Research points to origin of ripples in the Milky May (Forwarded)

News Service
University of Michigan
412 Maynard
Ann Arbor, MI 48109

Contact: Laura Bailey
Phone: (734) 647-1848

May 30, 2007

Research points to origin of ripples in the Milky May

ANN ARBOR, Mich. -- 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.

Related Links:

* More on Prof. Oey
http://www.astro.lsa.umich.edu/~msoey/
* More on Research Fellow Bensby
http://www.astro.lsa.umich.edu/~tbensby/