What is a galaxy? This is surprisingly difficult to answer

When is a collection of stars just a collection of stars and when is it a galaxy? It sounds like a silly joke or riddle, but instead it’s a real 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 backyard to observe galaxies, I see spirals emitting the bright light of billions of stars—in some cases 100 billion 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. I also know from graduate training that not all galaxies are spiral in shape. Some of them are elliptical in the shape of balls that have been squeezed from above. From this point of view, the question of what a galaxy is 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? In fact, what I just described is the basic definition of a globular cluster. These are collections—a hodgepodge, if you will—of tens of thousands to millions of stars that are gravitationally bound in a formation that is only a few light-years across. The important thing is that they live inside galaxies.

The fact that globular clusters are found only inside galaxies would seem to indicate that they are clearly a different cosmic phenomenon than the galaxies themselves. In addition, globular clusters are compact and galaxies are diffuse, more spread out across space. This also applies to dwarf spheroidal galaxies that are gravitationally bound to the Milky Way. They are smaller than our galaxies, but still, relatively speaking, large and spread out. They also tend to have a more diverse range 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 one large halo and then smaller dwarf spheroids in their own little sub-halos within. This is the general picture in this work. 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.

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

In some cases, subsequent observations made it clear that the cosmic object in question was indeed a galaxy, a dark matter shock. This work is ongoing and not always straightforward. Their inherent weakness makes these satellites a challenge to observe. These confusing objects are said to live in the “trough of uncertainty”. Blair Connthen at the Australian National University in Canberra and his colleagues called it va 2018 paper. They’re clearly not galaxies, but they’re clearly not galaxies.

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

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

Progress is being made on this issue. last month Vilém Cerny at Yale University and his colleagues have published a report containing the first large-scale examination of a large group of these objects. Their conclusion? They are a mixture of types, but more observations are needed. I’m not sure what to tell you about what we know, which is an exciting place to land. Our current position is the excitement in the middle of research, the part where we stand at the edge of what we know and overcome it.

what are you reading

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

what do you mean by that?

Too much Alfred Hitchcock!

what are you working on

I am preparing to launch my book in the USA Edge of spacetime on April 7th!

Chanda Prescod-Weinstein is an associate professor of physics and astronomy at the University of New Hampshire. She is an author A disordered universe and an upcoming book The Edge of Space-Time: Particles, Poetry, and the Cosmic Boogie Dream

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