Characterizing Precursors to Stellar Clusters with Herschel
  Context: Despite their profound effect on the universe, the formation of
massive stars and stellar clusters remains elusive. In the past decade,
compelling evidence has emerged that suggests Infrared Dark Clouds (IRDCs) may
be precursors to stellar clusters. However, the usual method for identifying
IRDCs is biased by the requirement that they are seen in absorption against
bright mid-IR emission, whereas dust continuum observations allow cold, dense
pre-stellar-clusters to be identified anywhere. Aims: We aim to understand what
physical properties characterize IRDCs, to explore the population of dust
continuum sources that are not IRDCs, and to roughly characterize the star
formation activity in dust continuum sources. Results: We present temperature
and column density maps in the Hi-GAL l=30 and l=59 SDP fields, as well as a
robust algorithm for cirrus subtraction and source identification using Hi-GAL.
We report on the fraction of Hi-GAL sources which are mid-IR-dark, -neutral, or
-bright in both fields. We find significant trends in column density and
temperature between mid-IR-dark and -bright pixels; mid-IR-dark pixels are
about 10 K colder and have a factor of 2 higher column density on average than
mid-IR-bright pixels. We find that Hi-GAL dust continuum sources span a range
of evolutionary states from pre- to star-forming, and that warmer sources are
associated with more star formation tracers. There is a trend of increasing
temperature with tracer type from mid-IR-dark at the coldest, to outflow/maser
sources in the middle, and finally to 8 and 24 micron bright sources at the
warmest. Finally, we identify five candidate IRDC-like sources on the far-side
of the Galaxy. These are cold (~ 20 K), high column density (N(H2) > 10^22
cm^-2) clouds identified with Hi-GAL which, despite bright surrounding mid-IR
emission, show little to no absorption at 8 micron. (abridged)
