(ORDO NEWS) — One of the characteristic features of modern cosmology is the description of the evolution of galaxies as a hierarchical process of collisions and associations with other systems.
The most easily accessible place in the Universe for observing such processes is our own galaxy, the Milky Way. Currently, one of our closest galactic neighbors, the Sagittarius dwarf galaxy, is in the process of being tidal ripped apart by the gravity of the Milky Way.
Two other nearby dwarf galaxies, the Large and Small Magellanic Clouds, are falling on our Galaxy. Meanwhile, the galaxy is surrounded by streams of globular clusters that indicate an earlier collision.
Information about even more ancient collisions can be obtained by studying the location and movement of objects in the Milky Way’s stellar halo – a roughly spherical structure of stars more than 10-12 billion years old.
Meanwhile, Andromeda, the closest large galaxy to us, is about 10 times farther than these dwarf galaxies; collision with it is expected in about 5 billion years.
In the new study, a team of astronomers led by Rohan P. Naidu used ESA’s Gaia space telescope and MMT multi-mirror telescope, USA to study in detail the history of the last major collision in the history of our Galaxy.
Observations show that 8-10 billion years ago the Milky Way unified with one dwarf galaxy. Known as the Gaia-Enceladus galaxy, the remains of this object today are composed of stars in the inner halo with characteristic velocities and chemical composition.
However, scientists still did not know the collision mechanism – was the collision “head-on”, or the galaxies first moved in orbit relative to each other.
To answer this question, astronomers performed computer simulations, which made it possible to establish that the Gaia-Enceladus galaxy contained about half a billion stars and did not revolve around the Milky Way, but approached it, moving retrograde (that is, opposite to the direction of rotation of the Galaxy).
The authors also concluded that about half of the modern stellar halo of the Milky Way and about 20 percent of its dark matter halo are formed from the remnants of the Gaia-Enceladus galaxy.
This study provides an almost complete picture of the growth of the Milky Way over the past ten billion years, the authors note.
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