- Hello, and welcome to this episode of the
- Physics World Weekly Podcast,
- which is sponsored by the Kavli Prize.
- The Kavli Prize honors scientists for breakthroughs
- in astrophysics,
- nanoscience,
- and neuroscience,
- transforming our understanding of the big, the small,
- and the complex.
- The vision for the Kavli Prize comes from
- Fred Kavli, a Norwegian American entrepreneur
- and philanthropist
- who turned his lifelong
- fascination with science
- into a lasting legacy for recognizing
- scientific breakthroughs and for supporting basic research.
- The 2026
- Kavli Prize in Astrophysics
- was announced on June 10. And I'm very
- pleased to be joined by the astronomer
- Amina Helmi,
- who shares this year's prize with Vasily
- Bolakarov
- and Rodrigo
- Ibadah.
- She joins me down the line from the
- Kapteine Astronomical
- Institute
- in The Netherlands,
- where she is professor of astronomy.
- I would also like to welcome
- Per Barth Lilia
- to the podcast.
- He is chair of the twenty twenty six
- Kavli Prize Committee in Astrophysics
- and speaks to us from the Institute of
- Theoretical
- Astrophysics
- at Norway's
- University of Oslo,
- where he is professor of cosmology.
- Amina and Per,
- welcome to the podcast,
- and congratulations,
- Amina, for sharing the 2026
- Kavli Prize in Astrophysics.
- Thank you, Hamish.
- Thank you for,
- inviting me,
- and thank you also to Per for selecting
- us to be the winners of this year
- for being the chair of the committee.
- It's an enormous pleasure and an honor to
- be here today.
- And, Per, before we chat about Amina's work
- and that of her fellow laureates,
- can you give us a citation
- for this year's prize
- and perhaps provide a little insight into why
- the committee
- came to its decision?
- Well, the citation,
- for for the award is that it it's
- given for uncovering the fossil evidence of past
- mergers
- proving that the Milky Way galaxy was built
- through hierarchical accretion.
- The three Kavli Prize laureates have fundamentally transformed
- our understanding
- of how massive galaxies
- have evolved over the history of the universe.
- In the hierarchical framework of galaxy formation,
- massive galaxies are thought to grow by accreting
- numerous smaller galactic systems
- from their surrounding circumgalactic
- environment.
- The laureates have provided clear observational evidence
- that major
- merger events have indeed occurred, that their signatures
- remain visible years later in our Milky Way
- galaxy.
- The committee,
- has received quite a number of nominations
- in all different fields covering
- space science and astronomy,
- astrophysics,
- instrument
- instrumentation.
- And,
- we had a tough job, of course,
- which, the committee always has of,
- narrowing down who we wanted to,
- to give the, to give the price to
- this year. But we found that,
- this is really fundamental work. It's also done
- by,
- quite young scientists who are still in their
- most active,
- period their career. And we thought this would
- be a very good,
- signal to them and to this community. And,
- also,
- in addition to that, really,
- a signal showing how important, especially the Gaia
- space mission is, and that it's important to
- really
- we also would like to
- show that this it's important to fund this
- until the final data releases,
- hope to come in 2030.
- And, Amina, can you set the scene for
- our listeners?
- Can you give us an overview of the
- Milky Way?
- What shape is it? How big is it?
- And how many stars does it contain?
- And and how old is it? Can can
- you give us a potted history of the
- Milky Way?
- Yes. Of course. So the Milky Way is,
- as we all know, is the galaxy that
- we live in.
- It's about 90,000
- light years in diameter.
- It has about a 100,000,000,000
- suns or stars.
- And and it is a typical galaxy. That's
- so that's quite interesting.
- It's a disk galaxy.
- The stars in the days the majority of
- these have formed in the last,
- about ten billion years,
- and it's still forming stars at about the
- rate of a few suns per year.
- So because it's it's a typical galaxy, it
- also gives us the the,
- the possibility of trying to understand how galaxy
- formation in general works.
- But the truth is it's our home in
- the universe, so that makes it also extra
- interesting.
- And and is it I mean, I would
- assume that because we're in the Milky Way,
- it's it's easy for us to study. But
- but but is that the case? I mean,
- you know, for example, we can't stand back
- from the Milky Way
- and and look at it from a distance
- and sort of see it whole.
- On the other hand, you know, it's it's
- very all stars are very close to us.
- So are there pros and cons in in
- studying the Milky Way?
- Yes. You're absolutely right. So one of the
- things we're still struggling, for example, is to
- understand how many spiral arms the Milky Way
- has because they are in the disk, and
- we're also in the disk. So we cannot
- raise above
- the plane and look at it from the
- outside.
