Giant elephant trunks from giant molecular clouds

We report the discovery of large elephant trunk (ET)-like objects, named giant elephant trunks (GETs), of molecular gas in star-forming complexes in the Scutum and Norma arms using the ¹²CO(J = 1--0)-line survey data with the Nobeyama 45 m telescope. In comparison with the CO maps of ETs in M 16 as derived from the same data, we discuss physical properties of the GETs. Their lengths are ∼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 the bow-shaped concave surface of giant molecular clouds (GMC) facing the H II regions, and point down-stream of the gas flow in the spiral arms. The molecular masses of the head clumps are ∼10³-10⁴ M_{⊙}, about three to four 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 ET 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 H II regions.