How was the mushroom-shaped GW 123.4-1.5 formed in the Galactic disk?

The unusual mushroom-shaped H I cloud GW 123.4-1.5 is hundreds of parsecs in size but does not show any correlations to H I shells or chimney structures. To investigate the origin and velocity structure of GW 123.4-1.5, we perform three-dimensional hydrodynamical simulations of the collision of a high-velocity cloud (HVC) with the Galactic disk. We also perform a parameter study of the density, radius, and incident angle of the impact cloud. The numerical experiments indicate that we can reproduce a mushroom-shaped structure which resembles GW 123.4-1.5 in shape, size, and position velocity across the cap of the mushroom, and the density ratio between the mushroom and surrounding gas. GW 123.4-1.5 is expected to be formed by the almost head-on collision of a HVC with velocity similar to 100 km s(-1) and mass similar to 10(5) M(circle dot) about 5 x 10(7) yr ago. A mushroom-shaped structure like GW 123.4-1.5 must be infrequent on the Galactic plane, because the head-on collision which explains the mushroom structure seems rare for observed HVCs. An HVC-disk collision explains not only the origin of the mushroom-shaped structure but also the formation of a variety of structures like shells, loops, and vertical structures in our Galaxy.