Stargazers call a prominent dark black region in the Southern Hemisphere's night sky the Coalsack. Even for naked-eye observers, the cloud of cold gas that makes up the Coalsack is hard to miss: It covers a part of the misty luminescence of the Milky Way, blocking out the band of distant stars in the disk of our galaxy with deep black shades that earned the Coalsack its name.
However, a newly discovered aspect of the Coalsack may soon have astronomers thinking of it more like a treasure chest. At an American Astronomical Society Meeting in Nashville last month, astronomers revealed evidence that the Coalsack has hot gases on its perimeter, a finding that means the Coalsack will likely provide many outstanding opportunities to learn more about interactions between regions of hot and cold gas, processes that are essential to star formation and distribution of the elements that make up life forms and the planets. Findings from the Coalsack also may help scientists refine their models of energy production in the galaxy.
"Every once in a while nature gives us a break and sets things up so that we can study the key processes fairly easily," said B-G Andersson, associate research scientist in the Krieger School of Arts and Sciences at Johns Hopkins and lead author of the AAS presentation. "For astronomers, this is a bit like finding a living dodo--instead of trying to make inferences about how the dodo walks, which is what we normally have to do, we can get direct, detailed observations of it walking."
The Coalsack is relatively nearby in cosmological terms, about 650 light-years away in the same spiral arm of the Milky Way Galaxy as Earth. Because it is close by, structures in the Coalsack can be studied in great detail. It's also nicely backlit from Earth's point of view by bright stars in the next arm of the Milky Way Galaxy, allowing scientists to use spectroscopy to acquire a fairly comprehensive sense of the chemical ingredients of the cloud.
The hot gas on the perimeter may indicate that the Coalsack is contained within a region of active massive star formation and supernovae known as the Upper Centaurus- Lupus super-bubble. This region has produced large hot stars that burn out quickly and die explosively, sometimes heating interstellar gas to high temperatures. Naked-eye observers can still see the darkness of the cold gas in the Coalsack because most of the surrounding hot gas is too warm to emit light in the visible portion of the electromagnetic spectrum.
Andersson and co-authors David Knauth and Robin Shelton of Johns Hopkins, S.L. Snowden of Goddard Space Flight Center and Peter Wannier of the Jet Propulsion Laboratory built the case for their finding with data taken by the orbiting Far Ultraviolet Spectroscopic Explorer and Roentgen Satellite observatories, known respectively as FUSE and ROSTAT. They had been using FUSE to study how cold gas clouds dissipate at their edges when their observations of the Coalsack came back with signs of oxygen VI, oxygen atoms with five of their eight surrounding electrons stripped away.
Astronomers know that considerable energy is required to knock that many electrons off an oxygen atom and as a result have long interpreted the presence of oxygen VI as a sign that very hot gases are entering a cooling process. How this cooling process occurs--through turbulent mixing of the gas with colder gas clouds or through conduction of electrons--is a topic of debate among astronomers that further study of the Coalsack may help resolve.
Tipped off by the FUSE readings, Andersson and his colleagues analyzed X-ray data from ROSAT, which surveyed X-ray emissions from the entire sky, and found that the perimeter of the Coalsack, particularly its southeast edge, lights up in X-rays--another sign of the hot gas cloud. Through further analysis, they were able to show that the two readings appeared to be coming from the same region around the Coalsack.
"If our model of the Coalsack is right, then you can use it to test various theories of oxygen VI generation, and this may help us better understand what are the mechanisms behind previously detected oxygen VI production in many more distant parts of the galaxy," Andersson commented. "This could tell us something about energy production in the galaxy, and that could in turn tell us more about star formation."
Andersson noted that astronomers have been studying the Coalsack since the early 19th century, and plenty of good data on many features of the cloud are already available.
"This is starting to look like a really good laboratory for conducting these kinds of experiments," he said. "There are certainly other cases where oxygen VI has likely been associated with a cloud, but never before have we had such a nearby cloud with a reliable distance determination or as many background stars behind the cloud to allow us to look at absorption readings."
This research was funded by NASA.