New recipe for

dwarf galaxies

ASTRONOMY NOW

Posted: 20 February, 2009

NASA’s Galaxy Evolution Explorer has uncovered a new recipe for making dwarf galaxies using pristine gas left over from the early Universe.

Dwarf galaxies are relatively small collections of stars that often orbit around larger galaxies like our Milky Way. They typically contain a population of just a few billion stars, compared with the sprawling cities of large galaxies, like the Milky Way, that contain several hundred billion stars.

The formation of galaxies usually requires the influence of the mysterious substance known as dark matter, or gases containing metals, but in the case of the galaxies spotted by the Galaxy Evolution Explorer (GALEX) in the constellation Leo, they appear to be forming out of gas that lacks both dark matter and metals. Though never seen before, the astronomers working on the project, led by David Thilker of the Henry A. Rowland Department of Physics and Astronomy at The Johns Hopkins University, suspect that this new type of dwarf galaxy may be common throughout the more distant and early Universe, when pristine gas was more pervasive.

Forming dwarf galaxies (in circles) are distinctively blue, indicating they are primarily detected in far ultraviolet light. The faint blue overlay traces the outline of the Leo Ring, a huge cloud of hydrogen and helium that orbits around two massive galaxies in the constellation Leo. Image: NASA/JPL-Caltech/DSS.

The galaxies were seen forming inside the Leo Ring, an expansive cloud of hydrogen and helium that traces an uneven path around two massive galaxies within the constellation Leo. The cloud is thought to be an ancient remnant of material that has remained relatively unchanged since the very earliest days of the Universe.

"This intriguing object has been studied for decades with world-class telescopes operating at radio and optical wavelengths," says Thilker. "Despite such effort, nothing except the gas was detected.  No stars at all, young or old, were found. But when we looked at the ring with the Galaxy Evolution Explorer, which is remarkably sensitive to ultraviolet light, we saw telltale evidence of recent massive star formation. It was really unexpected. We are witnessing galaxies forming out of a cloud of primordial gas."

In another recent study, Thilker and his team found the ultraviolet signature of young stars emanating from several clumps of gas within the Leo Ring. "We speculate that these young stellar complexes are dwarf galaxies, although, as previously shown by radio astronomers, the gaseous clumps forming these galaxies lack dark matter," he says. "Almost all other galaxies we know are dominated by dark matter, which acted as a seed for the collection of their luminous components - stars, gas, and dust. What we see occurring in the Leo Ring is a new mode for the formation of dwarf galaxies in material remaining from the much earlier assembly of this galaxy group."

The Leo Ring visible image represents the survey's blue, red, and infrared bands. The overlay indicating the location of hydrogen gas in the Leo Ring is based on observations made at the Arecibo Observatory in Puerto Rico. Image: NASA/JPL-Caltech/DSS.

Invisible dark matter, detected by its gravitational influence, is a major component of both giant and dwarf galaxies with one exception — tidal dwarf galaxies. This sub division of dwarf galaxies condense out of gas recycled from other galaxies and have been separated from most of the dark matter with which they were originally associated. They are produced when galaxies collide and their gravitational masses interact. In the violence of the encounter, streamers of galactic material are pulled out away from the parent galaxies and the halos of dark matter that surround them.

Because they lack dark matter, the new galaxies observed in the Leo Ring resemble tidal dwarf galaxies, but they differ in a fundamental way. The gaseous material making up tidal dwarfs has already been cycled through a galaxy. It has been enriched with metals — elements heavier than helium — produced as stars evolve. "Leo Ring dwarfs are made of much more pristine material without metals," says Thilker. "This discovery allows us to study the star formation process in gas that has not yet been enriched."

Thilker tells Astronomy Now: "It might be that the activity is stimulated by the influence of M96 on the ring." M96 is a spiral galaxy in Leo. "However, the interaction also could be irrelevant, and star formation may be able to quiescently proceed throughout the entire ring, even away from M96)." The team have planned a series of sensitive follow-up UV observations of the entire ring to check this hypothesis. "Self-gravity of the baryonic, gaseous matter in each HI clump may be just enough to initiate the compression process, leading to molecular gas and evidently star formation too," he adds.

Thilker also comments that the discovery of this new route to galaxy formation does not replace more conventional processes that rely on dark matter, but instead augments it, clarifying the fate of gaseous leftovers from cosmological structure formation. "Remember that the new detections are small dwarf galaxies, forming stars at an extremely mundane rate with very low efficiency," he says. "I think the bulk of evidence from previous studies still strongly supports the role of dark matter in shaping the well-known galaxy population."

The results of the new study appear in the 19 February issue of the journal Nature.