The Turbulent Gas Structure in the Centers of NGC 253 and the Milky Way

We compare molecular gas properties in the starbursting center of NGC 253 and the Milky Way Galactic center (GC) on scales of ∼1-100 pc using dendrograms and resolution-, area-, and noise-matched data sets in CO (1-0) and CO (3-2). We find that the size-line width relations in NGC 253 and the GC have similar slope, but NGC 253 has larger line widths by factors of ∼2-3. The σ 2/R dependency on column density shows that, in the GC, on scales of 10-100 pc the kinematics of gas over N > 3 × 1021 cm-2 are compatible with gravitationally bound structures. In NGC 253 this is only the case for column densities N > 3 × 1022 cm-2. The increased line widths in NGC 253 originate in the lower column density gas. This high velocity dispersion, not gravitationally self-bound gas, is likely in transient structures created by the combination of high average densities and feedback in the starburst. The high densities turn the gas molecular throughout the volume of the starburst, and the injection of energy and momentum by feedback significantly increases the velocity dispersion at a given spatial scale over what is observed in the GC.