Mining the archives of two major observatories, a team of astronomers has uncovered what could be a mother lode of "red nuggets" – a type of galaxy that could represent the initial building blocks for some of the enormous elliptical galaxies astronomers see throughout the universe.
Such ellipticals represent the final stages of galaxy evolution, where the vast collection of stars they contain are old and few if any new stars are forming. Some are thought to form through the mergers of large spiral galaxies such as the Milky Way.
But since the initial discovery of red nuggets, astronomers suggest that giant ellipticals also may form with red nuggets as the seeds that over time also grow through mergers with other galaxies.
Despite their small size, it's hard not to see why red nuggets can be so attractive, gravitationally speaking.
Red nuggets are compact galaxies. They can be as small as 10 percent of the Milky Way's size. But their small size belies the large number of stars they contain. The mass of all the stars a red nugget contains can run to more than 10 times the mass of all the stars in the Milky Way.
More important, current models of galaxy formation and evolution can't account for their existence, especially in the early universe, researchers say.
The team, led by Ivana Damjanov with the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., has uncovered nine of these nuggets at distances of between 2 billion and 6 billion light-years from Earth.
Although observations of the distant universe with the Hubble Space Telescope had yielded hints that red nuggets were out there, they were hard to distinguish from older, redder stars. Their existence as galaxies was confirmed in 2005 by a team led by French astrophysicist Emanuelle Daddi.
After that, there was an avalanche of observations "finding them at all kinds of high red shifts," Dr. Damjanov says, referring to units of measure that can be converted to distance. In essence, the most distant of these objects were more than 10 billion light-years away, corresponding to a time when the universe was less than 4 billion years old.
Under the assumption that cosmic fossils from those early years, such as globular clusters, exist in the local universe, others looked for relic red nuggets locally, but couldn't find any.
Damjanov and colleagues conducted their hunt at intermediate distances. Using data from the Hubble Space Telescope as well as the Sloan Digital Sky Survey, they found nine nuggets and have roughly 1,000 additional candidates identified.
With evidence that these nuggets exist at intermediate distances in hand, it's hard to imagine they don't exist in the local universe as well, Damjanov says.
Although all nine galaxies qualify as red nuggets, three of them were a bit odd, she explains. They have irregular shapes and still sport some young stars, while the other six all have smooth, elliptical shapes, host old stars, and show no star formation.
It may be that in this one sample, the team has snagged red nuggets at different stages of their evolution, she speculates.
The results have been accepted for publication in the Astrophysical Journal Letters.
Having such relatively close specimens means astronomers will be able to make detailed studies of the distribution of stars within these galaxies as a function of their distance from the galaxies' centers.
They also should be able to measure the motion of the stars as they orbit the center of these extremely dense objects, yielding more insights into the physics that govern them. Researchers also are interested in the environment surrounding them – whether they tend to populate voids between galaxy clusters or are found largely among dense collections of galaxies.
Such information may help solve a problem that stems from their existence so early in the universe's history.
Even these nuggets likely formed from mergers of smaller galaxies, Damjanov explains.
But to generate such densely packed objects, and to build them to respectable sizes, "you need a special type of merging," she says.
But mergers of any sort would have been sporadic, even in a younger, more compact, and denser universe. A special kind yielding such dense galaxies would be too rare to generate the number of red nuggets astronomers have detected in the early universe.
Which lobs the ball back onto the theorists' court.