Where is the gas in interstellar space?

Speaking at the Royal Astronomical Society National Astronomy Meeting in Preston on Tuesday 17 April, Professor Barstow will present a map of the local interstellar medium, the gas lying between the stars out to distances of about 300 light years from the Sun, made using the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite.

Professor Barstow and his team used FUSE to observe a group of white dwarf stars (compact remnants of stars like our Sun will be at the end of its life). The scientists intended to probe the structure of interstellar space in the vicinity of the Sun by searching for the imprint of oxygen in the ultraviolet light from the stars. However, all the oxygen detected was found to be in the atmospheres of the stars and no interstellar oxygen was found. This implies that, rather than being full of tenuous ionized gas, as expected, this region of interstellar space (the Local Cavity) is actually empty and was probably swept clear by an ancient supernova explosion a few million years ago.

Our present picture of the local interstellar medium is that the Sun and Solar system are embedded in and near the edge of a wispy diffuse cloud, known as the Local Cloud (or Local Fluff). This cloud, which is only 20-30 light years across, is itself in a larger much less dense region called the Local Bubble or Local Cavity.

The gas in the Local Cavity was expected to bear the scars of recent nearby events, such as supernova explosions, and radiation from hot young stars. These would make the cavity gas hot and ionized, with the electrons stripped from the constituent atoms, and should be detected by FUSE. The hot gas should emit also X-rays that are detected as a diffuse background in X-ray telescopes. However, if there is no hot gas present, then we need to find another explanation for this X-ray background. One novel suggestion is that it arises from the exchange of charged particles at the boundary between the Sun’s magnetic field and interstellar space.