  We have carried out a controlled comparison of the structural and kinematic
properties of the circumgalactic medium (CGM) around Milky Way mass galaxies in
the Illustris and IllustrisTNG simulations. Very striking differences are
found. At z=0, gas column density and temperature profiles at large radii
($\sim 100$ kpc) correlate strongly with disk gas mass fraction in Illustris,
but not in TNG. The neutral gas at large radii is preferentially aligned in the
plane of the disk in TNG, whereas it is much more isotropic in Illustris. The
vertical coherence scale of the rotationally supported gas in the CGM is linked
to the gas mass fraction of the galaxy in Illustris, but not in TNG. A tracer
particle analysis allows us to show how these differences can be understood as
a consequence of the different sub-grid models of feedback in the two
simulations. A study of spatially matched galaxies in the two simulations shows
that in TNG, feedback by supernovae and AGN helps to create an extended smooth
reservoir of hot gas at high redshifts, that then cools to form a thin,
rotationally-supported disk at later times. In Illustris, AGN dump heat in the
form of hot gas bubbles that push diffuse material at large radii out of the
halo. The disk is formed by accretion of colder, recycled material, and this
results in more vertically extended gas distributions above and below the
Galactic plane. We conclude that variations in the structure of gas around
Milky Way mass galaxies are a sensitive probe of feedback physics in
simulations and are worthy of more observational consideration in future.
