Puzzle Of Galactic Evolution Solved

Writing this week in the scientific journal Nature, Bart P. Wakker, a University of Wisconsin-Madison astronomer, and colleagues have chipped away at a three-decade-old mystery about the nature and role of what astronomers call high-velocity clouds. In the process, they've discovered a mechanism by which the galaxy is seeded with the stuff of stars and solved a long-standing question of galactic evolution.

Discovered 35 years ago, the clouds were an enigma because they behave differently than most galactic objects -- coursing through space at high speeds and not neatly rotating along with the rest of the galaxy. Moreover, scientists could never pinpoint their exact location, with distance estimates ranging from a few hundred light years to 10 million light years, an estimate that would place them well beyond the pale of the Milky Way.

But improved instrumentation, such as new large ground-based telescopes and the orbiting Hubble Space Telescope, recently allowed astronomers to determine that one such cloud lies about 20,000 light years from Earth in the halo of the Milky Way, a region high above the star-studded plane of the galaxy. And now, with the help of Hubble, Wakker and his colleagues have provided astronomers with measurements that reveal an inventory of some of the heavy elements in another high-velocity cloud which, they suggest, lies between 10,000 and 40,000 light years above the plane of the galaxy.

The new evidence, said Wakker, strongly suggests that some of the clouds play a key role in the chemical evolution of the galaxy by showering it with metal-poor gas that counteracts a buildup of heavy elements within the stars and gas found in the disk of the Milky Way.

Every star in the Milky Way was born of gas millions or billions of years ago, and they constantly turn hydrogen into helium or heavier elements like metals. They shed these metals back into interstellar space, a scenario that suggests recently-formed stars should be richer in metals than old stars. Yet astronomers observe that most stars in the disk of the galaxy have similar heavy element concentrations no matter how old or young they are.

"This rain of gas," Wakker said, "is material that has never been in the Milky Way before, suggesting, then, that metal production by stars is offset by this influx of metal-poor gas."

The new Hubble observations conform to a popular theory that there is a continual inflow of material into the galaxy to account for the continuing formation of stars, as well as their chemical composition. Competing theories -- ranging from the idea that, in the past, stars may have been more efficient at producing heavy elements to the notion of unknown processes at work -- can now be discarded, Wakker said.

"You don't need any other explanations anymore," he said, "because we now know that this gas is raining down onto the plane of the galaxy."

The finding also explains how the Milky Way can create, on average, a new star each year without running out of its supply of gas after a tenth of its lifetime.

The cloud observed by Wakker is estimated to contribute about one-fifth of a solar mass per year. But there are other such high-velocity clouds, Wakker noted, that can provide the balance of new gas needed for the galaxy to form a new star each year.

The origin of this accreting, low-metal gas remains a mystery. It could be gas left over from the formation of the so-called Local Group of galaxies that includes the Andromeda Nebula. Alternatively, it may be that the Milky Way is still forming, continuously gathering gas from near the edge of its sphere of influence. Or, said Wakker, the clouds might have been stripped away from passing dwarf galaxies.

###

NOTE TO PHOTO EDITORS: A high-resolution image is available for downloading at: http://www.news.wisc.edu/newsphotos/wakker.html