The formation of the HII regions N83 and N84 in the Small Magellanic
  Cloud triggered by colliding HI flows
  LHA 115-N 83 (N83) and LHA 115-N 84 (N84) are HII regions associated with the
early stage of star formation located in the Small Magellanic Cloud (SMC). We
have analyzed the new HI data taken with the Galactic Australian Square
Kilometre Array Pathfinder survey project at a high angular resolution of 30".
We found that the two clouds, having $\sim$40 km s$^{-1}$ velocity separation,
show complementary distribution with each other, and part of the HI gas is
dispersed by the ionization. In addition, the Atacama Large
Millimeter/submillimeter Array observations revealed clumpy CO clouds of
10$^{5}$ $M_{\odot}$ in total over an extent of 100 pc, which are also well
correlated with the HII regions. There is a hint of displacement between the
two complementary components, which indicate that the red-shifted HI cloud is
moving from the north to the south by $\sim$100 pc. This motion is similar to
what is found in NGC 602 (Fukui et al. 2020), suggesting a large scale
systematic gas flow. We frame a scenario that the two components collided with
each other and triggered the formation of N83, N84, and six O-type stars around
them in a time scale of a few Myr ($\sim$60 pc / 40 km s$^{-1}$). The
supersonic motion compressed the HI gas to form the CO clouds in the
red-shifted HI cloud, some of which are forming O-type stars ionizing the HII
regions in the last Myr. The red-shifted HI cloud probably flows to the
direction of the Magellanic Bridge. The velocity field originated by the close
encounter of the SMC with the Large Magellanic Cloud 200 Myr ago as proposed by
Fujimoto & Noguchi (1990).
