New stellar streams observed in the halo of nearby galaxy Andromeda that show an intensification of the density of stars represent the tell tale signs of past galactic collisions.

Stars spread around the halo of large galaxies (like our own Milky Way and Andromeda) typically display old age, few elements other than hydrogen and helium, and move about the halo at high speeds. Their characteristics record the early dynamics and chemical evolution of the galaxy, including evidence of past collisions between two or more galaxies in the form of stellar streams – local enhancements in the density of stars that move as a coherent group through a galaxy.

Using data from Subaru’s Suprime Cam for photometry and Keck II’s Deep Extragalactic Imaging Multi-Object Spectrograph (DEIMOS) for spectroscopy to study the large spiral galaxy Andromeda, an international team of astronomers discovered two previously unseen streams by making detailed measurements of the halo stars' spatial and velocity distributions. The observations revealed the presence of red giant stars moving with a common velocity in distinct bands.

The team discovered the two new bands in the previously uncharted northern side of the galaxy's halo, located 200,000 and 300,000 light years away from the galactic centre, respectively. The study also confirmed previous observations of other streams, including a little-studied and very diffuse stream to the southwest.

These streams are a result of mergers of dwarf galaxies associated with the formation of the stellar halo, and further study of the stars' chemical properties will provide more detailed information about their formation history. “Further observational surveys of an entire halo region in Andromeda will provide very useful information on galaxy formation, including how many and how massive individual dwarf galaxies as building blocks are and how star formation and chemical evolution proceeded in each dwarf galaxy,” says Mikito Tanaka of Tohoku University in Japan.

Since Andromeda shares similar characteristics with our own Milky Way Galaxy, understanding its evolutionary history will enable astronomers to learn more about the processes that defined the formation of our own Galaxy.

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