Clustering Properties of Far-Infrared Sources in Hi-GAL Science
  Demonstration Phase Fields
  We use a Minimum Spanning Tree algorithm to characterize the spatial
distribution of Galactic Far-IR sources and derive their clustering properties.
We aim to reveal the spatial imprint of different types of star forming
processes, e.g. isolated spontaneous fragmentation of dense molecular clouds,
or events of triggered star formation around HII regions, and highlight global
properties of star formation in the Galaxy. We plan to exploit the entire
Hi-GAL survey of the inner Galactic plane to gather significant statistics on
the clustering properties of star forming regions, and to look for possible
correlations with source properties such as mass, temperature or evolutionary
stage. In this paper we present a pilot study based on the two 2x2 square
degree fields centered at longitudes l=30 and l=59 obtained during the Science
Demonstration Phase (SDP) of the Herschel mission. We find that over half of
the clustered sources are associated with HII regions and infrared dark clouds.
Our analysis also reveals a smooth chromatic evolution of the spatial
distribution where sources detected at short-wavelengths, likely proto-stars
surrounded by warm circumstellar material emitting in the far-infrared, tend to
be clustered in dense and compact groups around HII regions while sources
detected at long-wavelengths, presumably cold and dusty density enhancements of
the ISM emitting in the sub-millimeter, are distributed in larger and looser
groups.
