Andromeda galaxy
February 24, 2014

Remnant Of Dwarf Galaxy Merger Observed In Andromeda II Star Stream

redOrbit Staff & Wire Reports - Your Universe Online

An international team of astronomers studying the satellite galaxies surrounding Andromeda has for the first time observed the remnant of a merger between two low-mass dwarf galaxies, according to new research published online Sunday in the journal Nature.

The study authors, who hail from the Niels Bohr Institute at the University of Copenhagen, the University of Cambridge’s Institute of Astronomy and the Max Planck Institute for Astronomy in Germany discovered a stream of stars in Andromeda II. The movement of the stars indicates they are the remnants of this type of merger.

They believe the galaxies in the early universe started off small, then grew larger and more massive over time by constantly colliding with neighboring baby galaxies in order to form larger ones. Larger, more massive galaxies often attract smaller galaxies due to gravitational pull, ultimately merging and becoming even bigger. However, not all of the smaller galaxies are consumed by their larger counterparts, the researchers said.

Some remain in an orbit around the bigger galaxy. The largest galaxy in our vicinity is the Andromeda Galaxy, a spiral galaxy that is located approximately 2.3 million light years away from Earth. Andromeda is surrounded by a swarm of more than 20 smaller galaxies, all of which have been given names such as Andromeda I, Andromeda II, and so forth. By studying the measurements of Andromeda II’s stars, the study authors made a startling discovery.

“Stars in a dwarf galaxy often move around at random, but this is not exactly the case for Andromeda II,” astrophysicist Nicola C. Amorisco of the Niels Bohr Institute’s Dark Cosmology Centre explained in a statement Sunday. “In particular we could see that a stream of stars is moving around differently than the rest in a very coherent way.”

“These stars are situated in an almost complete ring and are rotating around the center of the galaxy,” he added, explaining that Andromeda II is less than one percent the size of the Milky Way. “What we are seeing is the remains of a collision between two dwarf galaxies, which had a dramatic effect on the dynamics of the remnant.”

The galaxy’s rotating stream of stars studied by Amorisco and his colleagues is comprised entirely of old stars, and by studying their properties the investigators were able to figure out what happened during the galactic merger. These events were expected to occur during the galactic formation period, but had never been witnessed by astronomers. In fact, Andromeda II is the least massive known example of merging of galaxies to date.