ALMA Observations of Giant Molecular Clouds in M33. II. Triggered
  High-mass Star Formation by Multiple Gas Colliding Events at the NGC 604
  Complex
  We present the results of ALMA observations in $^{12}$CO($J=2-1$),
$^{13}$CO($J=2-1$), and C$^{18}$O($J=2-1$) lines and 1.3 mm continuum emission
toward a massive ($\sim 10^6 M_{\odot}$) giant molecular cloud associated with
the giant H II region NGC 604 in one of the nearest spiral galaxy M33 at an
angular resolution of 0''.44 $\times$ 0''.27 (1.8 pc $\times$ 1.1 pc). The
$^{12}$CO and $^{13}$CO images show highly complicated molecular structures
composed of a lot of filaments and shells whose lengths are 5 -- 20 pc. We
found three 1.3 mm continuum sources as dense clumps at edges of two shells and
also at an intersection of several filaments. We examined the velocity
structures of $^{12}$CO($J=2-1$) emission in the shells and filaments
containing dense clumps, and concluded that expansion of the H II regions
cannot explain the formation of such dense cores. Alternatively, we suggest
that cloud--cloud collisions induced by an external H I gas flow and the
galactic rotation compressed the molecular material into dense filaments/shells
as ongoing high-mass star formation sites. We propose that multiple gas
converging/colliding events with a velocity of a few tens km s$^{-1}$ are
necessary to build up NGC 604, the most significant cluster-forming complex in
the Local Group of galaxies.
