Stars Thought To Be Remnants Of Swallowed Galaxy Were Actually Once Part Of The Milky Way's Disk

The Milky Way and the location of the two patches of stars that once belonged to the galactic disk, but now sit above and below it after an encounter with the gravity of a satellite galaxy.  T.Mueller/NASA/JPL-Caltech

Two unusually dense collections of stars lying in the outer reaches of the Milky Way were previously thought to be remnants of dwarf galaxies captured by our own. However, a new analysis of their chemical composition shows the stars in each have always been part of the Milky Way, but originated in the galactic disk. Astronomers have concluded they were pulled from their original location by the gravity of a small satellite galaxy that interacted with the Milky Way.

Deep space may seem a serene environment, but it can host titanic encounters where galaxies cannibalize each other, or rip neighbors apart. There's no reason to think the Milky Way is immune, and indeed we are headed for a collision with Andromeda, our nearest large neighbor. Fortunately, the recent discovery that Andromeda is not actually larger than our galaxy means it will be a fair fight.

Even clashes with smaller galaxies can matter, however, and it appears one such case led to the formation of A13 and Triangulum-Andromeda (not to be confused with either the Triangulum or Andromeda Galaxies, which are vastly larger and further away). These collections of stars sit around 14,000 light-years above and below the plane of the Milky Way, in locations where stars are expected to be rare, forming what astronomers call an “overdensity”.

An examination, published in Nature of the chemical make-up of 14 stars within the two patches found both are very consistent in composition and resemble stars of the Milky Way disk. In contrast, the component elements are unlike those in any known dwarf galaxy. Although internal phenomena can throw the occasional star out of the galaxy, the only process known to be able to remove such large groups of stars is tidal interactions with substantial other galaxies.

“These findings are very exciting, as they indicate that the Milky Way Galaxy's disk as a whole can oscillate because of tidal interaction, and its dynamics are significantly more complex than previously thought,” Dr Luca Casagrande of the Australian National University said in a statement.

Triangulum-Andromeda's stars are estimated to be 6-10 billion years old. Casagrande told IFLScience that while the timing of the encounter that pulled them away from the disk is hard to calculate, it probably took place 4-8 billion years ago.

Although the size of the galaxy that disrupted these stars cannot be confirmed, Casagrande told IFLScience the team modeled an encounter with the progenitor of the Sagittarius dwarf galaxy, the closest satellite galaxy to the Milky Way, and found it sufficient to do the job.

It remains unknown whether the star collections will ever return to the plane of the galactic disk, or how being thrown out of the disk would affect planets around these stars.


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