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Milky way eats another galaxy early in life, reports team with Indian scientists

Last Updated 19 January 2020, 14:11 IST

Even in its infancy, the Milky Way has devoured a large satellite galaxy that seeded its halo with stars and forever changed the ways such fireballs move in the galaxy.

Observing a single star – Nu Indi – a team of astronomers including several Indian scientists not only reconstructed some of the violent past of the Milky Way but also to determine the time zone when the smaller galaxy merged with the bigger one.

Born soon after the Universe, Milky Way in its 13.5 billion years of life ate several galaxies as there are dozens of streams within the Milky Way that originated from such smaller ingested galaxies. One of them is the dwarf galaxy Gaia-Enceladus.

“The merger of Gaia-Enceladus with the Milky Way began around 13.2 billion years ago and continued for nearly the next two billion years. The merger took place between 11.6 and 13.2 billion years ago,” Anwesh Mazumder, a scientist at Tata Institute of Fundamental Research, Mumbai and one of the team members told DH.

On cosmic time scales, the colliding and merging of galaxies is not uncommon. Even if both galaxies involved are of very different sizes, such a collision leaves clear traces in the larger one. For example, the smaller galaxy introduces stars with a different chemical composition, the motion of many stars is altered, and myriads of new stars are formed.

Visible in the night sky, Nu Indi is located just 94 light-years (one light-year is the distance travelled by light in a year or nearly 9.5 trillion km) away in the southern constellation of Indus. It's a metal-poor star about three times the size of the Sun but only 85% of its mass.

Discovered from the ongoing Gaia galactic survey, the most accurate 3D map of the Milky Way's stars, it was studied using NASA's Transiting Exoplanet Survey Satellite.

The scientists discovered movement pattern of the stars in a region of the Milky Way where the merger happened; chemical composition of the star confirming its birth in the Milky Way and the time span for the merger.

“We obtained an age measure of a complete stream from one single star and also showed that mergers already took place early in the formation and evolution of the Milky Way and that the effect is still visible. A merger has a long-standing impact on the stellar evolution of the galaxy,” said Saskia Hekker a team member from the Max Planck Institute for Solar System Research, Gottingen.

“The result helps us better understand the current state of our own galaxy, and how historical mergers have affected the populations of stars within it,” noted team leader William Chaplin from the University of Birmingham.

The findings were published in the journal Nature Astronomy last week.

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(Published 19 January 2020, 14:11 IST)

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