Intermediate- and high-velocity clouds in the Milky Way - II. Evidence for a Galactic fountain with collimated outflows and diffuse inflows

We model the kinematics of the high- and intermediate-velocity clouds (HVCs and IVCs) observed in absorption towards a sample of 55 Galactic halo stars with accurate distance measurements. We employ a simple model of a thick disc whose main free parameters are the gas azimuthal, radial, and vertical velocities (vϕ, vR, and vz), and apply it to the data by fully accounting for the distribution of the observed features in the distance-velocity space. We find that at least two separate components are required to reproduce the data. A scenario where the HVCs and the IVCs are treated as distinct populations provides only a partial description of the data, which suggests that a pure velocity-based separation may give a biased vision of the gas physics at the Milky Way's disc-halo interface. Instead, the data are better described by a combination of an inflow component and an outflow component, both characterized by rotation with vϕ comparable to that of the disc and vz of $50\!-\!100\, {\rm km\, s}^{-1}$. Features associated with the inflow appear to be diffused across the sky, while those associated with the outflow are mostly confined within a bicone pointing towards (l = 220°, b = +40°) and (l = 40°, b = -40°). Our findings indicate that the lower ($|z| \lesssim 10\, {\rm kpc}$) Galactic halo is populated by a mixture of diffuse inflowing gas and collimated outflowing material, which are likely manifestations of a galaxy-wide gas cycle triggered by stellar feedback, that is, the galactic fountain