Simba: The average properties of the circumgalactic medium of $2 \leq z
  \leq 3$ quasars are determined primarily by stellar feedback
  We use the Simba cosmological hydrodynamic simulation suite to explore the
impact of feedback on the circumgalactic medium (CGM) and intergalactic medium
(IGM) around $2 \leq z \leq 3$ quasars. We identify quasars in Simba as the
most rapidly-accreting black holes, and show that they are well-matched in
bolometric luminosity and correlation strength to real quasars. We extract
Lyman-alpha (Ly-a) absorption in spectra passing at different transverse
distances (10 kpc $\lesssim b \lesssim$ 10 Mpc) around those quasars, and
compare to observations of the mean Ly-a absorption profile. The observations
are well reproduced, except within 100 kpc from the foreground quasar, where
Simba overproduces absorption; this could potentially be mitigated by including
ionisation from the quasar itself. By comparing runs with different feedback
modules activated, we find that (mechanical) AGN feedback has little impact on
the surrounding CGM even around these most highly luminous black holes, while
stellar feedback has a significant impact. By further investigating
thermodynamic and kinematic properties of CGM gas, we find that stellar
feedback, and not AGN feedback, is the primary physical driver in determining
the average properties of the CGM around $z\sim 2-3$ quasars. We also compare
our results with previous works, and find that Simba predicts much more
absorption within 100 kpc than the Nyx and Illustris simulations, showing that
the Ly-a absorption profile can be a powerful constraint on simulations.
Instruments such as VLT-MUSE and upcoming surveys (e.g., WEAVE and DESI)
promise to further improve such constraints.
