What is a galaxy? That’s a surprisingly difficult question to answer

When is a collection of stars just a collection of stars and when is it a galaxy? This sounds like a nerdy joke or riddle of some kind, but it is instead an actual scientific question that the astronomy community is struggling over.

Those of us who aren’t professional stargazers are so used to thinking of galaxies as easy-to-identify, complete objects, almost like finished works of art. When I use telescopes in my backyard to look at galaxies, I see spirals emitting the bright light of billions of stars – in some cases 100 billion of them or more. From my own research as a theoretical particle cosmologist, I know that these galaxies are permeated and surrounded by an invisible halo of dark matter that extends well beyond their visible regions. From my graduate training, I also know that not all galaxies have a spiral shape. Some of them are ellipticals, shaped like spheres that have been squished from the top. With this point of view, the question of what a galaxy is feels very straightforward.

But, as I wrote a few columns ago, how we organise our understanding of galaxies is always a work in progress. For example, while it is easy to categorise something that has a clear spiral structure and billions of stars, what about something that looks spheroidal and has millions of stars? Is that a galaxy? Actually, what I have just described is the basic definition of a globular cluster. These are collections – jumbles, if you will – of between tens of thousands and millions of stars that are gravitationally bound in a formation that is just a few light years across. Importantly, they live inside galaxies.

The fact that globular clusters are only found inside galaxies would seem to suggest they are obviously a distinct cosmic phenomenon from galaxies themselves. Besides, globular clusters are compact and galaxies are diffuse, more spread out across space. This is true even in the case of dwarf spheroidal galaxies that are gravitationally bound to the Milky Way. They are smaller than our galaxy, but still, relatively speaking, big and spread out. They also tend to have a more diverse range of stars, while globular clusters are comprised of more homogeneous populations. We also now know that dwarf spheroidals are contained in their own dark matter halos, while globular clusters aren’t.

Imagine a nesting doll of galaxies surrounded by dark matter. There is the Milky Way, with one big halo, and then smaller dwarf spheroidals in their own little sub-halos inside of it. That is the general picture at work here. In fact, for some astronomers, this is the thing that really separates the two categories: to be a galaxy is to be full of dark matter.

Until around 2005, this boundary seemed to work well, but then the Sloan Digital Sky Survey (SDSS) released its first dataset. The SDSS, tasked with scanning over a quarter of the night sky, was cataloguing a plethora of never-before-seen objects. Within that data, astronomers found observations of very faint, hard-to-see collections of stars near the Milky Way. These ultra-faint Milky Way satellites challenged the narrative that it is easy to distinguish between globular clusters and galaxies.

In some cases, follow-up observations made clear that the cosmic object in question was indeed a galaxy, chock full of dark matter. That work continues and isn’t always straightforward. Their inherent faintness makes these satellites an observational challenge. These confusing objects are said to live in a “trough of uncertainty”, as Blair Conn, then at the Australian National University in Canberra, and his colleagues called it in a 2018 paper. They aren’t obviously galaxies, but they aren’t obviously not galaxies.

Though we may have expected more data to shrink the trough of uncertainty, in some ways it has deepened. Recent sky surveys have muddied the waters further by revealing a population of even more faint objects, and we now know better than to presume that they aren’t galaxies. And we aren’t all in agreement about what is what: for example, a 2023 paper from a team led by Simon Smith at the University of Victoria in Canada declared the discovery of Ursa Major III, which the researchers called “the least luminous known satellite of the Milky Way”. As confident as this declaration may be, the authors face something of a battle making their case because observations count only 60 stars in the galaxy! That isn’t a typo – 60, not 60,000, 60 million or 60 billion. Just 60.

Small as Ursa Major III might be, it could have a big impact. Last year, a research team claimed that if it is indeed a galaxy, then it can be used to rule out a class of dark matter models. Figuring out whether Ursa Major III and other compact, ultra-faint Milky Way satellites like it are actually galaxies thus has the potential to shake up astrophysics, cosmology and particle physics.

Progress is being made on this question. Last month, William Cerny at Yale University and his colleagues published a report containing the first extensive investigation of a large group of these objects. Their conclusion? They are a mix of types, but more observations are needed. I have nothing certain to tell you about what we know, which is an exciting place to land. Our current position is mid-research excitement, the part where we stand at the edge of what we know and push past it.

What are you reading?

Poets, especially Cortney Lamar Charleston’s collection It’s Important I Remember and Camonghne Felix’s manifesto Let the Poets Govern.

What are you watching?

Too much Alfred Hitchcock!

What are you working on?

I’m getting ready for the US launch of my book The Edge of Space-Time on 7 April!

Chanda Prescod-Weinstein is an associate professor of physics and astronomy at the University of New Hampshire. She is the author of The Disordered Cosmos and the forthcoming book The Edge of Space-Time: Particles, poetry, and the cosmic dream boogie

Mysteries of the universe: Cheshire, England

Spend a weekend with some of the brightest minds in science, as you explore the mysteries of the universe in an exciting programme that includes an excursion to see the iconic Lovell Telescope.