Jet Feedback and the Photon Underproduction Crisis in Simba

TL;DR

This paper investigates how black hole jet feedback influences the intergalactic medium in the SIMBA simulation, addressing the Photon Underproduction Crisis by showing jets reduce diffuse baryons and align simulations with observations.

Contribution

It demonstrates that jet feedback significantly alleviates the Photon Underproduction Crisis in the SIMBA simulation, improving agreement with observed Lyα forest data.

Findings

Jet feedback reduces the diffuse IGM baryon fraction from 39% to 16% at z=0.

Jet feedback increases the WHIM baryon fraction from 30% to 70%.

SIMBA matches observed D_A better than MUFASA, highlighting feedback's role.

Abstract

We examine the impact of black hole jet feedback on the properties of the low-redshift intergalactic medium (IGM) in the SIMBA simulation, with a focus on the Ly$\alpha$ forest mean flux decrement $D_A$. Without jet feedback, we confirm the Photon Underproduction Crisis (PUC) in which $\Gamma_{\rm HI}$ at $z=0$ must be increased by $\times6$ over the Haardt & Madau value in order to match the observed $D_{A}$. Turning on jet feedback lowers this discrepancy to $\sim\times 2.5$, and additionally using the recent Faucher-Gigu\`ere background mostly resolves the PUC, along with producing a flux probability distribution function in accord with observations. The PUC becomes apparent at late epochs ($z \lesssim 1$) where the jet and no-jet simulations diverge; at higher redshifts SIMBA reproduces the observed $D_{A}$ with no adjustment, with or without jets. The main impact of jet feedback is to lower the cosmic baryon fraction in the diffuse IGM from 39% to 16% at $z=0$, while increasing the warm-hot intergalactic medium (WHIM) baryon fraction from 30% to 70%; the lowering of the diffuse IGM content directly translates into a lowering of $D_{A}$ by a similar factor. Comparing to the older MUFASA simulation that employs different quenching feedback but is otherwise similar to SIMBA, MUFASA matches $D_{A}$ less well than SIMBA, suggesting that low-redshift measurements of $D_{A}$ and $\Gamma_{\rm HI}$ could provide constraints on feedback mechanisms. Our results suggest that widespread IGM heating at late times is a plausible solution to the PUC, and that SIMBA's jet AGN feedback model, included to quench massive galaxies, approximately yields this required heating.