Cannibal galaxies and dark matter hurricanes

27 November 2018

Interview with

Wyn Evans, Institute of Astronomy at Cambridge University

The “GAIA” space observatory craft was launched by the European Space Agency in 2013. Since then it’s been scanning the skies with a 1 billion pixel camera. The aim is to image millions of stars in our Milky Way galaxy to understand how our cosmic neighbourhood has evolved and how it behaves. Cambridge Institute of Astronomy researcher Wyn Evans is one of the scientists working on the project and joined Chris Smith in the studio.

Wyn - We've known the galaxy is a messy eater for some time but this is the latest example of its guzzlings. The galaxy is called Antlia 2 but we call it a feeble giant - it’s feeble because it's got very few stars, but it's a giant because it's very extended. It's the least concentrated galaxy that we know.

Chris - Now what observations has Gaia made that enable you to know that our Milky Way galaxy ate this other entity back in history? And when did that happen?

Wyn - Gaia is providing precise positions and velocities of billions of stars in our galaxy. We look for stars that are moving in a coherent way, different from the normal pattern of rotation of stars in the Milky Way disk. That is how we found Antlia 2 and that collision happened billions of years ago.

Chris - In other words we've got this cloud, for want of a better word, of stars which are doing their own thing and you can see that they're moving in a trajectory that's slightly different than what we can really see as native stars to the Milky Way. And so that tells you that those have obviously come in with their own agenda, their own direction, movement, velocity, and that's how you could pick them out and tell that they've been eaten?

Wyn  - That's exactly it. The galaxy is a busy place and there are lots of stars doing lots of things. We're trying to pick out stars that are having a particular dance - a particular way of moving.

Chris - Now in its early history how would the Milky Way have ended up encountering this other galaxy that it then ended up consuming?

Wyn - The Milky Way is a big galaxy, it's a heavy eater, so it pulls in smaller galaxies by its gravitational pull. Antlia 2 or the feeble giant was just unlucky and fell into the gravitational field of the Milky Way Galaxy and was gradually eaten and  pulled apart.

Chris - And the stars that have been incorporated - do they now begin to adopt the same paths as the stars that are native to the Milky Way? Or will they continue on some bizarre trajectory and eventually leave again, or have they been captured indefinitely?

Wyn - They are still intact at the moment but, as the feeble giant falls towards the centre of the Milky Way, it will increasingly be torn apart and these storms will form part of that diffuse corona around the Milky Way called the stellar halo.

Chris - Now when you look at those stars I presume that one of the other things you can do is to is to tell quite a lot about them in terms of their size, how developed, how old they are and what they're burning and how fast. And so that tells you a bit about the history of the galaxy they came from. So what is their history?

Wyn - We can indeed do that. We can with spectroscopy measure the metallicity of these stars, which tells us something about the chemical history of the galaxy. It's harder to get ages of stars, that's really tough. So we know that the galaxy is very metal poor, so we do know that it was formed a long time ago - shortly after the Big Bang.

Chris - Okay well that's interesting. And so this tells us obviously a bit about the patch of space that these galaxies were in, and how they were formed, and what that was sort of like. What the arrangements must have been like in the early period of the universe then.

Wyn - Indeed these are fossils, if you like, akin to the kind of fossils that we find in rocks on the earth. They're telling us the conditions at the very early universe.

Chris - And do they actually fit with our models? Because obviously scientists like yourself are designing models of how the universe formed and evolved and is continuing to evolve. And those are only theories - you've got to then apply data like these that you've collected, in order to test those theories. So does it look like our models of the early universe are right?

Wyn - Well Antlia 2 is a very unusual galaxy and at present we can't fully explain it. It is so diffuse that the backbone of the galaxy, which is formed by the dark matter, must have what is called core - so as to protect the stars and enable them to remain intact and that is not currently predicted by our theories of galaxy formation.

Chris - Oh good because that gives you something to go after now doesn't it! Because if it all fitted all the theories life would be pretty boring.

Wyn - That's always the case with new data!

Chris - Now you mentioned dark matter. I started on that point because your colleagues are now saying the other thing that Gaia is detecting evidence for, possibly, is a hurricane of dark matter coming through. What's dark matter and why's there a hurricane of it passing through our cosmic neighborhood?

Wyn - Well dark matter forms the backbone of galaxies. It's probably some form of elementary particle and there's abundant astronomical evidence for its existence, but there's no direct detection of this mysterious particle. What we've done is we've discovered a stellar stream. And that itself is part of the disgorging of an earlier galaxy - it's a galactic meal, the remnants of an earlier effect of galactic cannibalism. So the stream is passing through the solar neighborhood and it's accompanied by a stream of dark matter.

Now that sounds rather terrifying but the interaction of the dark matter particles with ourselves is very low, so we can go about our daily lives drinking coffee, having toast, without any ill effects. But this will have an important consequence for direct dark matter detection experiments which are normally deep underground and are looking for tiny recoils.

Chris - And how will this help you to to try and fathom out what dark matter is?

Wyn - Well the stream is hitting the solar system head on. So there's a very large relative velocity between the sun, the earth, and these dark matter particles - so that will predict a large number of very high velocity recoil events in these direct dark matter detection experiments. So this is a precise signal that can now be sought in these experiments and if it is indeed fine, that is very convincing evidence for the existence of dark matter.

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