The ‘mega-merger’ that helped form the younger Milky Method: Astronomers simulate historic collision


Astronomers have simulated the massive collision between two galaxies roughly ten billion years ago that gave rise to the Milky Way’s halo and shaped its ‘inflated’ disk.

Recent observations from the Gaia satellite mission revealed many of the stars in the Milky Way’s halo are ‘invaders’ – or remnants of another, younger galaxy.

In the latest work, researchers have traced these stars to a galaxy called Gaia-Enceladus, showing how a mega-merger between the Milky Way and the smaller galaxy helped to shape our own during its early days.

An international team led by an astronomer from the University of Groningen used data from the second data release of the Gaia satellite mission last April to spot signs of an ancient merger in the Milky Way’s halo.

The latest Gaia observations allowed for a glimpse at roughly 1.7 billion stars.

‘We expected stars from fused satellites in the halo,’ said University of Groningen’s Professor of Astronomy, Amina Helmi.

‘What we didn’t expect to find was that most halo stars actually have a shared origin in one very large merger.’

The researchers found that the chemical signatures of many halo stars in the Milky Way were not the same as its ‘native’ stars.

‘And they are a fairly homogenous group, which indicates they share a common origin,’ Helmi said.

The team plotted the trajectory and chemical signature of these invading stars, revealing them to originate from the merger of the Milky Way and a smaller galaxy not much bigger than the Small Magellanic Cloud.

According to the team, this huge collision took place around 10 billion years ago.

This long-ago mega-merger was likely responsible for shaping the Milky Way’s thick disk during its early years, and gave it many of the stars that exist in its halo today.

‘The youngest stars from the Gaia-Enceladus are actually younger than the native Milky Way stars in what is now the thick disk region,’ Helmi says.

‘This means that the progenitor of this thick disk was already present when the fusion happened, and Gaia-Enceladus, because of its large size, shook it and puffed it up.’



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