A new study suggests that our Milky Way galaxy and its neighboring Andromeda galaxy are already interacting, long before their predicted collision billions of years from now.
According to research published in Nature Astronomy, the two galaxies' circumgalactic mediums (CGMs)—vast halos of gas and dust surrounding each galaxy—are likely overlapping, signaling the early stages of this cosmic encounter. This finding challenges the traditional timeline of galactic collisions and offers new insights into how galaxies evolve and interact over time.
The Role of the Circumgalactic Medium
Every galaxy, including the Milky Way and Andromeda, is surrounded by an expansive halo of gas and dust known as the circumgalactic medium (CGM), which contains up to 70% of the galaxy's visible mass. The CGM is crucial in regulating the flow of gases necessary for star formation and other galactic processes. While difficult to observe directly, the CGM has been studied through its ability to absorb light from distant objects, like quasars.
Thanks to recent advancements in imaging technology, scientists have been able to observe the CGM in greater detail. Nikole Nielsen, lead author from Swinburne University, explained, "We’re now seeing where the galaxy's influence stops, the transition where it becomes part of more of what’s surrounding the galaxy, and, eventually, where it joins the wider cosmic web and other galaxies." These advancements have allowed researchers to map the boundary between a galaxy's core and its circumgalactic halo for the first time.
Evidence of Overlapping Galaxies
The study revealed that the circumgalactic mediums of the Milky Way and Andromeda have likely begun to overlap. Previous models predicted that these galaxies would not interact until their physical collision in 4 billion years, but the outer halos of gas are already mingling, suggesting the interaction has started on a more subtle level.
"It’s highly likely that the CGMs of our own Milky Way and Andromeda are already overlapping and interacting," Nielsen noted. This early-stage interaction is invisible to the naked eye but indicates that the galaxies' outer atmospheres have started to influence each other well before their stars or central regions collide.
Defining Galactic Boundaries
One of the key outcomes of this research is the new understanding of where a galaxy ends. By using the W. M. Keck Observatory in Hawaii, researchers were able to peer 100,000 light-years into the edges of a distant spiral galaxy and observe the transition from the interstellar medium (gases and dust within the galaxy) to the circumgalactic medium.
"In the CGM, the gas is being heated by something other than typical conditions inside galaxies, likely heating from the diffuse emissions from the collective galaxies in the Universe and possibly shocks," explained Nielsen. This change helps scientists better define the boundary between galaxies and the surrounding cosmic web.
Understanding the Role of the CGM in Galactic Evolution
The circumgalactic medium is not only a boundary; it also plays a vital role in galactic evolution. The CGM regulates the inflow and outflow of gases, which impacts star formation and the life cycle of a galaxy. "The CGM plays a huge role in that cycling of gas," Nielsen said. "Being able to understand what the CGM looks like around galaxies of different types helps us observe how changes in this reservoir may actually be driving changes in the galaxy itself."
This study sheds light on how galaxies transition through different stages of development. Some galaxies continue to form stars, while others have stopped, and the CGM may be the key to understanding why these transitions occur.
Implications for the Future Collision of the Milky Way and Andromeda
While the physical collision between the Milky Way and Andromeda is still billions of years away, the discovery that their circumgalactic mediums are already overlapping has significant implications for how scientists view the early stages of galactic mergers. This interaction may provide clues about how galaxies behave long before their stars and central regions collide.
Emma Ryan-Weber, a professor at Swinburne University, emphasized the importance of this discovery: "It is the very first time that we have been able to take a photograph of this halo of matter around a galaxy." As researchers continue to observe the early interactions between galaxies, they will gain valuable insights into the processes that shape galactic evolution.
This early interaction suggests that the merging process between the Milky Way and Andromeda is already underway, even though the full collision is still far off. Understanding these subtle interactions will be crucial as scientists continue to study the cosmic collisions that shape the universe.