Milky Way's Halo Rife With Ancient Stars Torn from Other Galaxies

The Milky Way's ancient stars once belonged to other galaxies instead of being the earliest stars born inside the galaxy when it began to form about 10 billion years ago. Many of the Milky Way's ancient stars are remnants of other smaller galaxies ripped asunder by violent galactic collisions around five billion years ago, according to research that was part of the Aquarius Project, which uses the largest supercomputer simulations to study the formation of galaxies like the Milky Way.

Computer simulations revealed that the ancient stars, found in a stellar halo of debris surrounding the Milky Way, had been torn from smaller galaxies by the gravity generated by colliding galaxies.

Cosmologists predict that the early Universe was full of small galaxies which led short and violent lives, colliding with each other, leaving behind debris which eventually settled into more familiar looking galaxies like the Milky Way. The stellar halo preserves a record of a dramatic primeval period in the life of the Milky Way which ended long before the Sun was born.

"The computer simulations started from the Big Bang, around 13 billion years ago, and used the universal laws of physics to simulate the evolution of dark matter and the stars. These simulations are the most realistic to date, capable of zooming into the very fine detail of the stellar halo structure, including star "streams" -- which are stars being pulled from the smaller galaxies by the gravity generated by colliding galaxies.

One in one hundred stars in the Milky Way belong to the stellar halo, which is much larger than the galaxy's familiar spiral disk. These stars are almost as old as the Universe.

The research was conducted by Durham University's Institute for Computational Cosmology and their collaborators at the Max Planck Institute for Astrophysics, in Germany, and Groningen University, in Holland. The research was part of the Aquarius Project, which uses the largest supercomputer simulations to study the formation of galaxies like the Milky Way.

The artist’s illustration at the top of the page shows the hot gas halo enveloping the Milky Way and Magellanic Clouds. Our galaxy — and the nearby Large and Small Magellanic Clouds appear to be surrounded by an enormous halo of hot gas, several hundred times hotter than the surface of the Sun and with an equivalent mass of up to 60 billion Suns, suggesting that other galaxies may be similarly encompassed and providing a clue to the mystery of the galaxy’s missing baryons, according to a research team using data from NASA’s Chandra X-ray Observatory. In the artist’s rendering above our Milky Way galaxy is seen at the center of a cloud of hot gas. This cloud has been detected in measurements made with Chandra as well as with the European Space Agency’s XMM-Newton space observatory and Japan’s Suzaku satellite. The illustration shows it to extend outward over 300,000 light-years — and it may actually be even bigger than that.Image credit: NASA/CXC/M.Weiss; NASA/CXC/Ohio State/A.Gupta et al.

Image credit: Credit: Andrew Cooper/John Helly, Durham University

Source: The Daily Galaxy via Durham University, UK