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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