How was the mushroom - shaped GW 123 . 4 – 1 . 5 formed in the Galactic disk ?

The unusual mushroom-shaped HI cloud, GW 123.4–1.5, is hundreds of parsecs in size but does not show any correlations to HI 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 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 reproduce the mushroom-shaped structure which resembles GW 123.4–1.5 in shape, size, 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 ∼ 100 km s and mass ∼ 10 M⊙ about 5 × 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. HVC-disk collision explains not only the origin of the mushroomshaped structure but also the formation of a variety of structures like shells, loops, and vertical structures in our Galaxy. Subject headings: Galaxy : structure — ISM : cloud — ISM : individual : GW 123.4–1.5 — ISM : structure Astrophysical Research Center for the Structure and Evolution of the Cosmos (ARCSEC), Sejong University, Seoul 143-747, Korea; [email protected] National Astronomical Observatory of Japan, 2-21-1, Osawa, Mitaka, Tokyo, 181-8588 Japan Department of Astronomical Science, The Graduate University for Advanced Studies (SOKENDAI)