Discovery of a giant molecular loop in the central region of NGC 253

Abstract

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 ∼3 pc. We revealed that one of the non-rotating gas components in the central molecular zone shows a looplike distribution of ∼200 pc radius. The loop is associated with a star cluster, whereas the cluster is not inside the loop and is not likely as the driver of the loop 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. We show that the observed loop properties are similar to those in the Milky Way, and argue that recent magnetrohydrodynamics simulations lend support for the picture having the magnetic field strength of 100 µG. 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 ∼1 Myr.