Release of Lyα radiation by tidal (or ram pressure) interaction with a nearby galaxy. In this highly schematic depiction, the top panel shows an undisturbed H i halo (grey) surrounding a Lyα producing galaxy (dark spot). The Lyα photons escape gradually through the optically thick H i cocoon. In the bottom drawing, which is shown in the reference frame of the galaxy’s centre of mass, the encounter with a perturbing galaxy (e.g. in the form of a tidal field) leads to differential motion between the Lyα source and the gaseous halo in the part closest to the passing galaxy and at the opposite (far) end of the halo. If the relative velocities between gas and galaxy in a certain direction begin to exceed the thermal velocities, the photons drop out of the original resonance and may escape preferentially through these ‘velocity channels’. The Lyα emission will brighten up in these high-velocity directions and may boost the flux seen along this line of sight across a detection threshold. The lower optical depth and thus shorter path through the medium will also reduce the chances for absorption by dust.