The Last Galactic Firework: Timing the last significant merger with stars, globular clusters and $\omega$Centauri

TL;DR

This paper introduces a method to precisely date the last major merger in the Milky Way, linking it to globular cluster formation and the origin of omega Centauri, with implications for galaxy evolution.

Contribution

It provides the most precise dating of the last significant merger (GSE) at approximately 11.2 Gyr and links it to globular cluster formation and omega Centauri's origin.

Findings

Last major merger (GSE) occurred around 11.2 Gyr ago.

Omega Centauri likely is the GSE remnant.

Disc formation predates the last merger, around 13 Gyr ago.

Abstract

We present a robust method to empirically infer the timing of the last significant merger in the Milky Way which is tested against fully cosmological models of galaxy formation. We apply it to Milky Way subgiant stars with spectro-photometric ages, finding that the last significant merger (Gaia-Sausage-Enceladus, GSE), occurred $\sim11\,$Gyrs ago. This coincides with the birth of a coeval in-situ group of globular clusters (GCs), which constrains the merger-induced starburst (hereafter {\it Tain\'{a}}) to have occurred at $11.2\pm 0.1\,\rm{Gyr}$, the most precise dating of this merger event. The GSE's most metal-rich GCs were also born around this time ($\tau=10.9\pm0.1\,\rm{Gyr}$) and likely formed during the merger interaction prior to disruption of the GSE. We argue that $\omega$ Centauri is the most likely candidate for the surviving remnant of the GSE, and show that its stellar populations have final ages and metallicities consistent with the GSE GCs together with observational evidence it may have been affected by bar resonances. Furthermore, we argue that the mean metallicity for which stellar orbits transition from halo-like to disc-like kinematics shows an upward inflexion point at $[\rm{Fe/H}]\sim-1.33$, and this sets an upper-limit for the age when the disc was forming. To corroborate this, we identify proto-MW GCs with highly disc-like orbits that formed before the last significant merger (with ages up to $\tau=13.0\pm0.5\,\rm{Gyr}$). This places the disc formation time as far back as as $z_{\rm disc\, form}\gtrsim4$.