The Dense Filamentary Giant Molecular Cloud G23.0-0.4: Birthplace of
  Ongoing Massive Star Formation
  We present observations of 1.5 square degree maps of the 12CO, 13CO, and C18O
(J=1-0) emission toward the complex region of the supernova remnant (SNR) W41
and SNR G22.7-0.2. A massive (~5E5Msun), large (~84x15 pc), and dense (~10E3
cm^-3) giant molecular cloud (GMC), G23.0-0.4 with VLSR~77 km/s, is found to be
adjacent to the two SNRs. The GMC displays a filamentary structure
approximately along the Galactic plane. The filamentary structure of the dense
molecular gas, traced by C18O (J=1-0) emission, is also coincident well with
the distribution of the dust-continuum emission in the direction. Two dense
massive MC clumps, two 6.7 GHz methanol masers, and one HII/SNR complex,
associated with the 77 km/s GMC G23.0-0.4, are aligned along the filamentary
structure, indicating the star forming activity within the GMC. These sources
have periodic projected spacing of 0.18-0.26degree along the giant filament,
which is consistent well with the theoretical predictions of 0.22degree. It
indicates that the turbulence seems to dominate the fragmentation process of
the dense gaseous filament on large scale. The established 4.4 kpc distance of
the GMC and the long dense filament traced by C18O emission, together with the
rich massive star formation groups in the nearby region, suggest that G23.0-0.4
is probably located at the near side of the Scutum-Centaurus arm in the first
quadrant. Considering the large scale and the elongation structure along the
Galactic plane, we speculate that the dense filamentary GMC has relation to the
spiral density wave of the Milky Way.
