What is a galaxy? It’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 sort, but it is instead an actual scientific question that the astronomy community is grappling with.

Those of us who are not professional stargazers are so used to thinking of galaxies as easily identifiable, complete objects, almost like finished works of art. When I use telescopes in my garden 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 far beyond their visible regions. From my further education I also know that not all galaxies have a spiral shape. Some of them are elliptical, shaped like spheres that have been squeezed from the top. From this point of view, the question of what a galaxy feels is very simple.

But, as I wrote a few columns ago, how we organize our understanding of galaxies is always a work in progress. For example, while it’s easy to categorize something that has a clear spiral structure and billions of stars, what about something that looks spherical and has millions of stars? Is it a galaxy? Actually, what I’ve just described is the basic definition of a bullet heap. 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 only a few light-years across. Importantly, they live inside galaxies.

The fact that globular clusters are only found inside galaxies suggests that they are obviously a distinct cosmic phenomenon from the galaxies themselves. Also, globular clusters are compact and galaxies are diffuse, more spread out in space. This applies even to dwarf globular galaxies that are gravitationally bound to the Milky Way. They are smaller than our galaxy, but still relatively large and spread out. They also tend to have a more diverse selection of stars, while globular clusters consist of more homogeneous populations. We also now know that dwarf spheroids are contained within their own dark matter haloes, while globular clusters are not.

Imagine a nesting doll of galaxies surrounded by dark matter. There is the Milky Way, with a large halo, and then smaller dwarf spheroids in their own little subhalos inside it. It’s the general picture that works 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 limit seemed to work well, but then the Sloan Digital Sky Survey (SDSS) released its first data set. Tasked with scanning over a quarter of the night sky, the SDSS cataloged a host of objects never seen before. Within this data, astronomers found observations of very faint, hard-to-see clusters 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 it clear that the cosmic object in question was actually a galaxy full of dark matter. That work continues and is not always easy. Their inherent weakness makes these satellites an observational challenge. These puzzling 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’re not obviously galaxies, but they’re not obviously galaxies.

While we might have expected more data to shrink the bottom line of uncertainty, in some ways it has deepened. Recent sky surveys have further muddied the waters by revealing a population of even fainter objects, and we now know better than to assume they are not galaxies. And we don’t all agree on which is which: for example, a 2023 paper by a team led by Simon Smith at the University of Victoria in Canada declared the discovery of Ursa Major III, which scientists called “the least luminous known satellite of the Milky Way”. As confident as this statement may be, the authors face something of a battle making their case because observations only count 60 stars in the galaxy! That’s not a typo – 60, not 60,000, 60 million, or 60 billion. Only 60.

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

Progress is being made on this issue. Last month, William Cerny of Yale University and his colleagues published a report containing the first comprehensive survey of a large group of these objects. Their conclusion? They are a mixture 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 tension in the middle of research, the part where we stand on the edge of what we know and push past it.

What are you reading?

Poets, especially Cortney Lamar Charleston’s collection It is important that I remember and Camonghne Felix’s manifesto Let the poets rule.

What are you looking at?

Too much Alfred Hitchcock!

What are you working on?

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

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 Dreamboogie

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 program that includes an excursion to see the iconic Lovell Telescope.