLAMINGO simulations support this picture: at the virial radius, outflow entropy is mass-independent for isotropic thermal feedback but depends on the solid angle of directly heated gas for jet feedback. Above a critical halo mass $M_\rm{crit} \approx 10^{13.5\text{--}14}\, M_\odot$, virial shocks overwhelm the ceiling, predicting rejuvenation of star formation in the most massive galaxies, supported by new low-redshift evidence from star formation rates and morphologies.