Giant elephant trunks from giant molecular clouds

Abstract

Abstract 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 $^{12}$CO(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 $\sim\!\! 20$ 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 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 $\sim 10^{3}$–$10^{4}\, M_{\odot}$, 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.