Recent research suggests that the boundaries of the Milky Way may be much broader than previously thought, to the point of already being in contact with its nearest galactic neighbor, Andromeda. This discovery, published in Nature Astronomy, reveals a new definition for what separates interstellar space from the “circumgalactic medium” (CGM), a cloud of gas surrounding galaxies.
The CGM, which accounts for about 70% of a galaxy’s mass, has always been difficult to study directly. To analyze it, scientists rely on the absorption of light from distant objects, such as quasars. However, with advancements in observations and techniques, astronomers have gained more clarity about the behavior of these peripheral regions of galaxies.
Observations from the Hubble telescope had already predicted that the Milky Way would collide with Andromeda in about four billion years. However, the latest discoveries suggest that this collision may already be underway. “It is highly likely that the CGMs of our Milky Way and Andromeda are already overlapping and interacting,” said Nikole Nielsen from Swinburne University. She pointed out that the study provided a more detailed view of these interactions, with observations extending about 100,000 light-years into space. “We are now seeing where the galaxy’s influence ends… and where it merges with the broader cosmic web,” she added.
This transition region between the galaxy and the CGM reveals significant differences in the conditions of the gas, which is heated by external sources, including diffuse emissions from other galaxies. “This interesting change is important and provides some answers to the question of where a galaxy ends,” Nielsen added. These discoveries were made possible by using the W. M. Keck Observatory in Hawaii. Emma Ryan-Weber, also from Swinburne University, commented, “This is the first time we’ve been able to photograph this halo of matter around a galaxy.”
Studying the CGM is crucial for understanding galaxy evolution, especially how they accumulate and expel gases. “The circumgalactic medium plays a huge role in this gas cycle,” Nielsen said, explaining that analyzing different types of galaxies at various stages of star formation could shed light on the internal dynamics of these cosmic giants.
NASA, in turn, has confirmed that the merger between the Milky Way and Andromeda is inevitable. “Our findings are statistically consistent with a head-on collision,” said Roeland van der Marel from the Space Telescope Science Institute. Detailed Hubble measurements show that Andromeda is approaching at an impressive speed of 400,000 km/h, drawn by the gravitational pull between the galaxies and the invisible dark matter surrounding them.
Despite the inevitable collision, there is no immediate danger to the solar system. It is likely that the Sun will be pushed to a new region of the galaxy after the merger, but Earth is not expected to be destroyed. However, the complete impact will take about six billion years to occur, as the galaxies fully merge into a new elliptical structure. During this process, stars from both galaxies will be reconfigured into new orbits around a new galactic center.
Additionally, the Triangulum Galaxy, M33, may play a significant role in this merger, possibly colliding with the Milky Way even before Andromeda. The gravity of dark matter causes nearby galaxies to continue colliding, despite the universe’s expansion, as shown by the Hubble Space Telescope.
This series of photo illustrations shows the predicted merger between our Milky Way galaxy and the neighboring Andromeda galaxy.
- First Row, Left: Present day.
- First Row, Right: In 2 billion years the disk of the approaching Andromeda galaxy is noticeably larger.
- Second Row, Left: In 3.75 billion years Andromeda fills the field of view.
- Second Row, Right: In 3.85 billion years the sky is ablaze with new star formation.
- Third Row, Left: In 3.9 billion years, star formation continues.
- Third Row, Right: In 4 billion years Andromeda is tidally stretched and the Milky Way becomes warped.
- Fourth Row, Left: In 5.1 billion years the cores of the Milky Way and Andromeda appear as a pair of bright lobes.
- Fourth Row, Right: In 7 billion years the merged galaxies form a huge elliptical galaxy, its bright core dominating the nighttime sky.
A century ago, Edwin Hubble determined that Andromeda was a galaxy separate from the Milky Way, which changed the understanding of the universe. Since then, astronomers have studied Andromeda’s movement toward the Milky Way. Jay Anderson from the STScI explained that precise Hubble observations over five to seven years allowed the determination of M31’s lateral motion, eliminating doubts about the imminent collision.
Computer simulations suggest that after the impact, stars will be scattered into new orbits, and the Milky Way will lose its characteristic spiral shape. “In the worst-case scenario, M31 hits the Milky Way head-on, and the stars are scattered into different orbits,” said Gurtina Besla from Columbia University. Studying these mergers remains essential for understanding galaxy formation in the universe.
