The violent history of Andromeda, the big galaxy next door | Space | EarthSky
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The evidence for the two events comes from the relatively new field of galactic archaeology, that is, the use of the motions and properties of stars and star clusters – in this case, globular star clusters – to reconstruct a galaxy’s history. A statement from Gemini Observatory explained: Gas and dwarf galaxies in the vast cosmic web follow the gravitational paths laid out by dark matter – traversing filaments, they migrate slowly toward collections of dark matter and assemble into large galaxies. As dwarf galaxies are pulled in by gravity, they are also pulled apart, leaving behind long trailing streams of stars and compact star clusters. We are cosmic archaeologists, except we are digging through the fossils of long-dead galaxies rather than human history. By tracing the faint remains of these smaller galaxies with embedded star clusters, we’ve been able to recreate the way Andromeda drew them in and ultimately enveloped them at the different times. View larger. | Here is part of the evidence for 2 ancient migration events in the Andromeda galaxy. Astronomers studied this galaxy’s globular star clusters (lower right insets), indicated by colored circles. They are located in the outer halo of the Andromeda Galaxy, beyond the bright disk of the galaxy (upper left inset). The clusters separate into 2 groups: one associated with stellar streams and one not associated with stellar streams. The orbits of these groups of globular clusters are very different from each other, a result that points to 2 separate migration events in the history of the galaxy. The color of each circle indicates the line-of-sight velocity of the corresponding star cluster. Image via Australian National University/NSF’s National Optical-Infrared Astronomy Research Laboratory/Gemini Observatory. This is very weird and suggests that the extragalactic meals are fed from what’s known as the ‘cosmic web’ of matter that threads the universe. More surprising is the discovery that the direction of the ancient feeding is the same as the bizarre ‘plane of satellites,’ an unexpected alignment of dwarf galaxies orbiting Andromeda. More surprising is the discovery that the direction of the ancient feeding is the same as the bizarre ‘plane of satellites,’ an unexpected alignment of dwarf galaxies orbiting Andromeda. This deepens the mystery as the plane must be young, but it appears to be aligned with ancient feeding of dwarf galaxies. Maybe this is because of the cosmic web, but really, this is only speculation. We’re going to have to think quite hard to unravel what this is telling us. We’re going to have to think quite hard to unravel what this is telling us. The Milky Way is on a collision course with Andromeda in about four billion years. So knowing what kind of a monster our galaxy is up against is useful in finding out the Milky Way’s ultimate fate. Source: Two major accretion epochs in M31 from two distinct populations of globular clusters Via Australian National University Via Gemini Observatory