- But like you say,
- because we have access to,
- information about the individual stars, we can measure
- the properties, we can measure how fast they
- move, where they are located in space,
- We can measure ages and chemical compositions.
- All of that information is really only available
- at the moment for stars in our galaxy.
- And so that is what allows us to
- reconstruct the history in so much detail.
- And, Per, you mentioned that this year's laureates,
- have done,
- some really, really good work on,
- the hierarchical
- accretion,
- and how the Milky Way was formed
- by this process. Can can you give us
- a little more insight
- into hierarchical
- accretion?
- How does it happen, and and why is
- it important
- that we understand the Milky Way was was
- made
- by that process.
- Well, from the,
- nineteen eighties, early nineteen eighties,
- hierarchical,
- accretion,
- has been
- the common theory. But,
- that small structures form first in the universe
- just because
- fluctuations
- on that scale,
- in the density field, which
- finally became galaxies,
- stars, everything,
- was stronger on the small scales than on
- large scales. And then these merged. But we
- haven't really seen,
- this working in,
- this happening in in,
- real time. But now,
- Amina and her colleagues, they are showing us
- that we really see the remnants of recent
- and ongoing such merger events now.
- Thanks, Per. So, Amina, much of your research
- has focused on stellar streams.
- And I think there's actually a stellar stream
- named after you.
- So what is a stellar stream,
- and why do they support the idea of
- hierarchical
- accretion?
- Yes. So, indeed, there's a stream named after
- me.
- So so as Pero was saying, we think
- that galaxies or we thought, from the this
- model from the eighties that galaxies
- must have experienced mergers
- as they grew in mass, particularly
- at early times.
- And as a galaxy orbits another galaxy,
- it loses it feels the tidal forces of
- the bigger system, and
- that leads to the formation of stellar streams.
- So these are groups of stars that have
- lost their parent system,
- and that actually can remain coherent
- for even,
- the age of the universe and longer.
- So these are basically
- if you would,
- make a map of the sky, you will
- see,
- streams of stars, so groups of stars on
- very coherent structures.
- And these groups of stars also share similar
- motions because they are all lost at similar
- times, and therefore, they they move together through
- space.
- So they are actually the way to,
- discover,
- these mergers is by looking for such groups
- of stars that are moving together through space.
- So that's very important because that then helps
- us. If we find such groups of stars,
- then we basically know such stars
- formed together,
- and that pinpoints to a common origin.
- And you've all you've used a technique called
- chemodynamics
- to show that many stars in the Milky
- Way were born in dwarf galaxies.
- Can you explain this technique?
- Yes. Indeed. So,
- chemodynamics
- refers to the fact that we use both
- dynamical information, so say the motions of stars,
- and their chemical properties, the chemical composition. So
- the chemical composition of a star reflects the
- environment in which they were born.
- And so if stars are in a stream,
- then, you can ask whether such a stream
- was indeed formed in a dwarf galaxy.
- And one way to testing to test that
- is actually to look at the chemical compositions
- of the stars because if they they were
- born in a galaxy, they they would have
- similar
- DNA as it were. They will follow a
- very characteristic
- track
- in chemical space.
- So that's that's at the core of identifying,
- the stars associated
- to past mergers.
- I see. And and, Per, I'd like to
- ask you about,
- the other two laureates,
- this year.
- Vasiliy
- Belokharov
- of The UK's University of Cambridge
- is honored for his work on stellar streams.
- What are his main contributions
- to our understanding of the Milky Way?
- Well, there are several, but,
- I can mention,
- first of all, his discovery of what was
- called the field of streams where you see,
- several streams,
- being split up in,
- and
- some of these, at least those which were
- in the original paper, mainly come from the
- merger with the Sagittarius dwarf galaxy, which had
- been discovered by Ibotta earlier.
- And,
- it's showing that these are from several,
- rounds of the Sagittarius dwarf galaxy
- having orbited our own galaxy
- and and go coming through the the the
- disc of our galaxy.
- And,
- he also,
- independently of, Amina,
- discovered,
- what has been called the,
- this,
- Gaia Enceladus,
- sausage or
- Gaia and and so with this
- event, which is the really last merger of
- a big galaxy with our own galaxy,
- some 10,000,000,000 years ago. And,
- in in two different,
- somewhat different ways, Amina and,
- Vasili,
- discovered this
- simultaneously.
- Yeah. I I I was intrigued by this
- sausage.
- I mean, maybe, Amina, could you tell us
- a bit more about this this sausage event?
- I mean, I'm guessing it's called a sausage
- because it's sausage shaped.
- Is it a sausage say shaped structure of
- stars
- out there in the Milky Way?
