Galaxies are much bigger than we originally believed

Where does a galaxy end and deep space begin? A recently published study on a gas clouds provides a bit of clarity on this question.

Galaxies aren’t isolated entities floating in the void. They’re surrounded by an invisible cloud of gas called the circumgalactic medium (CGM), which engulfs the visible galactic disc.

This gaseous halo constitutes about 70% of the galaxy’s mass excluding dark matter. Elusive till now, we’ve only been able to observe it indirectly, through the light absorbed from a background object like a quasar.

Piercing through the cosmic veil

Imagine trying to visualize an entire forest through a narrow beam of light. That’s pretty much the challenge scientists faced while studying the gaseous cloud, until now.

However, a cutting-edge study has pierced through this cosmic veil, observing the CGM of a galaxy located a whopping 270 million light years away.

The observed gas cloud extends a colossal 100,000 light years into space, while the starlight from the galaxy’s disc, the part we see, extends only 7,800 light years from its center.

Galaxy’s gas in action

The lead author of the paper is Associate Professor Nikole M. Nielsen, a researcher with Swinburne University and ASTRO 3D, and an Assistant Professor at the University of Oklahoma.

With an ensemble of researchers from across the globe, they studied the connection between hydrogen and oxygen from the galaxy’s center to the far ends of space.

Galaxy’s influence

“We found it everywhere we looked, which was really exciting and kind of surprising,” says Dr. Nielsen. This observation traces out where the galaxy’s influence ceases and where it becomes a part of what’s surrounding the galaxy.

It’s like encountering a line in the cosmic sand, dividing the interstellar medium of the galaxy from its CGM.

Galaxy’s gas cloud conditions

“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,” says Dr Nielsen.

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

The Keck Cosmic Web Imager (KCWI) on the 10-meter Keck telescope in Hawaii made this groundbreaking discovery feasible.

“These one-of-a-kind observations require the very dark sky that is only available at the Keck Observatory on Mauna Kea,” said one of the paper’s authors, Swinburne’s Associate Professor Deanne Fisher.

The KCWI allows scientists to capture thousands of spectra simultaneously, changing how we measure and quantify the diffuse gas around galaxies.

Thanks to this instrument, it’s the first time we’ve managed to take a snapshot of a galaxy’s halo. This marks a revolutionary step in our understanding of galaxies.

Piecing together the galactic puzzle

The circumgalactic medium plays a huge role in that cycling of that gas.

According to Dr. Nielsen, 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 differences in this gas.

“This might drive the differences within the galaxies themselves, and changes in this reservoir may actually be driving the changes in the galaxy itself,” Dr. Nielsen explained.

How do galaxies evolve? How do they get their gas? Where does that gas go? The study adds another piece in solving the puzzle that is galaxy evolution. The CGM plays a vital role in the cycling of the gas, fueling galaxies’ growth over time.

By observing difference in gas around varied types of galaxies — star-forming, non-star-forming, and transitioning — it unfolds the granular picture of how galaxies change and interact, and the potential impact they might have on each other.

So, the next time you look up at the sky, try to imagine the cosmic dance unfolding in front of your eyes – galaxies intermingling, evolving, and forming the universe as we know it.

The study is published in the journal Nature Astronomy.

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