New Recipe for Dwarf Galaxies: Start with Leftover Gas

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Apparently, dwarf galaxies can spring out of thin air.

Astronomers using NASA’s Galaxy Evolution Explorer have spotted unexpected new galaxies in the constellation Leo that appear to be forming out of nothing more than pristine gas, probably leftover from the early universe.  The gas lacks both dark matter and metals — previously thought to be building blocks for galaxy formation.

Dwarf galaxies are relatively small collections of stars that often orbit around larger galaxies like our Milky Way. Though never seen before, the researchers say this new type of dwarf galaxy may be common throughout the more distant and early universe, when pristine gas was more pervasive. Their discovery appears in this week’s issue of the journal Nature.

The newly described dwarf galaxies are in 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 lead study author David Thilker of Johns Hopkins University in Baltimore. He added that no stars were ever seen in the gaseous regions before. 

“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.”

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,” Thilker said. “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 dwarf galaxies yet to be discovered that also lack dark matter.

Source: Caltech