The identification of filaments on far infrared and submillimiter
  images. Morphology, physical conditions and relation with star formation of
  filamentary structure
  Observations of molecular clouds reveal a complex structure, with gas and
dust often arranged in filamentary rather than spherical geometries. The
associations of pre- and proto- stellar cores with the filaments suggest a
direct link with the process of star formation. Any study of the properties of
such filaments requires a representative samples from different enviroments and
so an unbiased detection method. We developed such an approach using the
Hessian matrix of a surface-brightness distribution to identify filaments and
determine their physical and morphological properties. After testing the method
on simulated, but realistic filaments, we apply the algorithms to
column-density maps computed from Herschel observations of the Galactic Plane
obtained by the Hi-GAL project. We identified ~500 filaments, in the longitude
range of l=216.5 to l=225.5, with lengths from ~1 pc up to ~30 pc and widths
between 0.1 pc and 2.5 pc. Average column densities are between 10^20 cm^-2 and
10^22 cm^-2. Filaments include the majority of dense material with N_H_2 >
6x10^21cm^-2. We find that the pre- and proto-stellar compact sources already
identified in the same region are mostly associated with filaments. However,
surface densities in excess of the expected critical values for high-mass star
formation are only found on the filaments, indicating that these structures are
necessary to channel material into the clumps. Furthermore, we analyze the
gravitational stability of filaments and discuss their relationship with star
formation.
