In a groundbreaking study published in the journal Nature Astronomy, scientists have revealed that the boundary of our Milky Way galaxy stretches much farther into space than previously anticipated. Moreover, it is already in contact with our neighboring galaxy, Andromeda. The findings challenge previous assumptions about the configuration of our galactic home and shed light on the complex interactions within and between galaxies.

The research team, consisting of international scientists, proposes a revised definition for the boundary between interstellar space and the circumgalactic medium (CGM) - the gas cloud enveloping galaxies. The CGM, which comprises about 70 percent of a galaxy's mass, has proven elusive to scientists in the past, forcing them to rely on studying the light absorbed by celestial objects like quasars.

Earlier studies using the Hubble Space Telescope had predicted a head-on collision between the Milky Way and Andromeda in approximately four billion years. However, the latest data suggests that this collision may have technically already commenced. "It's highly likely that the CGMs of our Milky Way and Andromeda are already overlapping and interacting," stated Associate Professor Nikole Nielsen from Swinburne University.

This discovery offers valuable insights into the boundary between a galaxy's interstellar medium and its circumgalactic medium. Professor Nielsen explains, "We're now seeing where the galaxy's influence stops, the transition where it becomes part of what surrounds the galaxy, and eventually, where it joins the wider cosmic web and other galaxies."

Notably, the conditions within this gas cloud surrounding galaxies differ significantly from those within the galaxies themselves. According to Professor Nielsen, "In the CGM, the gas is being heated by something other than typical conditions inside galaxies. This likely includes heating from the diffuse emissions from collective galaxies in the universe and possibly some contribution due to shocks." These differences contribute to our understanding of the elusive concept of where a galaxy truly ends.

Excitingly, this study marks the first time scientists have been able to capture an image of the halo of matter surrounding a galaxy. Professor Emma Ryan-Weber from Swinburne University exclaims, "It is the very first time we have been able to take a photograph of this halo of matter around a galaxy." This breakthrough opens up new avenues for investigating the evolution of galaxies and the processes involved in gas accretion and expulsion.

By studying the circumgalactic medium, researchers hope to gain a deeper understanding of how galaxies evolve, as well as the mechanisms behind their gas accretion and expulsion. "The circumgalactic medium plays a huge role in the cycling of gas," affirms Professor Nielsen. "Therefore, understanding what the CGM looks like around galaxies of different types can help us observe how changes in this reservoir may drive changes within the galaxy itself."

The discovery that the Milky Way is already overlapping and interacting with Andromeda brings forth a new era of galactic exploration and paves the way for further revelations about the mysteries of the cosmos. As scientists uncover more about our place within the celestial tapestry, the allure and fascination with the vastness of space only continue to deepen.