X-ray bubbles in the circumgalactic medium of TNG50 Milky Way- and
  M31-like galaxies: signposts of supermassive black hole activity
  The TNG50 cosmological simulation produces X-ray emitting bubbles, shells,
and cavities in the circumgalactic gas above and below the stellar disks of
Milky Way- and Andromeda-like galaxies with morphological features reminiscent
of the eROSITA and Fermi bubbles in the Galaxy. Two-thirds of the 198 MW/M31
analogues inspected in TNG50 at z=0 show one or more large-scale, coherent
features of over-pressurized gas that impinge into the gaseous halo. Some of
the galaxies include a succession of bubbles or shells of increasing size,
ranging from a few to many tens of kpc. These are prominent in gas pressure,
X-ray emission and gas temperature, and often exhibit sharp boundaries with
typical shock Mach numbers of 2-4. The gas in the bubbles outflows with maximum
(95th pctl) radial velocities of 100-1500 km/s. TNG50 bubbles expand with
speeds as high as 1000-2000 km/s (about 1-2 kpc/Myr), but with a great
diversity and with larger bubbles expanding at slower speeds. The bubble gas is
at 10^6.4-7.2 K temperatures and is enriched to metallicities of 0.5-2 solar.
In TNG50, the bubbles are a manifestation of episodic, kinetic, wind-like
energy injections from the supermassive black holes at the galaxy centers that
accrete at low Eddington ratios. According to TNG50, X-ray, and possibly
gamma-ray, bubbles similar to those observed in the Milky Way should be a
frequent feature of disk-like galaxies prior to, or on the verge of, being
quenched. They should be within the grasp of eROSITA in the local Universe.
