2022MNRAS.515.4176M

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 (v_ϕ, v_R, and v_z), 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 v_ϕ comparable to that of the disc and v_z 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.