http://physicsweb.org/articles/news/10/6/3/1?rss=2.0 ===sample quote=== Levine and colleagues took previously obtained data from the hydrogen "21-cm emission lines" and applied a technique called unsharp masking to increase the contrast between high and low signal regions. They found that the spiral arms reach out as far as 25 kpc, or 80,000 light-years, away from the centre of the galaxy. "We have been able to trace the spiral structure of the entire disk of gas beyond the orbit of the Sun around the Galactic Center to the edge of the disk," explains Blitz. "This gives the clearest and most complete picture of the spiral structure of the disk so far, and shows that the spiral structure continues well beyond the radius at which we expect stars to be. This is not expected from theory, and will require a deeper understanding of the origin of spiral structure." ... The sharpened map of the galaxy also raises new questions, such as how gravity affects the spiral structure in galaxies. The spiral pattern in the Milky Way is like most other "grand design" spirals in that it is approximately logarithmic, or shaped like a cyclone. Scientists believe that the spiral structure is driven by the gravity of the disk. "But we are seeing spiral structure even where gravity seems to be too weak to drive it. Why? How?" asks Blitz. ===endquote=== http://arxiv.org/abs/astro-ph/0605728 The Spiral Structure of the Outer Milky Way in Hydrogen E.S. Levine, Leo Blitz, Carl Heiles 10 pages with 4 color figures. Accepted for publication in Science. Embargoed for discussion in the popular press until publication in ScienceXpress. Higher resolution versions of figures 3 and 4 are available at this http URL "We produce a detailed map of the perturbed surface density of neutral hydrogen in the outer Milky Way disk demonstrating that the Galaxy is a non-axisymmetric multi-armed spiral. Spiral structure in the southern half of the Galaxy can be traced out to at least 25 kpc, implying a minimum radius for the gas disk. Overdensities in the surface density are coincident with regions of reduced gas thickness. The ratio of the surface density to the local median surface density is relatively constant along an arm. Logarithmic spirals can be fit to the arms with pitch angles of 20-25 degrees."