Like bonfires on a cold winter night, galaxies are surrounded by plumes of gas and dust that waft into the shadows. This thin layer of material is known as the circumgalactic medium, a halo large enough to contain about 70 percent of the galaxy’s visible mass. Despite its dominant presence in the universe, little is known about the medium’s typical structure, making it difficult to tell where the edge of one cosmic bonfire ends and the next begins.

A new study of a single galaxy about 270 million light-years away has revealed interactions between the circumgalactic medium (CGM) and the galactic radiative disk, helping to define a boundary that suggests our own galaxy may extend beyond what we imagine.

If so, these implications could mean that the long-predicted future collision of the Milky Way with the Andromeda galaxy may already be underway, at least in terms of the onset of the intersection of the media around the two galaxies.

Australian and US astronomers used the highly sensitive Keck Cosmic Web Imager to capture several images focused on the relatively small spiral galaxy IRAS 08339+6517 (IRAS08 for short).

Most previous CGM studies used light powered by black holes in even more distant background galaxies. While the bright light can reveal many details in the environment, its narrow beam limits the results to only a small part of the halo.

By capturing a much larger area, about 90,000 light-years beyond the galaxy’s luminous disk, the team was able to analyse remarkable changes in the composition of the environment.

Some images of the field showed a filament of neutral hydrogen gas connecting IRAS08 to its even smaller neighbor via the cosmic web, so it’s not surprising that the team measured a large amount of hydrogen drifting in the void. What was unexpected was the discovery of hydrogen mixed with oxygen, a much heavier element stripped of its electrons, far beyond the filaments of cosmic gas.

“We found this everywhere we looked, and it was really exciting and a little bit unexpected,” said lead author Nicole Nielsen, an astrophysicist at Swinburne University in Australia.

Atoms floating in the intergalactic void actually have only two heat sources. One is the faint light of stars in distant galaxies. The other is a random collision with another atom. Both can be calculated to predict the expected ionization pattern of a mixture of hydrogen and oxygen. Using spectral data from scattering imaging, the researchers mapped changes in ionization intensity on a scale of a few thousand light-years, defining the region of influence of IRAS08’s own starlight that is slightly dimmed.

“In the CGM, the gas is heated by something different from the typical conditions inside galaxies; this probably includes heating by the diffuse radiation from the collective galaxies in the Universe and probably some contribution from shocks,” Nielsen says. “This interesting change is significant and provides some answers to the question of where the galaxy ends.”

This discovery not only better defines what a galaxy is, but also reveals how smoke and fire in the universe have combined and evolved over the ages. Since we are deep within the Milky Way, it is difficult to accurately define the boundaries of our own galactic borders. To summarize the results, we can imagine that the surrounding gas field heated by our stars may already be blending with the halo of our nearest neighbor.

“It is very likely that the CGMs of the Milky Way and Andromeda are already overlapping and interacting,” Nielsen says. The study was published in: Nature Astronomy.