Source clustering in the Hi-GAL survey determined using a minimum
  spanning tree method
  The aims are to investigate the clustering of the far-infrared sources from
the Herschel infrared Galactic Plane Survey (Hi-GAL) in the Galactic longitude
range of -71 to 67 deg. These clumps, and their spatial distribution, are an
imprint of the original conditions within a molecular cloud. This will produce
a catalogue of over-densities. The minimum spanning tree (MST) method was used
to identify the over-densities in two dimensions. The catalogue was further
refined by folding in heliocentric distances, resulting in more reliable
over-densities, which are cluster candidates. We found 1,633 over-densities
with more than ten members. Of these, 496 are defined as cluster candidates
because of the reliability of the distances, with a further 1,137 potential
cluster candidates. The spatial distributions of the cluster candidates are
different in the first and fourth quadrants, with all clusters following the
spiral structure of the Milky Way. The cluster candidates are fractal. The
clump mass functions of the clustered and isolated are statistically
indistinguishable from each other and are consistent with Kroupa's initial mass
function.
