ANN ARBOR—New research on the composition and age of a group of stars known as the Hercules Stream raises questions about one of the prevailing theories as to how our galaxy formed, and gives new clues about how these mysterious streams really were created.

One theory for the formation of our Milky Way galaxy is that it results from the mergers of numerous, smaller dwarf galaxies. Many astronomers believe that star streams such as Hercules?stars that move at the same velocity in a band or stream?are thought to be remnants of those mergers. New research from the University of Michigan doesn’t refute those ideas, but does suggest astronomers need to take a much closer look.

Thomas Bensby, U-M research fellow in astronomy, said it follows that if the Hercules Stream is actually a remnant of a dwarf galaxy that merged with others to make the Milky Way, one would expect its stars to result from different star formation histories than the stars in the Milky Way.

Bensby is principal investigator on the paper, “Disentangling the Hercules Stream” that recently appeared in the Astrophysical Journal Letters. The work was presented by Sally Oey, assistant professor and co-author of the paper, at the 210th Meeting of the American Astronomical Society this week in Hawaii.

Oey and Bensby wanted to test that theory and learn whether streams really are remnants of recently accreted dwarf galaxies. However, they found that the Hercules stream stars showed the same star formation histories as the stars in the Milky Way, which suggests that the stars in the stream were never part of another, smaller galaxy.

“There is no reason to believe that the star formation history in a different galaxy with completely different conditions should be similar to the star formation history in our galaxy,” Oey said. “People have been keen to point to such streams as evidence of mergers. Our work says, ‘not so fast.'”

A second aspect of the research is that these non-merger star streams are evidence of other activity happening in the Milky Way. One theory is that the bar of gas and dust spinning at the center of the Milky Way disturbed the stars and caused the streams, Bensby said.

“If so, then the Hercules Stream is an example of ripple motion that can probe the structure and properties of the bar, possibly spiral arms, and other major properties of our galaxy which are hard for us to study because we are embedded in the disk of our galaxy,” Oey said.

Researchers obtained high resolution spectra (spectral features in the starlight reveal the individual element abundances,) via the Magellan telescope in Chile. By looking at weak and faint lines in the spectra, they calculated the amount of barium and magnesium in the stars, which reveals the star’s formation history.

Other authors include S. Feltzing, Lund Observatory, Sweden, and B. Gustafsson, University of Uppsala, Sweden.

More on Prof. OeyMore on Research Fellow Bensby