Clouds, filaments and protostars: the Herschel Hi-GAL Milky Way
  We present the first results from the science demonstration phase for the
Hi-GAL survey, the Herschel key-project that will map the inner Galactic Plane
of the Milky Way in 5 bands. We outline our data reduction strategy and present
some science highlights on the two observed 2{\deg} x 2{\deg} tiles
approximately centered at l=30{\deg} and l=59{\deg}. The two regions are
extremely rich in intense and highly structured extended emission which shows a
widespread organization in filaments. Source SEDs can be built for hundreds of
objects in the two fields, and physical parameters can be extracted, for a good
fraction of them where the distance could be estimated. The compact sources
(which we will call 'cores' in the following) are found for the most part to be
associated with the filaments, and the relationship to the local beam-averaged
column density of the filament itself shows that a core seems to appear when a
threshold around A_V of about 1 is exceeded for the regions in the l=59{\deg}
field; a A_V value between 5 and 10 is found for the l=30{\deg} field, likely
due to the relatively larger distances of the sources. This outlines an
exciting scenario where diffuse clouds first collapse into filaments, which
later fragment to cores where the column density has reached a critical level.
In spite of core L/M ratios being well in excess of a few for many sources, we
find core surface densities between 0.03 and 0.5 g cm-2. Our results are in
good agreement with recent MHD numerical simulations of filaments forming from
large-scale converging flows.
