The Origin of Interstellar Turbulence in M33
  We utilize the multi-wavelength data of M33 to study the origin of turbulence
in its interstellar medium. We find that the HI turbulent energy surface
density inside 8 kpc is $\sim1-3~\times~10^{46}$ erg pc$^{-2}$, and has no
strong dependence on galactocentric radius because of the lack of variation in
HI surface density and HI velocity dispersion. Then, we consider the energies
injected by supernovae (SNe), the magneto-rotational instability (MRI), and the
gravity-driven turbulence from accreted materials as the sources of turbulent
energy. For a constant dissipation time of turbulence, the SNe energy can
maintain turbulence inside $\sim 4$ kpc radius (equivalent to $\sim
0.5~R_{25}$), while the MRI energy is always smaller than the turbulent energy
within 8 kpc radius. However, when we let the dissipation time to be equal to
the crossing time of turbulence across the HI scale-height, the SNe energy is
enough to maintain turbulence out to 7 kpc radius, and the sum of SNe and MRI
energies is able to maintain turbulence out to 8 kpc radius. Due to lack of
constraint in the mass accretion rate through the disk of M33, we can not rule
out the accretion driven turbulence as a possible source of energy.
Furthermore, by resolving individual Giant Molecular Clouds in M33, we also
show that the SNe energy can maintain turbulence within individual molecular
clouds with $\sim 1\%$ of coupling efficiency. This result strengthens the
proposition that stellar feedback is an important source of energy to maintain
turbulence in nearby galaxies.
