A Herschel study of G214.5−1.8: a young, cold, and quiescent giant molecular filament on the shell of a H i superbubble

We present an analysis of the outer Galaxy giant molecular filament (GMF) G214.5−1.8 (G214.5) using Herschel data. We find that G214.5 has a mass of ∼16 000 M⊙, yet hosts only 15 potentially protostellar 70-µm sources, making it highly quiescent compared to equally massive clouds such as Serpens and Mon R2. We show that G214.5 has a unique morphology, consisting of a narrow ‘Main filament’ running north–south and a perpendicular ‘Head’ structure running east–west. We identify 33 distinct massive clumps from the column density maps, 8 of which are protostellar. However, the star formation activity is not evenly spread across G214.5 but rather predominantly located in the Main filament. Studying the Main filament in a manner similar to previous works, we find that G214.5 is most like a ‘Bone’ candidate GMF, highly elongated and massive, but it is colder and narrower than any such GMF. It also differs significantly due to its low fraction of high column density gas. Studying the radial profile, we discover that G214.5 is highly asymmetric and resembles filaments which are known to be compressed externally. Considering its environment, we find that G214.5 is co-incident, spatially and kinematically, with a H I superbubble. We discuss how a potential interaction between G214.5 and the superbubble may explain G214.5’s morphology, asymmetry and, paucity of dense gas and star formation activity, highlighting the intersection of a bubble-driven interstellar medium paradigm with that of a filament paradigm for star formation.