Giant Elephant Trunks from Giant Molecular Clouds

Abstract

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 ∼ 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 ∼ 10 3 − 10 4 M ⊙ , 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.