The Milky Way is thought to have experienced several merger events throughout its history, but there are conflicting ideas regarding which is the most recent: the ‘last major merger’. A scenario put forward by Thomas Donlon and colleagues in 2019 proposes that the Virgo Radial Merger (VRM), an event responsible for an overdensity of stars in Virgo, occurred within the last 3 Gyr, making the VRM the Galaxy’s last major merger. A recent study adds further support and a refined timescale to this idea.

Donlon et al. look at a sample of accreted stars in the local (radius < 5 kpc) stellar halo, selecting comparatively metal-rich stars that are gathered into folds or ‘wrinkles’ in phase space. These folds are generated when a galaxy merges with the Milky Way, and their characteristics give information on the nature of the merger: in this case, a radial merger, where accreted stars join the disc of the Milky Way. Through comparison to a simulation of an analogue merger event in the FIRE-2 framework, they find that there are more folds than expected if the debris were to be old, since folds are expected to dissipate with age. Best agreement with the merger simulation is achieved at a time 1.5 Gyr ago, although reasonable fits encompass a merger event up to 3 Gyr ago.

银河系被认为在其历史上经历了几次合并事件，但对于最近的合并事件：“最后一次重大合并”，存在相互矛盾的观点。 Thomas Donlon 及其同事在 2019 年提出的一个假设是，室女座径向合并（VRM）是导致处女座恒星密度过高的事件，发生在过去 3 吉里内，使 VRM 成为银河系最后一次重大合并。最近的一项研究为这一想法提供了进一步的支持和完善的时间表。

唐伦等人。查看局部（半径 < 5 kpc）恒星晕中的吸积恒星样本，选择相对富含金属的恒星，这些恒星聚集在相空间中的褶皱或“皱纹”中。这些褶皱是在星系与银河系合并时产生的，它们的特征提供了有关合并性质的信息：在这种情况下，是径向合并，即吸积的恒星加入银河系圆盘。通过与 FIRE-2 框架中模拟合并事件的模拟进行比较，他们发现，如果碎片老化，褶皱会比预期的要多，因为褶皱预计会随着年龄的增长而消失。与合并模拟的最佳一致性是在 1.5 Gyr 前的时间实现的，尽管合理的拟合包括最多 3 Gyr 前的合并事件。