  We present a multi-wavelength investigation of a large-scale physical system
containing the W33 complex. The extended system (~50 pc x 37 pc) is selected
based on the distribution of molecular gas at [29.6, 60.2] km/s and of 88
ATLASGAL 870 micron dust clumps at d ~2.6 kpc. The extended system/molecular
cloud traced in the maps of 13CO and C18O emission contains several HII regions
excited by OB stars (age ~0.3-1.0 Myr) and a thermally supercritical filament
("fs1", length ~17 pc). The filament, devoid of the ionized gas, shows dust
temperature (T_d) of ~19 K, while the HII regions are depicted with T_d of
~21-29 K. It suggests the existence of two distinct environments in the cloud.
The distribution of Class I young stellar objects (mean age ~0.44 Myr) traces
the early stage of star formation (SF) toward the cloud. At least three
velocity components (around 35, 45, and 53 km/s) are investigated toward the
system. The analysis of 13CO and C18O reveals the spatial and velocity
connections of cloud components around 35 and 53 km/s. The observed positions
of previously known sources, W33 Main, W33 A and O4-7I stars, are found toward
a complementary distribution of these two cloud components. The filament "fs1"
and a previously known object W33 B are seen toward the overlapping areas of
the clouds, where ongoing SF activity is evident. A scenario concerning the
converging/colliding flows from two different velocity components appears to
explain well the observed signposts of SF activities in the system.
