WINSTON-SALEM, N.C. -- Where there is smoke, there is fire. And when a galaxy smokes, supernovas are usually involved, or so astronomers thought.
But now, in a stunning picture that reveals a distant galaxy entangled in a vast web of dust clouds, astronomers are rethinking how galaxies "pollute" their surrounding halos, ejecting dark clouds thousands of light years in extent and collectively capable of enmeshing entire galaxies.
"It's a lot cloudier than we expected in to be," said Blair D. Savage, a University of Wisconsin-Madison astronomer who, with graduate student Christopher Howk, captured the image of the galaxy NGC 891 with the WIYN Telescope, a state-of-the-art telescope operated by a consortium of universities and the National Optical Astronomy Observatories (NOAO) atop Kitt Peak, Ariz. NOAO is an arm of the National Science Foundation (NSF).
Situated 30 million light years from Earth, NGC 891 is a spiral galaxy like the Milky Way, yet its orientation is such that from Earth its central plane can be viewed edge-on. Using the 3.5 meter WIYN Telescope, which has a wide field of view, the Wisconsin astronomers discovered a galaxy-enveloping network of dust clouds that seem to emanate from many regions of the galaxy disk.
"What we found," said Savage "was a highly polluted atmosphere of this galaxy. The hundreds of clouds are irregularly distributed and have a wide variety of shapes and extents" with some found as far as 5,000 light years above the galaxy's central disk.
The galactic dust clouds are composed mostly of hydrogen, helium and a very small, solid grain of carbon and silicate dust. Astronomers think the extensive clouds of dust that permeate the space between stars within galaxies is, literally, stardust, the ejected remains of stars that died long ago in peaceful and violent events known as supernovas.
It was long believed -- and it may still be the case -- that supernova events, heating galactic space to temperatures of a million degrees or more, propelled some dust in chimney-like fashion up into the outer reaches or halos of galaxies. But finding a massive network of dust clouds in the halo of the galaxy was a surprise, said Savage, because it was believed the fragile interstellar dust grains would be consumed by the hot gases produced in violent supernova explosions.
The discovery of networks of clouds interwoven throughout the galactic halo of NGC 891 suggests that the picture is more complicated, or that other, gentler kinds of processes may be at work, said Howk.
"If the ejection process was just supernova-driven, we might expect to see just a few galactic chimneys" where dust in hot, over-pressurized regions is funneled up from the plane of the galaxy, Howk said. In fact, in some places the dust clouds appear isolated, while in other places they connect with active regions of the galaxy in chimney- like structures.
One possible explanation for the widespread distribution of the dust clouds, said Savage, is that the dust is also carried high into the halo by the gentle pressure of starlight. A similar process involving the pressure of sunlight produces the beautiful tails of dust observed to extend from comets as they orbit the sun.
The existence of the clouds in the halo of NGC 891, at the least, provides astronomers with a completely new way of studying the flow of matter from the disks into the halos of spiral galaxies, said Savage.
The telescope used to make the discovery, known as WIYN, is one of a new generation of ground-based telescopes making important fundamental contributions to understanding space and the objects that populate it. WIYN is operated by a consortium of universities including the University of Wisconsin-Madison, Indiana and Yale Universities, and the National Optical Astronomical Observatories, an arm of the National Science Foundation charged with managing and operating the suite of telescopes situated on Kitt Peak.
Contact: Blair D. Savage
Or: Christopher Howk
Editor's note: Images of the NGC 891 galaxy are posted at