For Release: 10:00 a.m. CST Jan. 8, 1999 An Atomic Hydrogen Mushroom Cloud Bursts out of our Galaxy New radio images catch a mushroom-shaped gas cloud bursting over 1,000 light-years out of the disk of our Milky Way galaxy, propelled by the power of 500 suns. These observations provide the first detailed view of the gas columns which appear to link the disk with altitudes high in our galaxy's atmosphere. They will help reveal how heat, radiation, dust, and gas could all rise away from their cradles, which are in clusters of igniting and dying stars, and in doing so contribute to the formation of our galaxy's hot gas halo. Although theories exist which model the chimney-like behaviour of gas column clouds, the preliminary measurements from these Dominion Radio Astrophysical Observatory (DRAO) images challenge these current scenarios. The images were unveiled today at the American Astronomical Society meeting in Austin, Texas by Drs. Jayanne English (STScI; formally of Queen's University), Russ Taylor (University of Calgary), Judith Irwin (Queen's University), and Sergey Mashchenko (University of Laval) who are participants the Canadian Galactic Plane Survey/Releve Canadien du Plan Galactique (CGPS/RCPG). The CGPS/RCPG project is not only a consortium of Canadian universities and institutes, it also has international components in Britain, China, and the United States. However the Dominion Radio Astrophysical Observatory, in Penticton, British Columbia, operated by the Herzberg Institute of Astrophysics of the National Research Council of Canada (with support from the National Science and Engineering Research Council), plays a significant role in this research. The resolution of DRAO images is more than a factor of 10 better than that in any previous study of the Milky Way. Emanating southward from the Perseus Arm, at a distance of about 12,000 light-years, this atomic hydrogen cloud (previously catalogued as GW 123.4-1.5) worms away from the midplane of our galaxy like other gas columns (which were first discovered by Carl Heiles (1984) and knicknamed "worms"). This cloud extends away from the disk for at least 1,100 light-years (340 pc) like mushroom-shaped smoke. (See Figure 1.) The power required to expel this gas from the disk of the Milky Way to this height is at least that of the radiation from 500 suns. Some astrophysicists propose that this cloud is a superbubble powered by many dying, exploding stars (supernovae) concentrated in a neighbourhood near the base of the cloud. Others suggest such gas columns are generated when bubbles from supernovae at random locations mix together or when fast moving gas clouds collide with the disk. DRAO is the first observatory to produce images that are detailed enough to discriminate between these and other theories. Additionally, the DRAO observations delineate delicate curly-ques along the surface of the cloud which suggests it is dissolving into the surrounding hot gas. Although gas clouds such as the mushroom may not be identical to open topped "chimneys", i.e. cavities in the interstellar medium, they may still be conduits for hot gas and radiation. For example, this cloud of neutral hydrogen gas and dust appears to have a narrow, hollow stem and the preliminary analysis indicates that gas flows along this dynamic structure. "Although the stem ends in a cap," emphasizes English, "this stretches into more rarefied parts of the galaxy's atmosphere and the cold cloud gas seems to be mixing with the surrounding warmer atmosphere, just like refrigerated cream curls in hot coffee. This turbulent mixing is causing the cap to crumble into fragments and grow wispy, producing gaps which allow energetic radiation and gas to escape to high galactic altitudes." Taylor notes "In addition to revealing that material is transported, the mushroom may also be employed as a probe of the galactic atmosphere, since the chaotic behaviour of the gas may depend on the characteristics of the environment." Remarkably, the general mushroom shape of this cloud was one of the morphologies predicted by theorists who study the intriguing question of how our galaxy acquired a halo of hot gas and energetic particles. (See Figure 2.) Many favour the notion that exploding stars could create an expanding bubble which would burst into a second bubble high in the Milky Way's atmosphere. In this superbubble (also called supershell) scenario the cloud's stem would be the first bubble and the cap would be identified as the second bubble. The theories also propose that the second bubble could possibly break apart, allowing the hot gas produced during star death to escape into the galaxy's halo. However the DRAO observations are the first which are detailed enough to constrain various theories and in fact challenge the details of this particular model. Therefore the astronomers expect to harvest insights from many different scenarios before a definitive picture evolves about the formation of worm-like gas clouds and what their contribution may be to the formation of structure in galaxies like the Milky Way. The high-sensitivity DRAO images, with ~3 light-yr (~1 arcmin = ~1 pc) angular resolution, are the product of 7 9-metre antennas on a 600 metre long track, plus one 26-metre single-dish antenna. For observations of neutral hydrogen, such as these, this telescope also provides information about the flow of gas towards or away from the sun (with a velocity resolution of about 1 kilometre per second). Irwin states "These CGPS/RCPG images are exquisite. They are allowing Canadian astrophysicists to characterize for the first time a unique mushroom-shaped gas cloud bursting from the Milky Way disk. Although it's formation history is currently unclear, the highly detailed DRAO observations will permit a rigorous examination of current theories and improve our understanding of how worm-like clouds interact with our galaxy's hot gas halo." CONTACTS: Dr. Jayanne English (410-338-4352; jenglish@stsci.edu) Dr. Russ Taylor (403-220-6633; russ@ras.ucalgary.ca) EDITORS: The 2 figures and the text are available over the Internet at http://nemesis.stsci.edu/~jenglish/ Figure 1. is the image of the mushroom cloud. Figure 2. is a diagram of the superbubble scenario. (For reviews of theories about gas columns see Tenorio-Tagle & Bodenheimer 1990, A & A, V.237, P.207 or Mac Low 1998, in publication.)