The “Milky Way and Andromeda are already overlapping and interacting.”

Personal Space

Researchers are suggesting that the outer boundary of our home Milky Way galaxy may stretch much farther into the vastness of space than initially thought — and is in fact already touching its closest neighbor, the galaxy Andromeda. As detailed in a paper published in the journal Nature Astronomy, the international team of scientists posits a new definition for the boundary between interstellar space and the “circumgalactic medium,” (CGM) the cloud of gas that surrounds galaxies.

Until now, this bubble of gases has been elusive to scientists, forcing them to analyze the light absorbed by celestial objects like quasars to study it, despite the CGM accounting for roughly 70 percent of a galaxy’s mass.

Previous Hubble observations had predicted that the Milky Way is destined for a “head-on” collision with Andromeda in a matter of four billion years.

The latest data, however, suggests that the collision may have techncially already started.

“It’s highly likely that the CGMs of our own Milky Way and Andromeda are already overlapping and interacting,” said Swinburne University associate professor Nikole Nielsen in a statement.

Galaxies Pressed

By using cutting-edge deep space imaging techniques, the team had a far more detailed look, peering some 100,000 light years into space.

“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,” said Nielsen. “But in this case, we seem to have found a fairly clear boundary in this galaxy between its interstellar medium and its circumgalactic medium.”

The conditions inside this cloud of gas surrounding galaxies are surprisingly different than those within the galaxies.

“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 the collective galaxies in the Universe and possibly some contribution is due to shocks,” Nielsen explained.

“It’s this interesting change that is important and provides some answers to the question of where a galaxy ends,” she added.

The team used the W. M. Keck Observatory on Mauna Kea in Hawaii to make their observation.

“It is the very first time that we have been able to take a photograph of this halo of matter around a galaxy,” said Swinburne University professor Emma Ryan-Weber in the statement.

The researchers are hoping to shed new light on how galaxies evolve and how they accrete and expel gases.

“The circumgalactic medium plays a huge role in that cycling of that gas,” Nielsen said. “So, being able to understand what the CGM looks like around galaxies of different types — ones that are star-forming, those that are no longer star-forming, and those that are transitioning between the two — we can observe [how] changes in this reservoir may actually be driving the changes in the galaxy itself.”

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