Modelling the expulsion of baryons from haloes: the role of feedback and of the cosmological constant

Abstract

The extent to which galactic-scale astrophysical processes conspire with the underlying cosmological model to expel baryons from haloes remains a central question in galaxy formation. We present an analytical model for the gas distribution within and beyond haloes, based on the balance between gravitational collapse, hydrostatic pressure, and cosmic expansion. Our model predicts the halo-centric distance enclosing a baryon mass fraction equal to the cosmic value $f_{\rm b} = \Omega_{\rm b}/\Omega_{\rm m}$ (`closure radius') in an arbitrary $\Lambda$CDM cosmology. We compare the predictions with the results of six variants of the EAGLE cosmological, hydrodynamical simulation, encompassing values of the cosmological constant ranging from 0 to 100 times its observed value in our Universe, $\Lambda_0$. Despite its simplicity, our model exhibits excellent agreement with the simulations for haloes with mass $M_{\rm 200c} > 10^{11} M_\odot$ in the redshift range $0<z<3$, suggesting that it captures the key astrophysical processes and highlighting its robustness to the cosmological parameters. Thus, it provides the first physical explanation for the empirical closure radius--halo mass relation previously observed in simulations. Furthermore, we find that dark energy plays a non-negligible role in baryon evacuation: the simulations reveal that in the fiducial cosmological model, the closure radius at $z<2$ is $\sim 30\%$ larger than in an Einstein-de Sitter universe. In cosmologies with $\Lambda \geq 10 \Lambda_0$, dark energy emerges as the dominant factor in this process -- suggesting that, as our Universe transitions towards $\Lambda$-domination, dark energy eventually becomes the primary driver of baryon evacuation from massive haloes.