Tiny galaxies once roared in the universe, say scientists

Astronomers led by the University of Colorado and Carnegie Observatories have shown that a miniature galaxy less than one-hundredth the size of the Milky Way is ejecting large quantities of gas and energy into huge regions of intergalactic space.

“This discovery suggests tiny galaxies that appear very faint and dormant today were once much brighter and more active,” said CU-Boulder graduate student Brian Keeney. “It also indicates similar galaxy systems may have been primarily responsible for the chemical evolution of the universe in the very early stages of galaxy evolution,” said Keeney, who presented the results of the research at the American Astronomical Society Meeting held in Nashville, Tenn., May 25 through May 29.

CU-Boulder teamed up with the Carnegie Institution in Washington, D.C., and East Tennessee State University using the Hubble Space Telescope and ground-based telescopes to make a series of observations. Ray Weymann of the Carnegie Institution led a team that used the electromagnetic spectrum from the brightest quasar in the sky, 3C273, to discover a dense cloud of gas in the far reaches of intergalactic space.

Subsequent observations of the cloud showed it contained elements formed in stars and ejected into space by supernova explosions, he said. There was no known source nearby that could have contributed the ancient elements to this gas.

After several years of searching for the source of this intergalactic “pollution”, a team led by CU-Boulder Professor John Stocke and Weymann discovered a tiny “dwarf galaxy” so small that it had been previously overlooked.

Better images and a detailed spectral analysis obtained by Stocke and Keeney at the Apache Point 3.5-meter Telescope in New Mexico showed strong evidence that this tiny galaxy was responsible for forming the gas cloud.

Some of the strongest evidence is the abundance of elements in the gas cloud and of the stars in the galaxy match, Keeney said.

In addition, an unusual “overabundance” of the element silicon in the gas cloud suggests that thousands of supernovas — the type created when massive stars die –were the source of the gas cloud. A spectral analysis of the dwarf galaxy by Stocke and Keeney showed the dwarf galaxy probably experienced a massive “burst” of star formation some 2 billion to 3 billion years ago, and the ejected gas cloud has since traveled 250,000 light-years to to the location where it is today.

The event may have created thousands of supernovas of the type that create the overabundance of silicon, said Keeney. “Two to three billion years is plenty of time for stars in the ‘starburst galaxy’ to die and create supernovas, and for the gas to reach its current location between us and 3C273.

“Because the large numbers of supernovas made by the dwarf’s starburst blew all of the gas into the surrounding intergalactic space, there likely will be no further star formation in the galaxy,” Keeney said. Theoretical models predict the dwarf galaxy will continue to fade to only about 10 percent of its current brightness. After another few billion years, the dwarf is expected to be so faint that it will be comparable to the smallest and faintest galaxies, known as “dwarf spheroidals.”

Not only are these small objects the most numerous of all galaxy types today, but there also may have been a much larger number of them in the past, said Stocke. Current theories of galaxy formation suggest in the early history of the universe, all stars were formed in tiny galaxies like this one, most of which then merged together and became incorporated into larger galaxies.

“So our own Milky Way probably was created by mergers of smaller galaxies like this one,” said Keeney. “If this is correct, and if all dwarf spheroidals went through an active starburst phase, a large portion of intergalactic space could have been enriched with gas without any help from more massive galaxies like the Milky Way.

“They may be tiny,” Keeney said, “but they are so numerous that their collective effects may be more important in the chemical evolution of the universe than much larger galaxies like our own.”

Project team members include Keeney, Stocke and Kevin McLin of CU-Boulder’s astrophysical and planetary sciences department, Weymann of the Carnegie Institution and Professor Mark Giroux of East Tennessee State University.

Additional observations were made with the Carnegie Institution’s Las Campanas 2.6-meter telescope in Chile and the Wiyna 3.5-meter telescope at Kitt Peak, Ariz.