One of the keys to the puzzle appears to be ‘metal-poor disc stars’:
‘…[O]ver the past two years, researchers have rewritten nearly every major chapter of the galaxy’s history. What happened? They got better data.
On April 25, 2018, a European spacecraft by the name of Gaia released a staggering quantity of information about the sky. Critically, Gaia’s years-long data set described the detailed motions of roughly 1 billion stars. Previous surveys had mapped the movement of just thousands. The data brought a previously static swath of the galaxy to life. “Gaia started a new revolution,” said Federico Sestito, an astronomer at the Strasbourg Astronomical Observatory in France.
Astronomers raced to download the dynamic star map, and a flurry of discoveries followed. They found that parts of the disk, for example, appeared impossibly ancient. They also found evidence of epic collisions that shaped the Milky Way’s violent youth, as well as new signs that the galaxy continues to churn in an unexpected way.
The Gaia satellite has revolutionized our understanding of the Milky Way since its launch in December 2013. Taken together, these results have spun a new story about our galaxy’s turbulent past and its ever-evolving future. “Our picture of the Milky Way has changed so quickly,” said Michael Petersen, an astronomer at the University of Edinburgh. “The theme is that the Milky Way is not a static object. Things are changing rapidly everywhere.”…’
— via WIRED