Star Formation Thresholds: The View from Inside the Galaxy

We explore the relationship between the total gas surface density and star formation rate surface density, a.k.a., the ``Kennicutt-Schmidt relation,'' in a Galactic context. Specifically, we probe the origins of thresholds in the behaviour of the K-S relation at 10 M_⊙ pc^-2 and 100-200 M _⊙ pc^-2 using images from the Herschel Hi-GAL and Gould Belt surveys. In both cases, pervasive filamentary structures are seen, possibly due to turbulent motions. The Hi-GAL image supports the view that at ~10 M_⊙ pc^-2 gas becomes molecular, leading to the formation of clouds that harbour star formation. The GBS images suggest the 100-200 M_⊙ pc^-2 threshold originates from the nature of filaments being stable until a critical column density of ~160 M_⊙ pc^-2 is reached. Therefore, the transition between non-star-forming and star-forming gas in clouds (and galaxies) may be set universally by the dynamical properties of filaments.