and more, the surviving dwarf population provides a fossil record of the early formation conditions when our own Milky Way formed.

 

The least massive, dimmest type of dwarf galaxies are called dwarf spheroidal galaxies. They only have 1/100,000th of the mass of the Milky Way and are merely about a factor of 10 more massive than rich globular star clusters. Dwarf spheroidal galaxies are up to 60,000 times fainter than spiral galaxies such as the Milky Way. Not surprisingly, they are hard to detect. Many were found only in recent years. Invisible to the naked eye, the nine closest dwarf spheroidal galaxies are satellites of the Milky Way with distances from 65,000 to 880,000 light years.

 

Once believed to be simple, old systems, astronomers found these dwarfs to be unexpectedly complex. Owing to advances in instrumentation and detector technology, ground-based 4 to 10 meter (150 to 400 inch) class telescopes of the National Science Foundation's National Optical Astronomy Observatories in Arizona and Chile, of the European Southern Observatory in Chile, and of the W.M. Keck Observatory in Hawaii played a leading role in this research. The superior resolution of the Hubble Space Telescope, operated by Nasa and the European Space Agency, E sa, revolutionized the field by resolving more distant dwarf spheroidals into stars and thus made it possible to study the evolution of dozens of dwarf galaxies in unprecedented detail. These studies led to the determination of distances, age structure and times of major star formation events, element abundances, and kinematics for many dwarf galaxies.

 

“Thanks to the efforts of many researchers in this field, we realize now that even the least massive galaxies show a wide range of evolutionary histories, and that no two dwarfs are alike,” Grebel said. While some low-mass galaxies stopped forming stars 10 billion years ago, others continued to form stars until a few hundred million years or are still active. Some dwarfs converted gas into stars at a fairly constant or gradually declining rate, while others show distinct bursts of star formation. Also the chemical evolution of these low-mass galaxies varies widely. Another puzzling and seemingly contradictory feature is the lack of star-forming material, i.e., gas, although some dwarf spheroidal galaxies experienced star formation in the recent past and thus should have significant amounts of gas. The strong stellar winds from massive stars and supernova explosions following star formation events are insufficient to remove all gas from dwarf galaxies.

 

“Despite the amazing diversity of individual dwarf galaxies common global properties are beginning to emerge that may hold the key to understanding the evolution of these important low-mass objects,” Grebel said. With very few exceptions dwarf galaxies of all types show more metal enrichment the more luminous and massive they are. This suggests that they undergo similar physical processes, and that more massive galaxies can hold on longer to both their gas and metals and use them in subsequent star formation episodes.

 

Furthermore, there is a rough trend for a dwarf galaxy to have a larger fraction of young stars the farther it is from a massive galaxy. The strong gravitational pull as well as ram pressure from the massive galaxy may strip star-forming gases from the nearby low-mass companion galaxies, whereas the more distant companions can retain this material for longer periods and continue to form stars. In accordance with this idea gas-rich dwarf galaxies are not found in the immediate vicinity of a massive galaxy unless they are at least a 100 times more massive than dwarf spheroidals and can thus offer more resistance to the destructive influence of the massive galaxy.

 

“Among the dwarf satellites of the Milky Way and of the Andromeda spiral galaxy at a distance of 2.5 million light years we see galaxies that appear to represent different stages of the proposed evolution from low-mass gas-rich dwarf galaxies toward gas-deficient dwarf spheroidals,” Grebel pointed out.

 

Ongoing studies of these convenient nearby probes, which cover a range of galaxy masses and distances, include also their stellar mass loss history through the tidal effects of the massive galaxies. With forthcoming astrometric space missions such as Nasa 's Space Interferometry Mission and the Global Space Astrometry Mission of the European Space Agency crucial information on companion orbits and overall kinematics will be obtained. Studies of dwarf galaxies in more distant galaxy groups are also on the way to investigate whether the suggestive scenario of mass- and stripping-dominated dwarf galaxy evolution is universally valid.

 

 

 

Editor’s note:

You may contact Eva Grebel at (+49 6221) 528 225 or grebel@mpia-hd.mpg.de.

 

This release is also available on the World Wide Web at http://www.mpia-hd.mpg.de/~grebel/aas196_pressrel_1.html