The fragmentation and stability of hierarchical structure in Serpens
  South
  Filamentary structures are ubiquitous in molecular clouds, and have been
recently argued to play an important role in regulating the size and mass of
embedded clumps through fragmentation and mass accretion. Here, we reveal the
dynamical state and fragmentation of filamentary molecular gas associated with
the Serpens South protocluster through analysis of wide (~4 x 4 pc)
observations of NH3 (1,1) and (2,2) inversion transitions with the Green Bank
Telescope. Detailed modeling of the NH3 lines reveals that the kinematics of
the cluster and surrounding filaments are complex. We identify hierarchical
structure using a dendrogram analysis of the NH3 emission. The distance between
neighbour structures that are embedded within the same parent structure is
generally greater than expected from a spherical Jeans analysis, and is in
better agreement with cylindrical fragmentation models. The NH3 line width-size
relation is flat, and average gas motions are sub- or trans-sonic over all
physical scales observed. Subsonic regions extend far beyond the typical 0.1 pc
scale previously identified in star-forming cores. As a result, we find a
strong trend of decreasing virial parameter with increasing structure mass in
Serpens South. Extremely low virial parameters on the largest scales probed by
our data suggest that the previously observed, ordered magnetic field is
insufficient to support the region against collapse, in agreement with large
radial infall motions previously measured toward some of the filaments. A more
complex magnetic field configuration in the dense gas, however, may be able to
support the filaments.
