Radial velocity (left-hand panels) and density (right-hand panels) projection of the gas particles within a spherical region with a radius of 500 pc, assuming the equatorial plane aligned with the stellar galactic disc. We report five different redshifts, from top to bottom, two before the cavity creation, one when the cavity is at its maximum extent, and one after the cavity closes. The entire set spans a time interval of about 100 Myr.