  We have mapped four selected about 0.5 deg x 0.5 deg-sized fields containing
Spitzer 8-micron dark regions with APEX/LABOCA at 870 micron. Selected
positions in the fields were observed in C17O(2-1) to obtain kinematic
information. The obtained LABOCA maps are used in conjunction with the Spitzer
IR images. The total number of clumps identified in this survey is 91, out of
which 40 (44%) appear dark at 8 and 24 micron. The remaining clumps are
associated with mid-IR emission. Many of the identified clumps are massive
enough to allow high-mass star formation, and some of them already show clear
signposts of that. Seven clumps associated with extended-like 4.5 micron
emission are candidate extended green objects (EGOs). Filamentary dust "ridges"
were found towards the Spitzer bubbles N10/11 in one of our fields, which
conforms to the triggered high-mass star formation in the system. The relative
number of IR-dark and IR-bright clumps suggest that the duration of the former
stage is about 1.6x10^5 yr. The mass distribution of the total sample of
clumps, and that separately constructed for the IR-dark and IR-bright clumps,
could be fitted at the high-mass end with the power-law function dN/dlogM ~
M^(-0.8...-0.7). The C17O observation positions appear to be dominated by
non-thermal motions, and the data also revealed some potential sites of strong
CO depletion. In G11.36+0.80, which is the best example of a filamentary IRDC
in our sample, the clumps appear to be gravitationally bound. The fragmentation
of the filament can be understood in terms of a "sausage"-type fluid
instability, in agreement with the results for other IRDCs. The formation of
filamentary IRDCs might be caused by converging turbulent flows, and the same
process may play a role in exciting the fluid perturbations responsible for the
fragmentation of the clouds into clumps.

Astrophysics of Galaxies