APEX/SABOCA observations of small-scale structure of infrared-dark
  clouds I. Early evolutionary stages of star-forming cores
  Infrared-dark clouds (IRDCs) harbor the early phases of cluster and high-mass
star formation and are comprised of cold (~20 K), dense (n > 10$^4$ cm$^{-3}$)
gas. The spectral energy distribution (SED) of IRDCs is dominated by the
far-infrared and millimeter wavelength regime, and our initial Herschel study
examined IRDCs at the peak of the SED with high angular resolution. Here we
present a follow-up study using the SABOCA instrument on APEX which delivers
7.8" angular resolution at 350 micron, matching the resolution we achieved with
Herschel/PACS, and allowing us to characterize substructure on ~0.1pc scales.
Our sample of 11 nearby IRDCs are a mix of filamentary and clumpy morphologies,
and the filamentary clouds show significant hierarchical structure, while the
clumpy IRDCs exhibit little hierarchical structure. All IRDCs, regardless of
morphology, have about 14% of their total mass in small scale core-like
structures which roughly follow a trend of constant volume density over all
size scales. Out of the 89 protostellar cores we identified in this sample with
Herschel, we recover 40 of the brightest and re-fit their SEDs and find their
properties agree fairly well with our previous estimates (<T> ~ 19K). We detect
a new population of "cold cores" which have no 70 micron counterpart, but are
100 and 160 micron-bright, with colder temperatures (<T> ~ 16K). This latter
population, along with SABOCA-only detections, are predominantly low-mass
objects, but their evolutionary diagnostics are consistent with the earliest
starless or prestellar phase of cores in IRDCs.
