Physical Properties of Large-scale Galactic Filaments

The characterization of our Galaxy’s longest filamentary gas features has been the subject of several studies in recent years, producing not only a sizable sample of large-scale filaments, but also confusion as to whether all these features (e.g., “Bones,” “Giant Molecular Filaments”) are the same. They are not. We undertake the first standardized analysis of the physical properties (H₂ column densities, dust temperatures, morphologies, radial column density profiles) and kinematics of large-scale filaments in the literature. We expand and improve upon prior analyses by using the same data sets, techniques, and spiral arm models to disentangle the filaments’ inherent properties from selection criteria and methodology. Our results suggest that the myriad filament-finding techniques are uncovering different physical structures, with length (11-269 pc), width (1-40 pc), mass (3× {10}³ {M}_⊙ {--}1.1× {10}⁶ {M}_⊙ ), aspect ratio (3:1-117:1), and high column density fraction (0.2%-100%) varying by over an order of magnitude across the sample of 45 filaments. We develop a radial profile-fitting code, RadFil, which is publicly available. We also perform a position-position-velocity (p-p-v) analysis on a subsample and find that while 60%-70% lie spatially in the plane of the Galaxy, only 30%-45% concurrently exhibit spatial and kinematic proximity to spiral arms. In a parameter space defined by aspect ratio, dust temperature, and column density, we broadly distinguish three filament categories, which could indicate different formation mechanisms or histories. Highly elongated “Bone-like” filaments show the most potential for tracing gross spiral structure (e.g., arms, spurs), while other categories could be large concentrations of molecular gas (giant molecular clouds, core complexes).