Washington: An international team of astronomers has identified two new tidal streams in the Andromeda Galaxy, the remnants of dwarf galaxies consumed by our large galactic neighbor.
Analysis of the stars in Andromeda’s tidal streams and other components of its extended halo is yielding new insights into the processes involved in the formation and evolution of massive galaxies, according to Puragra Guhathakurta, professor of astronomy and astrophysics at the University of California, Santa Cruz.
In the currently favored “Lambda Cold Dark Matter” paradigm of structure formation in the universe, the outer halos of large galaxies like our Milky Way Galaxy and the neighboring Andromeda Galaxy (also known as M31) are built up through the merger and dissolution of smaller “dwarf” satellite galaxies.
“This process of galactic cannibalism is an integral part of the growth of galaxies,”
The smooth, well-mixed population of halo stars in these large galaxies represents the aggregate of the dwarf galaxy victims of this cannibalism process, while the dwarf galaxies that are still intact as they orbit their large parent galaxy are the survivors of this process.
“The merging and dissolution of a dwarf galaxy typically lasts for a couple billion years, so one occasionally catches a large galaxy in the act of cannibalizing one of its dwarf galaxy satellites,” Guhathakurta said.
“The characteristic signature of such an event is a tidal stream: an enhancement in the density of stars, localized in space and moving as a coherent group through the parent galaxy,” he added.
Tidal streams are important because they represent a link between the victims and survivors of galactic cannibalism - an intermediate stage between the population of intact dwarf galaxies and the well-mixed stars dissolved in the halo.
In a project led by collaborators Mikito Tanaka and Masashi Chiba of Tohoku University, Japan, the researchers used the Subaru 8-meter telescope and Suprime-Cam camera to map the density of red-giant stars in large portions of the Andromeda Galaxy, including the hitherto uncharted north side.
This led to the discovery of two tidal streams to the northwest (streams E and F) at projected distances of 60 and 100 kiloparsecs (200,000 and 300,000 light years) from Andromeda’s center.
The study also confirmed a few previously known streams, including the little-studied diffuse stream to the southwest (stream SW), which lies at a projected distance of 60 to 100 kiloparsecs (200,000 to 300,000 light years) from Andromeda’s center.