University of California-Berkeley<br /><br />Media Contacts:<br /><br />Robert Sanders<br />(510) 643-6998, (510) 642-3734<br />rls@pa.urel.berkeley.edu<br /><br />Additional Resources:<br /><br />Barry Welsh<br />(510) 642-0305, bwelsh@ssl.berkeley.edu<br /><br />FOR IMMEDIATE RELEASE: Thursday, May 29, 2003<br /><br />3-D map of local interstellar space shows sun lies in middle of hole piercing <br />galactic plane<br /><br />Nashville, Tenn. -- The first detailed map of space within about 1,000 light <br />years of Earth places the solar system in the middle of a large hole that <br />pierces the plane of the galaxy, perhaps left by an exploding star one or two <br />million years ago.<br /><br />The new map, produced by University of California, Berkeley, and French <br />astronomers, alters the reigning view of the solar neighborhood. In that <br />picture, the sun lies in the middle of a hot bubble -- a region of <br />million-degree hydrogen gas with 100-1,000 times fewer hydrogen atoms than the <br />average gas density in the Milky Way -- and is surrounded by a solid wall of <br />colder, denser gas.<br /><br />Instead, said astronomer Barry Welsh of UC Berkeley's Space Sciences Laboratory, <br />the region around the sun is an irregular cavity of low-density gas that has <br />tunnels branching off through the surrounding dense gas wall. Welsh and his <br />French colleagues suspect that the interconnecting cavities and tunnels, <br />analogous to the holes in a sponge, were created by supernovas or very strong <br />stellar winds that swept out large regions and, when they encountered one <br />another, merged into passageways.<br /><br />"When we started mapping gas in the galaxy, we found a deficit of neutral gas <br />within about 500 light years, suggesting that we are in a bubble-shaped cavity <br />perhaps filled with hot, ionized gas," Welsh said. "But the Local Bubble is <br />shaped more like a tube and should be called the Local Chimney."<br /><br />If this system of interlocking, gaseous cavities is characteristic of the entire <br />galaxy, it presents a dramatic confirmation of a 30-year-old theory of the Milky <br />Way, Welsh said.<br /><br />Welsh is presenting the findings on Thursday, May 29, at the American <br />Astronomical Society meeting in Nashville, Tenn.<br /><br />At the moment, the origin of the cavities is anybody's guess, Welsh said. The <br />local cavity has been around for a few million years and could easily have been <br />caused by a supernova punching through the top and bottom of the galactic disk, <br />the intense stellar winds from 10 or so hot stars, a powerful gamma-ray burst, <br />or even a large star moving through the area. Each of these could theoretically <br />sweep dense gas out of the region, leaving only tenuous, ionized hydrogen.<br /><br />Three recently developed satellites could shed light on the mystery. The Cosmic <br />Hot Interstellar Plasma Spectrometer (CHIPS) satellite, built at UC Berkeley's <br />Space Sciences Laboratory, was launched last December to look for hot, <br />500,000-degree Celsius gas near our solar system. The UC Berkeley-built SPEAR <br />(Spectroscopy of Plasma Evolution from Astrophysical Radiation) instrument, to <br />be launched later this year as the primary payload of the Korean KAISTSAT-4 <br />satellite mission, will detect the presence of warm gas -- about 250,000 degrees <br />Celsius -- in the solar neighborhood. NASA's Far Ultraviolet Spectroscopic <br />Explorer (FUSE) satellite is also currently searching for this hot gas.<br /><br />With only ground-based telescopes at their disposal, Welsh and his colleagues <br />could not look directly for cold neutral hydrogen (H), since the density is <br />about 10 times too low for radio telescopes to detect. Instead, they looked for <br />a surrogate -- cold neutral sodium, which is found wherever cold, dense hydrogen <br />is found. Using five separate telescopes, they searched for the cavity walls <br />where the density of cold neutral sodium becomes high enough to detect.<br /><br />"We used several ground-based telescopes, including the Observatoire de Haute <br />Provence in France, the European Southern Observatory in Chile and the Lick <br />Observatory in California, to detect atoms of gas in interstellar space towards <br />over a 1,000 nearby stars," said Dr. Rosine Lallement, the project leader at the <br />Centre National de la Recherche Scientifique (CNRS) in Paris. "In collaboration <br />with Dr. Barry Welsh at UC Berkeley, this project has taken over five years to <br />accumulate and analyze all the data."<br /><br />A key factor in mapping the local interstellar space was recent data from the <br />European Hipparcos satellite, which has provided highly accurate distances to <br />nearby stars, improving significantly over distances obtained through <br />ground-based measurement of parallax.<br /><br />By locating stars that showed no absorption by sodium and those that did, they <br />were able to construct a three-dimensional picture of the edge of the <br />low-density region surrounding our solar system. The 1,005 stars they looked at <br />were all hot, blue Type A and B stars, because it's easier to pick out sodium <br />absorption lines from their spectra.<br /><br />"Eventually, our measurements towards more distant stars started to pick up <br />large numbers of sodium atoms, indicating that we had stumbled across a dense <br />neutral-gas boundary, or 'wall,' to our local cavity," Welsh said. The nearest <br />wall is 175-190 light years from Earth, in the direction of the center of our <br />galaxy.