New tidal streams found in Andromeda reveal history of galactic mergers

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Images associated with press release

Figure 1.
Traditional view of the Andromeda galaxy, showing only its bright bulge and inner disk and extending out to a projected distance of only about 20 kiloparsecs (65,000 light years) from the galaxy's center. By contrast, the stellar halo of the Andromeda galaxy in which the Tidal Streams E, F, and SW were found is known to extend more than 150 kiloparsecs (500,000 light years) from Andromeda's center. Two of Andromeda's inner dwarf satellite galaxies, M32 (directly below Andromeda's center) and NGC 205 (to the upper right of Andromeda's center), are also visible in this image. Both satellites are still largely intact in spite of their tidal interaction with Andromeda. Image credit: Robert Gendler.

Figure 2.
False-color map of the density of red giant stars in Andromeda, constructed from Subaru/SuprimeCam images. The stellar density enhancements in Streams E, F, and SW are indicated. The map extends out to a projected distance of 100 kiloparsecs (300,000 light years) from Andromeda's center. The central inset shows the traditional view of Andromeda's bright bulge and disk and two inner dwarf satellites. Image credit for star count map: Mikito Tanaka (Tohoku University, Japan).

Figure 3.
Distribution of line-of-sight velocities of stars in the Stream SW field. The filled portion of the histogram corresponds to Andromeda red giant stars while the open portion corresponds to foreground Milky Way stars. The concentration of red giant stars (at a velocity of -370 kilometers per second) is characteristic of tidal streams. The mean velocity of the Andromeda galaxy is -300 kilometers per second, so this shows that Stream SW is moving towards us at 70 kilometers per second relative to the parent galaxy. In addition to the red giants in Stream SW, there is a population of red giants with a broad distribution of velocities that represents the smooth halo of Andromeda built from the dissolved dwarf galaxy victims of the cannibalism process.

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