Discovery of a giant molecular loop in the central region of NGC 253
  NGC 253 is a starburst galaxy of SAB(s)c type with increasing interest
because of its high activity at unrivaled closeness. Its energetic event is
manifested as the vertical gas features in its central molecular zone, for
which stellar feedback was proposed as the driving engine. In order to pursue
details of the activity, we have undertaken a kinematic analysis of the ALMA
archive data of $^{12}$CO($J$=3-2) emission at the highest resolution $\sim$3
pc. We revealed that one of the non-rotating gas components in the central
molecular zone shows a loop-like structure of $\sim$200 pc radius. The
loop-like structure is associated with a star cluster, whereas the cluster is
not inside the loop-like structure and is not likely as the driver of the
loop-like structure formation. Further, we find that the bar potential of NGC
253 seems to be too weak to drive the gas motion by the eccentric orbit. As an
alternative, we frame a scenario that magnetic acceleration by the Parker
instability is responsible for the creation of the loop-like structure. We show
that the observed loop-like structure properties are similar to those in the
Milky Way, and argue that recent magneto-hydrodynamics simulations lend support
for the picture having the magnetic field strength of $\gtrsim$100 $\mu$G. We
suggest that cluster formation was triggered by the falling gas to the
footpoint of the loop, which is consistent with a typical dynamical timescale
of the loop $\sim$1 Myr.
