Gas Accretion Can Drive Turbulence in Galaxies
  The driving of turbulence in galaxies is deeply connected with the physics of
feedback, star formation, outflows, accretion, and radial transport in disks.
The velocity dispersion of gas in galaxies therefore offers a promising
observational window into these processes. However, the relative importance of
each of these mechanisms remains controversial. In this work we revisit the
possibility that turbulence on galactic scales is driven by the direct impact
of accreting gaseous material on the disk. We measure this effect in a
disk-like star-forming galaxy in IllustrisTNG, using the high-resolution
cosmological magnetohydrodynamical simulation TNG50. We employ Lagrangian
tracer particles with a high time cadence of only a few Myr to identify
accretion and other events, such as star formation, outflows, and movement
within the disk. The energies of particles as they arrive in the disk are
measured by stacking the events in bins of time before and after the event. The
average effect of each event is measured on the galaxy by fitting explicit
models for the kinetic and turbulent energies as a function of time in the
disk. These measurements are corroborated by measuring the cross-correlation of
the turbulent energy in the different annuli of the disk with other time
series, and searching for signals of causality, i.e. asymmetries in the
cross-correlation across zero time lag. We find that accretion contributes to
the large-scale turbulent kinetic energy even if it is not the dominant driver
of turbulence in this $\sim 5 \times 10^{9} M_\odot$ stellar mass galaxy.
Extrapolating this finding to a range of galaxy masses, we find that there are
regimes where energy from direct accretion may dominate the turbulent energy
budget, particularly in disk outskirts, galaxies less massive than the Milky
Way, and at redshift $\sim 2$.
