New Recipe for Dwarf Galaxies: Start with Leftover Gas

Credit: NASA/JPL-Caltech/DSS
Seeing Baby Dwarf Galaxies

Thursday, February 19, 2009

There is more than one way to make a dwarf galaxy, and NASA's Galaxy Evolution Explorer has found a new recipe. The spacecraft has, for the first time, identified dwarf galaxies forming out of nothing more than pristine gas likely leftover from the early universe. Dwarf galaxies are relatively small collections of stars that often orbit around larger galaxies like our Milky Way.

The findings surprised astronomers because most galaxies form in association with a mysterious substance called dark matter or out of gas containing metals. The infant galaxies spotted by the Galaxy Evolution Explorer are springing up out of gas that lacks both dark matter and metals. Though never seen before, this new type of dwarf galaxy may be common throughout the more distant and early universe, when pristine gas was more pervasive.

Astronomers spotted the unexpected new galaxies forming inside the Leo Ring, a huge cloud of hydrogen and helium that traces a ragged path around two massive galaxies in the constellation Leo. The cloud is thought likely to be a primordial object, an ancient remnant of material that has remained relatively unchanged since the very earliest days of the universe. Identified about 25 years ago by radio waves, the ring cannot be seen in visible light.

"This intriguing object has been studied for decades with world-class telescopes operating at radio and optical wavelengths," said David Thilker of Johns Hopkins University, Baltimore, Md. "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 a recent study, Thilker and his colleagues 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 said. "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."

Our local universe contains two large galaxies, the Milky Way and the Andromeda galaxy, each with hundreds of billions of stars, and the Triangulum galaxy, with several tens of billions of stars. It also holds more than 40 much smaller dwarf galaxies, which have only a few billion stars. Invisible dark matter, detected by its gravitational influence, is a major component of both giant and dwarf galaxies with one exception — tidal dwarf galaxies.

Tidal 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," said Thilker. "This discovery allows us to study the star formation process in gas that has not yet been enriched."

Large, pristine clouds similar to the Leo Ring may have been more common throughout the early universe, Thilker said, and consequently may have produced many dark-matter-lacking, dwarf galaxies yet to be discovered.

The results of the new study reporting star formation in the Leo Ring appear in the February 19, 2009, issue of the journal Nature.

Caltech leads the Galaxy Evolution Explorer mission and is responsible for science operations and data analysis. NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the mission and built the science instrument. The mission was developed under NASA's Explorers Program managed by the Goddard Space Flight Center, Greenbelt, Md. South Korea and France are the international partners in the mission.

Hidden galaxies

photo:Hubble Space Telescope images of five of the newly identified UCDs and a normal dwarf galaxy (dEN) are shown along with their positions in the Fornax galaxy cluster. The positions of the two UCDs not pictured are also shown. Michael Hilker; Background: Arna Karick (Univ. of Melbourne), Curtis Schmidt Telescope; Insets: HST

June 4, 2003

An in-depth study of the nearby Fornax galaxy cluster revealed that some objects previously believed to be foreground stars belong to a new class of galaxy designated "ultra-compact dwarf galaxies" or UCDs. Members of this newly discovered type of galaxy compress their stars into a region so small each system is only two to twenty percent the size of a typical dwarf galaxy. UCDs are 500 times smaller than a normal-sized galaxy, such as the Milky Way with its 100 billion stars spanning about 100,000 light-years.

Astronomers had long thought that existing galaxy surveys were missing special galaxy types, such as very diffuse or very compact galaxies. "Fornax is one of the closest galaxy clusters, yet it is difficult to tell whether a galaxy that appears small is a tiny member of the cluster or is a giant galaxy that lies in the same direction but is much farther away," says Michael Gregg, one of the project's co-leaders from the University of California at Davis and the Lawrence Livermore National Laboratory. The team of astrophysicists published their findings of UCDs in the May 29 edition of Nature.

The scientists used the Anglo-Australian Telescope, Hubble Space Telescope, Very Large Telescope, and Keck Telescope to detect and measure these dense new galaxies. "Our Fornax Cluster Survey used new instruments to measure the distances to about 14,000 objects in the direction of the cluster, enabling us to separate cluster members from background galaxies and foreground stars," says Michael Drinkwater, the team's other co-leader from the University of Queensland in Australia.An instrument known as the Two Degree Field Spectrograph on the Anglo-Australian Telescope targeted the Fornax cluster, which lies about 60 million light-years from Earth and contains 300 known galaxy members. The spectrograph, which can look at up to 400 targets simultaneously, measured 3,500 objects for the study. The scientists found that 1,000 of those are background galaxies. Of the remaining 2,500 objects that were believed to be regular foreground stars, seven actually turned out to be UCDs.

photo:The two insets are Hubble Space Telescope images of a normal dwarf galaxy (upper left) and a recently discovered ultra-compact dwarf galaxy (lower right). The object running diagonally in the middle is a star trail that would result from a tidal disruption of the halo of a normal dwarf galaxy, producing a compact dwarf galaxy. The background shows the Fornax galaxy cluster, where seven ultra-compact dwarf galaxies have been found. Background: Arna Karick (University of Melbourne), Michigan Curtis Schmidt Telescope; Insets: HST


The researchers used the Hubble Space Telescope to precisely measure the galaxies' sizes. The Very Large Telescope and Keck Telescope were then used to determine the speeds of the stars orbiting within the galaxies. With the sizes and star speeds in hand, the astronomers were then able to determine the galaxies' masses and confirm their classification as UCDs.

UCDs may help scientists understand how galaxies in densely populated regions are transformed and destroyed. The team hopes to make additional observations to determine whether UCDs are dense nuclei of once-larger galaxies that have been stripped to the core by close interactions with giant galaxies in the cluster.