2019ApJ...871...17U

We utilize the multi-wavelength data of M33 to study the origin of turbulence in its interstellar medium. We find that the H I turbulent energy surface density inside 8 kpc is ∼1-3 x 10⁴⁶ erg pc^–2, and has no strong dependence on galactocentric radius because of the lack of variation in H I surface density and H I 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 ∼4 kpc radius (equivalent to ∼0.5 R₂₅), 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 H I 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 cannot 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 ∼1% of coupling efficiency. This result strengthens the proposition that stellar feedback is an important source of energy to maintain turbulence in nearby galaxies.