New observations of the globular cluster Terzan 5 suggest that it is a remnant of a pre-existing proto-galaxy which contributed to the formation of the Milky Way bulge, courtesy F. R. Ferraro, University of Bologna

November 28, 2009 - Mauna Kea, Hawaii

New observations made by the W. M. Keck Observatory of a globular cluster within the Milky Way Galaxy once thought to be "common" now reveal the remnant of a proto-galaxy that merged with similar systems to form the Galactic bulge.

The finding, published in the Nov. 26 issue of Nature, identified two distinct groups of stars within the Milky Way Galaxy’s globular cluster Terzan 5. The two stellar populations have different ages and iron abundances, which are rare features among globular clusters, suggesting that Terzan 5 could be a surviving remnant of a pre-existing galaxy.

The discovery opens a new window on the formation mechanisms of galaxies, according to the study’s lead author, Francesco Ferraro of the University of Bologna in Italy. Ferraro says it could be the first observational evidence to confirm that the bulge of a galaxy originates from the merging of pre-formed, internally evolved systems of stars.

The Terzan 5 region located in the central bulge of the Galaxy - a region that has been hard to study because of its high concentration of interstellar dust - has two, distinct stellar populations: a bright one whose stars are centrally concentrated and a second one, whose stars are fainter.

Spectral data taken with the Keck II telescope and its NIRSPEC instrument also demonstrated that the brighter branch is roughly three times richer in metals, specifically, iron, which is formed in supernovae, said team member R. Michael Rich, of the University of California at Los Angeles. Scientists say it is one of the most metal rich stars known to exist.

The anomaly, not previously observed, suggests it is the remnant of a disrupted dwarf galaxy that merged with the Milky Way late during its evolution.

European Southern Observatory’s Very Large Telescope also helped with the discovery.