An international research team of astronomers from Tohoku University, The University of Tokyo, National Astronomical Observatory of Japan (NAOJ), The University of California, and other universities has discovered stellar streams, the remnants of dwarf galaxy mergers, in a vast region surrounding the galactic disc of Andromeda, so called a stellar halo. The data from the team's observations using both Subaru and Keck II telescopes led to this discovery and provided detailed spatial and velocity distributions of the stellar streams.

 

 Stars older than 10 billion years are distributed in a halo surrounding a young bright galactic disc. These stars are called halo stars and considered to give important information on galaxy formation and its evolution process because their elemental compositions and space motions reflect the early chemical and dynamical evolution of a galaxy. In the current theory of galaxy formation, large galaxies are built up through the merger and dissolution of smaller galaxies by the tidal force, and such remnants could be seen in a halo.

 

 The merger and dissolution of dwarf galaxies typically takes a couple billion years, and there is a possibility to catch the scene. A group of halo stars called stellar streams often can be seen after the merger process, and these stars move at the same velocity in a gravitational field in galaxies. They are also called tidal streams because the stream structure is formed by the tidal force. Andromeda is an excellent test bed for finding such streams: it provides an external perspective of the nearest large spiral galaxy similar to our own and yet is close enough for individual stars to be studied in great detail.

 

 Based on this scientific motivation, a research team led by Mikito Tanaka, a research fellow at Astronomical Institute of Tohoku University, has conducted photometric observations of a wide range of Andromeda's halo fields using Suprime-Cam camera of Subaru telescope through V and I-band. Since Andromeda, the closest big galaxy to us, has a huge apparent size as seen from our position, mapping its entire halo needs enormous number of Subaru nights. Instead, the team looked at specific portions of Andromeda's minor axis fields including the hitherto uncharted north side and some fields at the major axis. This survey led to the discovery of two stellar streams to the northwest (Streams E and F) at projected distances of 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 200,000 to 300,000 light years from Andromeda's center.

 

 Then, a team led by Puragra Guhathakurta, a professor at UC Santa Cruz, followed up with a spectroscopic survey of several hundred red giant stars in Streams E, F, and SW, using the Keck II 10-meter telescope and DEIMOS spectrograph at the W. M. Keck Observatory. The spectrograph spreads out the light from each star into a spectrum, which allows astronomers to measure the velocity of the star and distinguish Andromeda red giant stars from foreground stars in the Milky Way. The spectral data confirmed the presence of coherent groups of Andromeda red giant stars moving with a common velocity.

 

 This stream structure could be the remnants of large galaxies such as the Milky Way or the Andromeda galaxy that were formed by the merger of small galaxies. These findings explain a scenario of galaxy formation through galaxy mergers, which is expected to be in a standard theory. It will be important to measure metal content of stars that build up streams, and to gain information about the chemical evolution history. "By observing wider range of the halo, we aim to clarify important issues in the galaxy formation process including the mass of dwarf galaxies involved with the large galaxy formation and the chemical evolution process in smaller galaxies," said Tanaka.

 

 The research results produced by Subaru telescope have been published in the January 10, 2010 issue of The Astrophysical Journal by American Astronomical Society (AAS), and results by DEIMOS spectrograph of Keck II have been presented at the 215th AAS Meeting on January 7, 2010 and chosen as a lecture given at a press conference.

 

[Contact] Masashi Chiba, Professor

Astronomical Institute, Tohoku University

E-mail: chiba *at* astr.tohoku.ac.jp

 

Mikito Tanaka, Research fellow

Astronomical Institute, Tohoku University

E-mail: mikito *at* astr.tohoku.ac.jp