ALMA Observations of Giant Molecular Clouds in M33 I: Resolving Star
  Formation Activities in the Giant Molecular Filaments Possibly Formed by a
  Spiral Shock
  We report molecular line and continuum observations toward one of the most
massive giant molecular clouds (GMCs), GMC-16, in M33 using ALMA with an
angular resolution of 0$''$44 $\times$ 0$''$27 ($\sim$2 pc $\times$ 1 pc). We
have found that the GMC is composed of several filamentary structures in
$^{12}$CO and $^{13}$CO ($J$ = 2-1). The typical length, width, and total mass
are $\sim$50-70 pc, $\sim$5-6 pc, and $\sim$10$^{5}$ $M_{\odot}$, respectively,
which are consistent with those of giant molecular filaments (GMFs) as seen in
the Galactic GMCs. The elongations of the GMFs are roughly perpendicular to the
direction of the galaxy's rotation, and several H$\;${\sc ii} regions are
located at the downstream side relative to the filaments with an offset of
$\sim$10-20 pc. These observational results indicate that the GMFs are
considered to be produced by a galactic spiral shock. The 1.3 mm continuum and
C$^{18}$O ($J$ = 2-1) observations detected a dense clump with the size of
$\sim$2 pc at the intersection of several filamentary clouds, which is referred
to as the $"$hub filament,$"$ possibly formed by a cloud-cloud collision. A
strong candidate for protostellar outflow in M33 has also been identified at
the center of the clump. We have successfully resolved the parsec-scale local
star formation activity in which the galactic scale kinematics may induce the
formation of the parental filamentary clouds.
