The presence of a very small galaxy neighboring the Milky Way is having big reverberations for astronomers’ models of the universe. The galaxy, named Seque 2, is the most “lightweight” galaxy ever found, according to scientists from the University of California-Irvine, who conducted new measurements of the dwarf galaxy and published a concluding study on it on June 10 in the Astrophysical Journal. The scientist additionally note that the galaxy offers important clues to how iron, carbon, and other elements vital to human life originally formed, and it vindicates some previous models that had hitherto been in serious doubt.
Seque 2 numbers only around a thousand stars, bound together by dark matter, and it orbits the Milky Way at about 114,000 light years from the Sun. Scientists first caught sight of it in 2009, while scanning large areas of the sky during the Sloan Digital Sky Survey, which produced 3-D maps of more than 93,000 different galaxies.
This year, scientists from the University of California-Irvine conducted new measurements of Segue 2, using the W. M. Keck Observatory in Hawaii. After observing the galaxy through the observatory’s pair of 10-meter telescopes—two of the highest-resolution telescopes on Earth—and then estimating the weight of 25 of its most prominent stars, the team determined that Seque 2 is 10 times less dense than researchers had initially thought.
The team also confirmed that a halo of dark matter holds the stars together. This finding proves that Segue 2 fits the definition of “galaxy”—if there were no dark matter holding its stars in place, then astronomers would only consider Segue 2 a star “cluster.”
No matter what one calls it, however, Segue 2 is very difficult to view from Earth. Its total luminosity is a mere 900 times that of the sun, compared to the Milky Way, whose light output is the sun times 20 billion. According to Evan Kirby, a California-Irvine doctoral student and the study’s lead author, this observatory’s telescope array is the only two telescopes on Earth powerful enough to have determined observed the galaxy’s density.
Scientists had been expecting to find it for many years, though. Earlier models had predicted that the Milky Way would have dwarf galaxies orbiting it, but telescopes had failed to find any such galaxies despite repeated searches. Astronomers had begun to question if maybe the models were flawed.
According to Kirby and his colleagues, the sightings of Segue 2 prove that the models were right, after all. Now the question is whether more dwarf galaxies will be found. There could be thousands more, suggests co-author James Bullock, who said that Segue 2 could turn out to be just “the tip of the iceberg.”
The discovery could aid astronomers in their attempt to better understand how galaxies evolve over time. A number of recent studies have sought to collect data on galaxies throughout the universe, part of a an attempt to gain a better understanding of how our own Milky Way galaxy came to be over the course of billions of years.