A Tubular Cavity Penetrating the Local Disk

On May 29, 2003, a team of astronomers (including Barry Welsh, Rosine Lallement, Francoise Crifo, Daphne Sfeir, and and Jean-Luc Vergely) presented the first detailed map of interstellar space within 1,000 ly (300 pc) of the Solar System. They also announced confirmation of the hypothesis that the so-called Local Bubble of the spiral disk's gas clouds actually pierces the disk of the galaxy, perhaps as the result of exploding stars around one or two million years ago (CNRS press release). The tubular cavity of hot, low-density gas was found to be irregular with tunnels branching off through the surrounding dense gas wall, which is suspected of having interconnecting cavities and tunnels like an interstellar sponge, as first suggested by astronmers nearly 30 years ago (Cox and Smith, 1974). The astronomers hypothesize that energetic supernova explosions created fast-moving expanding bubbles of hot gas that collided with the surrounding cold gas of interstellar space, which in turn became compressed into thin shells. Eventually, these shells of cold gas met other expanding hot cavities and broke up to form small tunnels or pathways between the expanding voids (U.C. Berkeley press release; and Welsh et al, 1999). Such "hot chimneys" have been detected in other galaxies.

© U.C. Berkeley, CNRS







Larger illustration.







Regions of extremely low density but

probably hot, ionized gas are shown

in white, while dark areas are large

condensations of cold and dense

gas which surround the local cavity.

These dark "walls" are broken in

several places by low density

"interstellar tunnels" that link the

local cavity with other nearby

bubble cavities such as the

Pleiades and GSH 238+00+09 (more ).



By locating stars that showed no absorption by sodium and those that did, the astronomers were able to construct a three-dimensional picture of the edge of the low-density region surrounding the Solar System by observing 1,005 hot and bluish type A and B stars, whose sodium absorption lines are easy to detect from their spectra. Eventually, measurements towards more distant stars picked up large numbers of sodium atoms, indicating the presence of a dense neutral-gas boundary, or "wall," around the local cavity of hot and sparse, ionised gas. The nearest wall is 175 to 190 ly light years from the Solar System, in the direction of the galactic center. By measuring the motion of sodium gas atoms detected along the 1,005 sight-lines, the astronomers hope to say soon whether the gas wall that surrounds the Local Chimney coming towards the Solar System, or is moving away to enlarge the local cavity. If the wall is approaching the Solar System, then it is likely that a distant explosive force is pushing it towards the Solar System. If the wall is expanding away from the Solar System, however, then a supernova explosion may have taken place near the Solar System about a million years ago.

The Local Chimney has been around for a few million years. It could easily have been caused by a supernova punching through the top and bottom of the galactic disk, the intense stellar winds from 10 or so hot stars, a powerful gamma-ray burst, or even a large star moving through the area. In theory, any of these could have swept dense, neutral gas out of the region, leaving only very tenuous, hot, ionized hydrogen gas.

Assuming that the Local Bubble or Chimney was created by the supernovae of young, massive stars during the past few million years, some astronomers have been looking for their probable source among the 27 member B stars of the Pleiades moving group, which are located towards the nearest part of Gould's Belt of massive hot, OB-type stars (more discussion in pdf). The youngest stars in the galactic region surrounding around the Solar Neighborhood are associated with "subgroup B1" of the Pleiades (M 45) stellar moving group, and astronomers hypothesize that the more massive stars born in this group may have already exploded as 20 or so supernovae over the past 10 to 20 million years as the entire group of stars moved through a nearby region of the Local Bubble (Berghoefer and Breitschwerdt, 2002). The center of mass of these stars is located 135 +/- 15 pc away from the Sun, whereas the distance of the Gould or Gould's Belt of young bright stars in this direction is around 160 pc, whose origins may be 30 to 60 million years old. According to (Asiain et al, 1999, in pdf) the Pleiades B1 subgroup may only be 20 +/- 10 million years old.

Some astronomers wonder if the Local Chimney's system of interlocking, gaseous cavities is characteristic of the entire Milky Way. Astronomers looking at an updated map of the cold, local interstellar gas in the galactic plane see an inner empty region with finger-like extensions or tunnels that poke through the surrounding wall and reach into nearby regions of the galaxy, such as the Lupus-Norma, Auriga-Perseus and Scorpius-Centaurus associations of young stars. These regions are known to contain large amounts of hot and ionized low-density gas that can be detected at radio and ultraviolet wavelengths. The local void of gas extends out of the galactic disk and stretches into the overlying galactic halo region. Thus, the cavity appears as a tube-like "chimney" that links the gas in the galactic disk with that of the halo.

Unknown Artist, FIMS,

Space Science Lab, NASA





Larger illustration.





Supernova explosions can break

through the galactic disk and

inject hot gas into a galactic

corona, perhaps driving a

"galactic fountain", wherein the

gas cools and condenses, and

eventually recycles matter into

the galactic disk (more ).



Galactic chimneys have been widely observed in other galaxies and are thought to be responsible for venting the hot, ionized gas expelled in supernova explosions into galaxy halos. Active vents seen in other galaxies have been called "galactic fountains" while the gas falling back into the disk have been dubbed "galactic rain." Astronomers searching for signs of hot gas in the Local Chimney with the NASA FUSE satellite have detected hot gas that is coming towards the Solar System, which could be part of a fountain that is falling back onto the galactic plane.