<br /><br />"This thin shell of dense gas surrounding the local void is broken in many <br />places," said Dr. Francoise Crifo, an astronomer at the Paris Observatory. "In <br />several directions in the galaxy, our local cavity seems to be linked with other <br />similar empty regions by pathways or tunnels in the interstellar medium."<br /><br />The existence of a network of tunnels of hot gas that thread interstellar space <br />was first suggested nearly 30 years ago by Don Cox and Barry Smith of the <br />University of Wisconsin. In their model, energetic galactic supernova explosions <br />create fast-moving expanding bubbles of hot gas that collide with the <br />surrounding cold gas of interstellar space, which in turn becomes compressed <br />into thin shells. Eventually, these shells of cold gas meet other expanding hot <br />cavities and break up to form small tunnels or pathways between the expanding <br />voids, Welsh said.<br /><br />"If you look at the new map of cold, local interstellar gas in the galactic <br />plane, you see an inner empty region with finger-like extensions or tunnels that <br />poke through the surrounding wall and reach into nearby regions of our galaxy, <br />such as the Lupus-Norma, Auriga-Perseus and Scorpius-Centaurus associations of <br />young stars," he said. "These regions are known to contain large amounts of hot <br />and ionized low-density gas that can be detected at radio and ultraviolet <br />wavelengths."<br /><br />The new results also show that the local void of gas extends out of the galactic <br />disk and stretches into the overlying galactic halo region. In this view of the <br />galaxy, the cavity appears as a tube-like "chimney" that links the gas in the <br />galactic disk with that of the halo. Galactic chimneys have been widely observed <br />in other galaxies and are thought to be responsible for venting the hot and <br />ionized gas expelled in supernova explosions into galaxy halos. Active vents <br />seen in other galaxies have been labeled "galactic fountains" and the gas <br />falling back into the disk, "galactic rain."<br /><br />"We have been searching for signs of hot gas in this local chimney using the <br />NASA FUSE satellite, and so far, we have found tentative signs of hot gas that <br />seems to be coming towards us, which could be part of a fountain that is falling <br />back onto the galactic plane," Welsh said.<br /><br />The results of the research are soon to be published in the journal Astronomy <br />and Astrophysics, along with a 3-D map of the distribution of cold sodium gas <br />out to 1,000 light years. Lallement noted, however, that the project is not yet <br />complete.<br /><br />"We also have information on the motion of the atoms of sodium gas we have <br />detected along the 1,005 sight-lines," she said. "Soon we will be able to say <br />whether the gas wall that surrounds our local void is coming towards the sun and <br />squeezing our local interstellar space, or whether it is moving away from us <br />such that the local void is getting larger."<br /><br />"Either of these scenarios is fascinating," Welsh said. "If the wall is <br />approaching us, it means that a distant explosive force is pushing it towards <br />us. If it is expanding away from the sun, then it seems possible that a <br />supernova explosion took place about a million years ago that was located <br />relatively close to our sun."<br /><br />Other colleagues are Daphne Sfeir, who published a preliminary map in 1999 using <br />350 stars, and J. L. Vergely of ACRI, a high-tech company based in <br />Sofia-Antipolis, France. The group used data from two other telescopes -- the <br />Kitt Peak telescope in Arizona and the recently destroyed Mt. Stromlo telescope <br />in Australia.<br /><br />The research was supported by a National Aeronautics and Space Administration <br />contract with the Johns Hopkins University.<br /><br />IMAGE CAPTIONS:<br /><br />[Image 1:<br />http://www.berkeley.edu/news/media/releases/2003/05/images/planar.jpg (52KB)<br />Hi-res JPEG file:<br />http://www.berkeley.edu/news/media/download/2003/05/planar.jpg (1.3MB)<br />Layered Photoshop file:<br />http://www.berkeley.edu/news/media/download/2003/05/planar.psd ]<br />New map of the cold and dense interstellar gas surrounding the local cavity in <br />the plane of the galaxy. White areas represent regions of extremely low gas <br />density (which are probably filled with hot and ionized gas), whereas dark areas <br />reveal where large condensations of cold and dense gas occur. Notice that the <br />local cavity is surrounded by many of these condensations, but this "wall" is <br />broken in several places by low density 'interstellar tunnels' that link the <br />local cavity with other nearby bubble cavities such as the Pleiades and GSH <br />238+00+09.<br /><br />[Image 2:<br />http://www.berkeley.edu/news/media/releases/2003/05/images/polar.jpg (36KB)<br />Hi-res JPEG file:<br />http://www.berkeley.edu/news/media/download/2003/05/polar.jpg (1.5MB)<br />Layered Photoshop file:<br />http://www.berkeley.edu/news/media/download/2003/05/polar.psd ]<br />The local cavity, when viewed side-on, reveals a tube-like chimney that extends <br />from the galactic plane up into the lower halo region of our galaxy. Such <br />chimneys can act as vents for energetic hot gas produced in supernovas.