ATOMS: ALMA Three-millimeter Observations of Massive Star-forming
  regions -- VI. On the formation of the "L" type filament in G286.21+0.17
  Filaments play an important role in star formation, but the formation process
of filaments themselves is still unclear. The high-mass star forming clump
G286.21+0.17 (G286 for short) that contains an "L" type filament was thought to
undergo global collapse. Our high resolution ALMA band 3 observations resolve
the gas kinematics of G286 and reveal two sub-clumps with very different
velocities inside it. We find that the "blue profile" (an indicator of gas
infall) of HCO+ lines in single dish observations of G286 is actually caused by
gas emission from the two sub-clumps rather than gas infall. We advise great
caution in interpreting gas kinematics (e.g., infall) from line profiles toward
distant massive clumps in single dish observations. Energetic outflows are
identified in G286 but the outflows are not strong enough to drive expansion of
the two sub-clumps. The two parts of the "L" type filament ("NW-SE" and "NE-SW"
filaments) show prominent velocity gradients perpendicular to their major axes,
indicating that they are likely formed due to large-scale compression flows. We
argue that the large-scale compression flows could be induced by the expansion
of nearby giant HII regions. The "NW-SE" and "NE-SW" filaments seem to be in
collision, and a large amount of gas has been accumulated in the junction
region where the most massive core G286c1 forms.
