New evidences in IRDC G333.73+0.37: colliding filamentary clouds,
  hub-filament system, and embedded cores
  To unravel the star formation process, we present a multi-scale and
multi-wavelength study of the filamentary infrared dark cloud (IRDC)
G333.73+0.37, which hosts previously known two HII regions located at its
center. Each HII region is associated with a mid-infrared source, and is
excited by a massive OB star. Two filamentary structures and a hub-filament
system (HFS) associated with one HII region are investigated in absorption
using the Spitzer 8.0 $\mu$m image. The $^{13}$CO(J = 2-1) and C$^{18}$O(J =
2-1) line data reveal two velocity components (around $-$35.5 and $-$33.5 km
s$^{-1}$) toward the IRDC, favouring the presence of two filamentary clouds at
different velocities. Nonthermal (or turbulent) motions are depicted in the
IRDC using the C$^{18}$O line data. The spatial distribution of young stellar
objects (YSOs) identified using the VVV near-infrared data traces star
formation activities in the IRDC. Low-mass cores are identified toward both the
HII regions using the ALMA 1.38 mm continuum map. The VLT/NACO adaptive-optics
L$^{\prime}$-band images show the presence of at least three point-like sources
and the absence of small-scale features in the inner 4000 AU around YSOs NIR31
and MIR 16 located toward the HII regions. The HII regions and groups of YSO
are observed toward the central part of the IRDC, where the two filamentary
clouds intersect. A scenario of cloud-cloud collision or converging flows in
the IRDC seems to be applicable, which may explain star formation activities
including HFS and massive stars.
