Unraveling the inner substructure of new candidate hub-filament system
  in the HII region G25.4NW
  We present multi-scale and multi-wavelength data of the Galactic HII region
G25.4-0.14 (hereafter G25.4NW, distance ~5.7 kpc). The SHARC-II 350 micron
continuum map displays a hub-filament configuration containing five parsec
scale filaments and a central compact hub. Through the 5 GHz radio continuum
map, four ionized clumps (i.e., Ia-Id) are identified toward the central hub,
and are powered by massive OB-stars. The Herschel temperature map depicts the
warm dust emission (i.e., Td ~23-39 K) toward the hub. High resolution Atacama
Large Millimeter/submillimeter Array (ALMA) 1.3 mm continuum map (resolution
~0".82 X 0".58) reveals three cores (c1-c3; mass ~80-130 Msun) toward the
ionized clumps Ia, and another one (c4; mass ~70 Msun) toward the ionized clump
Ib. A compact near-infrared (NIR) emission feature (extent ~0.2 pc) is
investigated toward the ionized clump Ia excited by an O8V-type star, and
contains at least three embedded K-band stars. In the direction of the ionized
clump Ia, the ALMA map also shows an elongated feature (extent ~0.2 pc) hosting
the cores c1-c3. All these findings together illustrate the existence of a
small cluster of massive stars in the central hub. Considering the detection of
the hub-filament morphology and the spatial locations of the mm cores, a global
non-isotropic collapse (GNIC) scenario appears to be applicable in G25.4NW,
which includes the basic ingredients of the global hierarchical collapse and
clump-fed accretion models. Overall, the GNIC scenario explains the birth of
massive stars in G25.4NW.
