Like lumps of dough stirred by a giant gravitational spoon, clusters and streams of stars travel in silent unison on a far-flung path around the center of our Milky Way galaxy.
These galactic denizens have been carving their present courses for millions and even billions of years, but only recently have astronomers realized that they are non-natives -- relics of intergalactic ingredients added to the Milky Way after the initial recipe had been written.
Two papers in Thursday's issue of the journal Nature combine to make a strong case that a lot of the stars in the Milky Way were absorbed during collisions with other, probably smaller, galaxies. These stellar clumps -- cosmic versions of fossil evidence -- provide the clues that these so-called dwarf galaxies were absorbed.
The smaller galaxies were no match for our Milky Way, gravitationally speaking. They more or less dissolved into the batter of what researchers call the galactic halo, a comparatively sparse region of stars swirling about in the Milky Way's flat outer disk.
What's left of the swallowed-up dwarfs are the clumps or streams of stars. By studying how these conglomerations orbit the galactic center, researchers now see that they had entered the picture after the initial formation.
(A separate and compatible theory holds that a giant bulge of stars that inhabits the central region of the Milky Way formed in a completely different manner, during the collapse of a single dust cloud.)
One group of researchers, led by graduate student Amina Helmi of the Leiden Observatory in the Netherlands, discovered two such streams of stars. Helmi and her colleagues calculate that some 8 percent of the galactic halo is made up of material from the collision that left these stars behind.
"Our findings may be the first fossil evidence directly showing our own galaxy formed by the amalgamation of many smaller building blocks," Helmi said. She added that the research is one step toward figuring out how our galaxy, and others, formed and evolved.
First, a stream of data
Helmi and her colleagues used computers to simulate what would happen during collisions of the Milky Way with smaller galaxies. The models indicated there should be leftover signatures of the collisions, in the form of lumps of energy (i.e. groupings of stars). The researchers started searching for this evidence in catalogues of older stars in the vicinity of our sun.
"The ability of predicting and then confirming your own predictions is a wonderful experience, and this is basically what we did by finding this first fossil evidence of a disrupted satellite in the solar neighborhood," Helmi said.
Along with similar findings in the past, the results raise the question of whether the entire galactic halo is made up of material accreted via mergers with lesser galactic counterparts.
A second, similar finding
In another paper appearing Thursday, Young-Wook Lee of Yonsei University in Korea, led a group of researchers who looked at a cluster of stars known as Omega Centauri -- the brightest globular cluster in the Milky Way.
Within Centauri, the researchers found that certain metal-rich stars were about two billion years younger than some of their neighbors, implying that the globular cluster must have experienced at least two periods of star formation. The researchers suggest that the cluster was formed in a separate dwarf galaxy, which was then sucked into the Milky Way, stimulating a new burst of star formation.
"This suggests mergers and accretion of small galaxies were important in the formation of our Milky Way galaxy," Lee said.
Dwarf galaxies are not wimpy
Sidney van den Bergh, of the Dominion Astrophysical Observatory in Canada, points out in a separate Nature commentary that the remnant star clusters were not complete wimps. The massive clutches of stars at the heart of the dwarf galaxies put up a good fight when the Milky Way exerted its strong gravitational influence. Researchers call this process "tidal disruption."
"Such galactic nuclei are so dense that they can survive the tidal disruption following their capture by a larger galaxy," van den Bergh said, explaining why the streams and clusters remained.
"It now appears that the [new research] supports a picture of galactic formation in which parts of the outer Milky Way were built up from accreted dwarf galaxies," said van den Bergh, who was not involved in the research.
Past studies have shown at least one example of a merger that is still in progress. A cluster of stars known as Sagittarius, studied in 1993, appear to be a small galaxy currently being absorbed by the Milky Way. Researchers suspect that Sagittarius will one day resemble the star streams and clumps being reported Thursday.
Separately, the new work by Yonsei University's Lee uncovered a curiosity that, along with the Sagittarius discovery, challenges the popular notion of all galaxies being far, far away. Some, as we now understand, are right in our backyard.
"In addition to its importance in the study of galaxy formation," Lee said, "our discovery also suggests that Omega Centauri should be considered to be the nearest galaxy [to] the sun (~15,000 light-years), since the Sagittarius dwarf galaxy (which, until our discovery, was considered to be the nearest) is much further (~78,000 light-years) from the Sun."
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