How Astronomers Define Where a Galaxy Ends and Interstellar Space Begins

For us, galaxies are where most of the interesting stuff happens in the cosmos. Stars, planets, and black holes are all usually found inside these bustling cosmic neighborhoods. But space isn't just galaxies. In between galaxies are vast expanses of interstellar space — galactic deserts where you might be lucky to stumble across a few hydrogen atoms or, at best, a wandering rogue planet ejected from its home galaxy. 

So, where does a galaxy end and interstellar space begin? There seem to be a number of plausible ways astronomers might define the 'edge' of a galaxy — perhaps where concentrations of gas and dust drop below a certain threshold or even via a galaxy's gravitational influence. 

Nushkia Chamba is a postdoctoral fellow at NASA's Ames Research Center who specializes in studying the outskirts of galaxies, as well as galaxy formation and evolution in general. Here, she helps explain how astronomers have defined the edge of a galaxy and what factors contribute to where a given galaxy's edge is likely to be situated. 

"The short answer is that the ‘boundary’ of a galaxy has been traditionally defined using fixed levels of brightness (or technically called surface brightness isophotes)," says Chamba. However, Chamba has spearheaded recent efforts to more robustly define the edge of a galaxy. "Our work in 2020 and 2022 was the first to discuss an alternative idea of “edge of a galaxy” as a definition of size." 

Specifically, Chamba and her team developed a physically motivated criteria for the boundary of a galaxy based on the required gas density for star formation. This means the outermost region of a galaxy where star formation is still, or likely to take place, is where Chamba and her colleagues have defined the 'edge' of a galaxy.

It's kind of like defining the edge of a city where new housing is still being developed — implying that this 'edge' is a shifting, dynamic thing.  

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Since the boundary of a galaxy is defined as a function of where star formation can occur, astronomers have been interested in how a galaxy's location with respect to other galaxies might influence its size and, therefore, its ability to sustain star formation in its outer regions. 

"We have been trying to understand how galaxies regulate their size during their life and how their environment impacts their growth, depending on if they are located in a crowded cluster with 1000s of galaxies or are nearly isolated," says Chamba. 

Chamba was part of a team that, this year, found that galaxies in crowded environments were up to 50 percent smaller in size than those in near isolation.

"This is consistent with our current understanding of how galaxies undergo gas removal due to environmental interactions and processes which can stop their star formation and growth, compared to more isolated galaxies that can retain their gas for longer periods of time," she says. 

"Many factors can influence the edge of a galaxy, the environment, the morphology (shape and size), the mass of a galaxy in stars (we are yet to investigate how the dark matter affects star formation, which is an open question in astrophysics), but also at which epoch most of the star formation of a galaxy occurred, as the edge of a galaxy not only changes due to their inherent structure and environment but it also changes with time," Chamba explains. 

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When it comes to small, or 'dwarf' galaxies, they are more impacted by their environment, where they are more likely to lose material — gas and dust — as they move through a crowded galactic neighborhood, which in turn affects the galaxy's ability to support new star formation. 

Recent research by Chamba and colleagues has also shed light on the role that supernovas might play in the dispersion of gas within a galaxy — where 'stellar feedback' can influence the inflows and outflows of gas in a galaxy which either supports or negates star formation. 

The ability for astronomers to observe concentrations of gas, and therefore where star formation is possible, in far-off galaxies has been made possible by the development of deep imaging techniques, where telescopes such as the James Webb Space Telescope have been optimized for observing very faint light. 

Read More: How Galaxies Live, Breathe and Die

Chamba says that researchers are working on how more mysterious mechanisms, such as the dispersion of dark matter, might influence star formation in galaxies. Based on the current cosmological framework, it is generally agreed that the size of galaxies — how much material they are composed of — would be set by the properties of the dark matter halo, as it exerts its gravitational influence on surrounding baryonic matter — the stuff we are made of. 

There is still a lot to learn about the factors that drive the evolution of galaxies through time, and Chamba is at the forefront of that endeavor.

Read More: Black Hole Theory Finally Explains How Galaxies Form

Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:

• Postdoctoral fellow at NASA's Ames Research Center. Nushkia Chamba 

• Monthly Notices of the Royal Astronomical Society. A physically motivated definition for the size of galaxies in an era of ultradeep imaging

• Astronomy & Astrophysics. The edges of galaxies: Tracing the limits of star formation

• Astronomy & Astrophysics. The impact of environment on size: Galaxies are 50% smaller in the Fornax Cluster compared to the field

• Astronomy & Astrophysics. Strong size evolution of disc galaxies since z = 1. Readdressing galaxy growth using a physically motivated size indicator

• Eprint arXiv:2408.13311. A Break In the Size-Stellar Mass Relation: Evidence for Quenching and Feedback in Dwarf Galaxies

• Springer International Publishing. Outskirts of Galaxies

Conor Feehly is a New Zealand-based science writer who covers a wide range of topics, including astronomy and neuroscience, with an eye for research at the intersection of science and philosophy. He received a master's in science communication degree from the University of Otago. Conor is a regular contributor to Discover Magazine, with his work also appearing in New Scientist, Nautilus Magazine, Live Science, and New Humanist among others.