Jet Feedback and the Photon Underproduction Crisis in Simba
  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.
