Giant Elephant Trunks from Giant Molecular Clouds
  We report the discovery of large elephant trunk (ET)-like objects, named
giant elephant trunk (GET), of molecular gas in star forming complexes in the
Scutum and Norma arms using the $^{12}$CO(J=1-0)-line survey data with the
Nobeyama 45-m telescope. In comparison with the CO maps of ETs in M16 as
derived from the same data, we discuss physical properties of the GETs. Their
lengths are $\sim 20$ to 50 pc, an order of magnitude larger than ETs. GETs
show a cometary structure coherently aligned parallel to the galactic plane,
and emerge from bow-shaped concave surface of giant molecular clouds (GMC)
facing the HII regions, and point down-stream of the gas flow in the spiral
arms. The molecular masses of the head clumps are $\sim 10^3 -10^4 M_\odot$,
about 3 to 4 times the virial masses, indicating that the clumps are
gravitationally stable. Jeans masses calculated for the derived density and
assumed kinetic temperature are commonly sub-solar. We suggest that the GET
heads are possible birth sites of stellar clusters, similarly to ETs' globules,
but at much greater scale. We discuss the origin of the GETs by Rayleigh-Taylor
instability due to deceleration of GMCs by low density gas stagnated in the
galactic shock waves as well as by pressure of the HII regions.
