A Galactic Molecular Cloud Clump Catalog from Hi-GAL Data: Method and
  Initial Results Comparison to BGPS
  As the precursors to stellar clusters, it is imperative that we understand
the distribution and physical properties of dense molecular gas clouds and
clumps. Such a study has been done with the ground-based Bolocam Galactic Plane
Survey (BGPS). Now the Herschel infrared GALactic plane survey (Hi-GAL) allows
us to do the same with higher quality data and complete coverage of the
Galactic plane. We have made a pilot study comparing dense molecular gas clumps
identified in the Hi-GAL and BGPS surveys, using six $2^\circ \times 2^\circ$
regions centered at Galactic longitudes of $\ell = 11^\circ$, 30$^\circ$,
41$^\circ$, 50$^\circ$, 202$^\circ$, and $217^\circ$. We adopted the BGPS
methodology for identifying clumps and estimating distances, leading to 6198
clumps being identified in our substudy, with 995 of those having
well-constrained distances. These objects were evenly distributed with Galactic
longitude, a consequence of Hi-GAL being source confusion limited. These clumps
range in mass from 10$^{-2}$ M$_\odot$ to 10$^{5}$ M$_\odot$, and have
heliocentric distances of up to 16 kpc. When clumps found in both surveys are
compared, we see that distances agree within 1 kpc and ratios of masses are of
order unity. This serves as an external validation for BGPS and instills
confidence as we move forward to cataloging the clumps from the entirety of
Hi-GAL. In addition to the sources that were in common with BGPS, Hi-GAL found
many additional sources, primarily due to the lack of atmospheric noise. We
expect Hi-GAL to yield $2\times10^5$ clumps, with 20% having well-constrained
distances, an order of magnitude above what was found in BGPS.
