In 2005, astronomers discovered a dense grouping of stars in the constellation Virgo. It looked like a star cluster, except that further research showed that some stars were moving towards us, and some were receding. This discovery was unexpected and suggested that the stream was not just a star cluster.
A 2019 study showed that the group of stars is not a star cluster at all; it is the shell of a dwarf spheroidal galaxy that has merged with the Milky Way. It is called Virgo Density or Virgo Stellar Stream.
The new research shows how and when the merge occurred and finds other shells from the same merge.
The study is published under the title 'Milky Way Shell Structure Shows Radial Impact Time'. The lead author is Thomas Donlon II, a graduate student at Rensselaer University. The article was published in The Astrophysical Journal.
In the article, the authors write: 'In this paper, we first define the shell substructure in the Milky Way and assert that these shells are indeed associated with a radial merging event.
There are different types of fusions, and the fusion that resulted in the Virgo Density was the so-called radial fusion.
Every time a dwarf galaxy collides with the center of the galaxy, it flies out from the other side only to return back to the center. Each time it reaches the farthest point, it leaves behind several of its stars, forming shells.
Astronomers created a model using observational data and calculated how many times the dwarf galaxy bounced back and forth and when it first merged with the Milky Way.
Their computer simulations show that the merger began when the dwarf galaxy first passed through the center of the Milky Way 2.7 billion years ago.
Mergers of galaxies are not uncommon. Huge galaxies like the Milky Way have grown by merging with much smaller galaxies. The Milky Way is currently in the process of two mergers. In the process of merging with the Sagittarius dwarf spheroidal galaxy, as well as the Small and Large Magellanic clouds.
All these mergers have left their mark on the Milky Way. The halo of our galaxy is a region of spherical stars that surrounds the spiral arms of the Milky Way.
Most of these stars are not 'native' to the galaxy, but 'immigrants' from other galaxies that have merged with the Milky Way.
Over time, the tidal forces of the galaxy transform immigrants into long streams of stars. And the streams move in concert with each other through the halo. Astronomers call these tidal mergers and they are the subject of much research.
But this merger was different. Radial mergers like the one observed are much stronger, and a dwarf galaxy can repeatedly move back and forth, leaving stellar envelopes.
Article published by Universe Today.
Sources: Photo: Four shell structures found in the Milky Way. (Rennselaer University).
