A dwarf galaxy is a type of galaxy that’s much smaller than our own Milky Way and can have as few as 1,000s of stars. They are incredibly common throughout the Universe and can often be found in galaxy clusters or as companions to larger galaxies.
Tucana II is a dwarf galaxy known to contain large amounts of dark matter, a mysterious substance that can be inferred via its gravitational influence, but which cannot be directly observed.
It is also at the heart of a study headed by Professor Anna Frebel and Anirudh Chiti, astronomers at MIT in Cambridge, Massachusetts, USA.
Professor Frebel and Chiti have been studying Tucana II and made a number of discoveries about this dwarf galaxy that put a question mark over what we know about the early Universe.
What is a dwarf galaxy?
Dwarf galaxies are a few thousand stars with a dark matter halo around them. We know of star clusters in the Milky Way that they have up to a million stars, but they don’t have a dark matter halo pushing the stars together, they just hold each other.
Dark matter acts like normal matter, which means we can measure its gravity.
When you have a galaxy, its stars literally feel this gravity. Based on simple laws of physics we can deduce that this dark matter halo is there.
What’s so special about Tucana II?
We were looking at the faint dwarf galaxy Tucana II. As we often do – we looked for something and then found something different.
I’ve studied many of the 30 small galaxies orbiting the Milky Way trying to work out the chemical composition of their stars so that we can learn what has happened over billions of years. The stars tell us that story.
Usually I use spectroscopy to turn the starlight into rainbows to study, but because these small galaxies are far away, you don’t get enough photons with your telescope.
That’s a big problem because it means we can only study the brightest stars, and characterising an entire galaxy with only one star is far from ideal.
So, along with my PhD student Anirudh Chiti, we developed a new technique to get information on composition based on specialised images rather than spectra.
The motivation was to move beyond the one, two, three stars per galaxy and turn it into 10, 20 and 30. We used the SkyMapper Telescope to do this.
How did you use SkyMapper?
SkyMapper has a specific filter that lets you see purple – we selected near-ultraviolet light around the wavelength of a very strong calcium absorption line near 400nm.
The light that we see there is a reflection of how much calcium might be in the star and we can use that as a proxy of its overall chemical content.
The fewer calcium and heavy elements are in that star, the less absorption and the brighter the star will shine when observed with that filter.
This is exciting because it means that the stars we are interested in will light up like traffic lights.
You may ask why we are looking for low calcium stars. The oldest stars in the Universe have the least amount of heavy elements because they formed at a time when not much of that junk had been made yet.
Tucana II is around 13 billion years old.
What did you find?
Because SkyMapper surveys the entire southern hemisphere sky, each image is broad.
We ran these through our algorithm and out popped seven member stars of Tucana II really far out, and we said “What’s going on?”
The stars staggered out to almost 10 times the size of the core. That was surprising, so we had to check our technique and numbers, but we concluded they were all part of the same galaxy.
It made us rethink what these small galaxies might be like. This is also the first time that a dark matter halo has been extensively mapped very far away from its galactic centre.
It turns out that the entire galaxy is three to five times as massive as previously thought.
What do the stars at Tucana II’s edge tell us?
They confirm that Tucana II is really old. The stars further out had a more primitive chemical make-up than those in the core.
And that aligns with the theoretical simulation in a follow-up paper that suggests Tucana is the product of a merger of two of the very first galaxies.
When you smash galaxies together, chaos ensues and stars get scattered around.
How does this affect our understanding of the early Universe?
It offers up a great, new prospect for people modelling early star formation, early galaxy formation and early element production.
The idea of getting the beginning of the story of galaxy formation right, which began a few hundred million years after the Big Bang, is going to be huge!
We know what the end is: that’s today. So having the beginning right is an important step because then we can interpolate everything in between.
This interview originally appeared in the May 2021 issue of BBC Sky at Night Magazine.