Universe Today
It is a scientific consensus that our galaxy, the Milky Way, grew to its current size by merging with smaller galaxies. Some of those mergers are still taking place, including the "nearest" galaxy to our own: the Canis Major Dwarf Galaxy. Unfortunately, the history of those mergers was not well understood for a long time. But new data from NASA's Hubble Space Telescope show definitive evidence that a dwarf galaxy merged with the Milky Way during its earliest phases of evolution.
The research was led by astronomers from the Osservatorio di Astrofisica e Scienza dello Spazio di Bologna (OAS). They were joined by researchers from the Kapteyn Astronomical Institute, the Italian National Institute for Astrophysics (INAF), the Escola de Enxeñaría de Telecomunicación, the Instituto de Astrofísica de Canarias (IAC), Edinburgh University's Astrophysics Research Institute, the Space Telescope Science Institute (STScI), and multiple universities and observatories. The results are reported in a paper that appeared in Nature Astronomy.
Today, the Milky Way measures about 200,000 light-years in diameter and contains around two hundred billion stars. However, over time, it grew by forming new stars from clouds of dust and gas (predominantly in its spiral arms) and through mergers with other galaxies. Mergers not only add stellar populations but also trigger additional star formation by forcing gas clouds together, compressing them to the point of gravitational collapse. The most recent, between the Milky Way and the Sagittarius dwarf galaxy, began over 6 billion years ago and is still happening today.
This is a Hubble telescope image of NGC 2623, a late-stage merger between two spiral galaxies. Credit: By ESA/Hubble, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=63401428
Scientists have learned much about the history of our galaxy thanks to immense astronomical surveys like the ESA's Gaia mission. However, the farther back in time scientists look, the harder it is to detect the telltale signs of mergers. Evidence of the earliest, in fact, may have been erased over billions of years. Hubble was instrumental in this regard, allowing the team to examine some of the Milky Way's globular clusters, which contain some of the oldest stars in our galaxy.
These ancient stars may provide evidence of mergers, in which the Milky Way collected stars from dwarf and larger galaxies. "Our home is the Milky Way galaxy, but we do not know how our house was built," said lead author Davide Massari, First Researcher at the OAS di Bologna. "In this paper we discover where the first significant batch of bricks came from: a dwarf galaxy that we call LKH."
Researchers have learned that the Milky Way galaxy consumed the Gaia-Sausage-Enceladus dwarf galaxy about 10 billion years ago, with smaller mergers occurring between the two. This ancient merger significantly affected the structure of the Milky Way's stellar disk. To look back even farther, the team analyzed Hubble data from 39 globular clusters located 20,000 light-years from the center of the Milky Way, where evidence of the most ancient mergers is likely to be preserved.
Using Hubble's sensitive observations, the team determined each cluster's precise age and heavy-element abundance (metallicity). From this, they identified a third population of globular clusters in the inner regions of our galaxy, which are older than the group collected in the Gaia-Sausage-Enceladus merger but younger than those formed in the Milky Way. Accordingly, these clusters came from a separate, earlier merger in which the Milky Way absorbed a dwarf galaxy of 500 solar masses - a significant mass for our galaxy at the time.
These images of galaxy mergers show the tell-tale streamers of gas and stars that come from mergers. Credit: NASA/ESA/Hubble
They named this dwarf galaxy Low-energy-Kraken-Heracles (LKH) in honor of three earlier research papers that suggested a merger took place early in our galaxy’s history. Said Chiara Zerbinati, a PhD candidate at the University of Bologna and a co-author on the study:
Thanks to the high resolution and depth of Hubble imaging, we could measure the age and the metal content of these clusters with unprecedented precision. Coupled with measurements from Gaia, this made it possible to distinguish a population of globular clusters that are different from the others. These are the clusters that were born in LKH, and they tell us when that galaxy was devoured by ours, and how massive it was.
These latest observations effectively extend our knowledge of the Milky Way's merger history to 11.8 billion years ago, just 2 billion years after the Big Bang. Such a large, early merger in our galaxy's history has profound implications for the evolution of the Milky Way. “Some past studies have argued that the earliest phases of our galaxy’s evolution were defined by stars born only in our galaxy,” said Massari. “Here, we have shown that stars born in external galaxies also need to be considered.”
The team plans to continue studying globular clusters within the Milky Way in the hopes of characterizing all the massive mergers that our galaxy has experienced. “Hubble is observing globular clusters that have never been studied before, and this will help us characterize the merger events that are far back in time in the Milky Way galaxy’s history,” said co-author Fernando Aguado-Agelet, a researcher from the University of Vigo and the University of La Laguna in Spain.
