The Milky Way Swallowed a Neighbor 11 Billion Years Ago express
About 11 billion years ago, the Milky Way suffered a head-on collision. Not with an asteroid, but with an entire smaller galaxy. It slammed into ours and was torn apart like a puff of flour hit by a fist. Astronomers call this cosmic scar Gaia-Sausage-Enceladus (after the shape of the orbits of its remaining stars). But when exactly did the impact happen?
The authors found a way to clock it. They turned to special stars, subgiants, which live short lives and shine just before becoming red giants. Their ages can be determined with high precision from their spectra and brightness. It turned out that the last major merger occurred 11 billion years ago, give or take 100 million years. This is the most accurate chronology in the history of galactic archaeology.
Immediately after the impact, a whole group of globular clusters, dense stellar knots that still orbit the center, lit up in the Milky Way. The authors named this burst of star formation Tynya (from the Tupi-Guarani word for "star"). Its age—11.2 billion years — matches the merger time perfectly. So the catastrophe didn't just destroy the intruder; it also shook our galaxy, forcing it to give birth to new stars.
But the biggest surprise is the Milky Way's disk. The flat pancake we live in began to assemble not billions of years after the Big Bang, but almost immediately. Analysis of stellar motions showed that the transition from chaotic halo orbits to orderly disk orbits occurs at a certain metal concentration. This inflection point corresponds to an age of about 13 billion years. So the disk already existed then — long before the collision with Gaia-Sausage. Our galaxy turned out to be much older and more resilient than we thought.
And one final detail: the giant globular cluster Omega Centauri, visible to the naked eye from the southern hemisphere, is most likely the surviving core of that very victim galaxy. Its stars have the right age and composition, and its orbit bears the scars of gravitational perturbations from the Milky Way's bar. So you can even spot the "culprit" with binoculars.
Original abstract
as written by the authors arXiv:2604.14502
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 $$z_{\rm disc\, form}\gtrsim4$$.
Subgiants are stars in retirement: they've just burned all the hydrogen in their cores and are swelling up, getting ready to become red giants. This phase lasts only a few million years, a blink of an eye by stellar standards, so their ages can be determined with record precision.
In the novel "The Hammer of God," Arthur C. Clarke and Michael McCollum described a galactic merger that awakens an ancient civilization. Here, the merger only awakened star formation.