* Astronomy

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RE: GW 123.4-1.5

Blobrana · L

Title: How was the mushroom-shaped GW 123.4--1.5 formed in the Galactic disk?
Authors: Chang Hyun Baek, Takahiro Kudoh and Kohji Tomisaka

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 kms and mass ~ 10^5 solar masses about 5 x 10^7 r 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 mushroom-shaped structure but also the formation of a variety of structures like shells, loops, and vertical structures in our Galaxy.

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A mushroom-shaped hydrogen cloud rearing 1000 light years above the plane of our galaxy is the aftermath of a massive gas cloud that dive-bombed the Milky Way, new computer simulations suggest. The work explains why the cloud is unlike any other found so far.
The cloud, called GW 123.4-1.5, was discovered in 1999 by Jayanne English of the University of Manitoba in Winnipeg, Canada, and colleagues, who found it in a radio survey of the galactic plane. Two explanations for its familiar shape were offered at the time that the mushroom is a bubble of gas blown out by exploding stars in the Milky Way, or that it is the remnant of a gas cloud that punched through the galaxy.
Now, 3D computer simulations led by Chang Hyun Baek of Sejong University in Seoul, Korea, bolster the latter possibility.
The simulations show that the cloud's size, shape and velocity can be produced if a hydrogen cloud with 100,000 times the mass of the Sun slams into our spiral-shaped galaxy at 100 kilometres per second and punches through to the other side. The event would have had to occur about 50 million years ago to match the present-day observations.

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Credit: Jayanne English et al/U Manitoba/CGPS

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Title: The Galactic Worm GW 123.4-1.5: A Mushroom-shaped H I Cloud
Authors: English, Jayanne; Taylor, A. R.; Mashchenko, S. Y.; Irwin, Judith A.; Basu, Shantanu; Johnstone, Doug

The Dominion Radio Astrophysical Observatory's Synthesis Telescope provides the highest resolution data (1' and 0.82 km s^-1) to date of an H I worm candidate. Observed as part of the Canadian Galactic Plane Survey, mushroom-shaped GW 123.4-1.5 extends only a few hundred parsecs, contains ~10^5 Msolar of neutral hydrogen, and appears unrelated to a conventional shell or chimney structure. Our preliminary Zeus two-dimensional models use a single off-plane explosion with a modest (~10^51 ergs) energy input. These generic simulations generate, interior to an expanding outer blast wave, a buoyant cloud whose structure resembles the morphology of the observed feature. Unlike typical model superbubbles, the stem can be narrow because its width is not governed by the pressure behind the blast wave or the disk scale height. Using this type of approach, it should be possible to more accurately model the thin stem and other details of GW 123.4-1.5 in the future.

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