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2008: Halo 2: Opposite Spin

January 2008

A team of scientists, led by University Distinguished Professor Timothy C. Beers, have used data from the Sloan Digital Sky Survey to demonstrate that the halo surrounding the Milky Way comprises two stellar components rotating in opposite directions about the center of the Galaxy.

 halo surrounding the Milky Way

The paper "Two Stellar Components in the Halo of the Milky Way", appeared in Nature (450, 1020) in mid-December 2007. The principal investigators include Daniela Carollo, a recent JINA-sponsored visitor at MSU, and presently a PhD candidate at The Australian National University, Timothy C. Beers, Young Sun Lee (current MSU graduate student), Sivarani Thirupathi (current MSUpostdoctoral researcher), and Brian Marsteller (recent MSU PhD recipient, presently a postdoctoral researcher at the University of California, Irvine).

While the inner part of the halo rotates in the same direction as the disk of the Galaxy, the outer part appears to rotate "backwards." The halo components also differ in their chemical makeup and orbital characteristics, prompting the team to conclude that the two components were likely formed in different ways and perhaps at different times in the history of the Galaxy.

"Although it was once considered a single component, an analysis of more than 20,000 stars from the Sloan Digital Sky Survey (SDSS-II) has shown that the halo is clearly divisible into two, broadly-overlapping structural components,? Beers explained. "This discovery is important because it provides critical information on the formation of the first objects in the Universe and in our own galaxy."

The inner part of the halo is more flattened and dominates the population of stars up to 50,000 light years from the Milky Way's center. (The Sun is around 25,000 light years from the Galactic center). While the inner halo rotates at a speed no more than about 50 km/s in the same direction as the disk of the Galaxy (itself spinning along at 220 km/s), the outer halo rotates in the opposite sense, at a speed of around 70 km/s. The outer halo is almost spherical, and begins to dominate the nature of the region of the Milky Way beyond about 65,000 light years from the Galactic center. It probably extends out to beyond 300,000 light years.

The stellar material in the inner halo was likely to have formed first, from numerous massive sub-Galactic structures that collided in the same direction as presently observed Galactic rotation. The outer halo is likely to have formed somewhat later, from the mergers of many smaller sub-Galactic structures many billions of years ago. The researchers believe that the merged galaxies came in opposite the Galaxy's rotation and - as more galaxies were acquired - they "remembered" their orbits and stayed in place in opposition to the clockwise galactic rotation.

The two structures also vary dramatically in their chemical properties. "Based on our analysis, we find that the outer-halo stars are three times more metal poor than the inner-halo stars," according to Beers. "Metal poor" (or low metallicity) refers to the relative abundances of elements heavier than helium; elements that are created during the lives and deaths of stars. The earliest generations of stars have the lowest metallicities, since they formed from gas that had little previous processing.

"The difference in the metallicities of the two halo populations indicates that the lowest metallicity stars in the Galaxy may be associated with the outer halo," according to Beers. The lowest metallicity stars provide the clearest chemical "snapshot" of the early Universe.

Thus, the new discovery will help astronomers refine future searches for these "jewels of the night sky", the stars that have recorded the elemental abundances created only a few hundred million years after the Big Bang, using MSU's access to the SOAR 4.1 m telescope, and other telescopes.