J plots: a new method for characterizing structures in the interstellar
  medium
  Large scale surveys have brought about a revolution in astronomy. To analyse
the resulting wealth of data, we need automated tools to identify, classify,
and quantify the important underlying structures. We present here a method for
classifying and quantifying a pixelated structure, based on its principal
moments of inertia. The method enables us to automatically detect, and
objectively compare, centrally condensed cores, elongated filaments and hollow
rings. We illustrate the method by applying it to (i) observations of
surface-density from Hi-GAL, and (ii) simulations of filament growth in a
turbulent medium. We limit the discussion here to 2D data; in a future paper we
will extend the method to 3D data.
