“The Merger History of the Milky Way – What Gaia Revealed” with Eva Grebel (ZAH, Heidelberg University, Germany)

The ESA Cornerstone Mission Gaia is revolutionizing our understanding of the assembly history of the Milky Way. Cosmological models suggest that Milky Way-like galaxies are made up in part of stars that formed in situ and in part of stars that formed in other, smaller galaxies and that were subsequently accreted. Most of the more massive merger events should have occurred more than nine or ten billion years ago. Such galactic cannibalism is believed to be the typical mode of growth of more massive galaxies. The surviving satellite galaxies, which all host ancient stellar populations, hold important clues to the properties of those early building blocks.

In combination with massive ground-based photometric and spectroscopic surveys, Gaia is confirming and refining the cosmological picture. Gaia data have uncovered numerous stellar tidal streams in our Galaxy, not all of which have known progenitors. Many come from disrupted dwarf galaxies, others from dissolving globular clusters – Gaia permits us to trace the detailed assembly history of our Galaxy, revealing the type of objects, their numbers, their properties, and the time of accretion. The most spectacular discovery is arguably that of the fairly massive dwarf galaxy Gaia-Enceladus or Gaia Sausage, which merged with the Milky Way about 10 Gyr ago. This event contributed many globular clusters and likely triggered the formation of the thick disk. In fact, Gaia data suggest that possibly half of our globular clusters come from merger events. Also, Gaia reveals the orbits of the surviving satellites, providing clues to their origins and future merger history.

Eva Grebel is professor of astronomy at Heidelberg University and director of the “Astronomisches Rechen-Institut”. She leads a collaborative research center (Sonderforschungsbereich) on the Milky Way system funded by the German Research Foundation (DFG).  She is a member of the Heidelberg Academy of Sciences, of the Hector Fellow Academy, and of the German National Academy of Sciences (Leopoldina).  For her work on galactic archaeology, she has received a number of scientific awards. A more detailed CV of Prof. Grebel can be found at https://wwwstaff.ari.uni-heidelberg.de/mitarbeiter/grebel/CV.html

This webinar was recorded on June 10, 2021