- Yes. So so the,
- the way,
- the name of the sausage comes from,
- the distribution of stars actually in their velocity
- space.
- So if you look at stars in the
- solar vicinity,
- and you plot the velocities,
- the rotation of velocity against the velocities
- of the oscillations
- in the plane itself, what you see is
- a structure that is very elongated,
- and that's where the name sausage came from.
- And and it reflects how actually was,
- identified
- by,
- Vasili
- in in his work.
- Complimentary
- to that or independently
- of that,
- we identified it in a different way. We
- also looked at the kinematics of stars, but
- as I was explaining before,
- we looked at the chemistry,
- of the stars. And what we found was
- actually that stars that have peculiar kinematics, they're
- slightly retrograde,
- so they they go counterclockwise
- in the opposite direction of the vast majority
- of stars in the disk.
- What we found is that the chemistry
- was clearly telling us that the stars were
- not born in the Milky Way, but were
- born elsewhere
- in a different
- galaxy, in a different system.
- So in that way, the two pictures kind
- of,
- came together, and the system was really identified
- as such as and in fact, we could
- tell when the merger happened and how it
- happened
- and,
- that there was a disk at the time.
- So very many things actually emerged for that
- from that from those studies.
- And I suppose the the story of the
- Milky Way's formation
- isn't over.
- And,
- I I think one of the reasons we
- know that is,
- thanks to the work of the third, laureate,
- Rodrigo,
- Ibata
- of France's
- University
- of Strasbourg.
- Per, can you can you tell us a
- bit about his work
- and,
- his discovery that the Milky Way is still
- accreting
- dwarf galaxies?
- Well, the early work,
- is really all the way back to his
- PhD thesis, I think, which, was discovered in
- 1994,
- of him together with Irvin and Gilmore,
- of the Sagittarius Dwarf Galaxy,
- which is directly on the other side of
- our of the center of our galaxy.
- It's,
- he identified it, I think, through,
- so called carbon stars. These are,
- extremely red, the very luminous stars at the
- end of their lives,
- and which,
- we're we're seeing on on through
- the bulge of our own galaxy on the
- other side of our galaxy, and and this
- was identified. And
- it's going on
- a polar orbit and has left these,
- several,
- streams after it.
- So this is
- probably the most recent merger because it's ongoing
- now.
- Probably this is,
- Sagittarius
- dwarf
- will finally be totally merged into our galaxy.
- Maybe
- a substantial part of it has already,
- been mixed, other stars, and it has been
- mixed into the stars of our galaxy, but
- it's still ongoing.
- So this was,
- his early work.
- What we are also very,
- really like is his,
- much more recent work on
- stellar streams in the Andromedae galaxy, the nearest
- really big galaxy,
- from,
- to ours,
- a bit bigger than our own galaxy.
- Where he has also identified similar,
- collision events, in the in that galaxy. So
- it's not something only happening our in our
- own galaxy. It happens in other galaxies too.
- I see. And and are we confident
- that other galaxies are also
- being built or have been built use, by
- hierarchical
- accretion? Is does this seem to be a
- universal thing in in galaxy formation?
- Well, that it's still a subject under study,
- of course. But,
- for example,
- the European Space Agency
- is now planning this is Araki's mission to
- be launched in 2030,
- which will look look for,
- features in the halos,
- dwarf galaxies and stellar streams and around 80
- nearby galaxies.
- So this,
- can be interesting in the next decade.
- And, Amina, I wanted to ask you about
- the sort of the significance of knowing
- that,
- that this
- accretion process is occurring.
- Does this tell us something about, you know,
- mysteries of of the universe like like dark
- matter?
- You know, for example, does, hierarchical
- accretion,
- does it sort of agree with the idea
- that,
- the universe is permeated
- by dark matter and that dark matter is
- involved
- in this accretion process? Does it tell us
- something about that?
- Yeah. I guess there are multiple ways in
- which it tells us about,
- dark matter. One of the things is, of
- course, if,
- you know, the lambda called dark matter model
- is correct, the mergers
- must have been important in the history of
- galaxies. So finding the mergers is a first
- step.
- Right? And so this and and what we're
- finding for the Milky Way is indeed consistent
- with the expectations
- of this cosmological
- model of hierarchical
- merging.
- On the other hand, these streams,
- because they are composed of stars on very,
- similar orbits, they're very sensitive
- to actually the nature of dark matter.
- The cold dark matter model predicts that the
- halos of galaxies
- have
- tiny,
- substructures
- in them in the form of dark satellites,
- dark subhalos.
- And if these dark subhalos come close to
- stellar streams,
- then they cause a perturbation.
- It is as if these streams are moving
- in a on on a bumpy road as
- it were, and you would see the bumps
- if such,
- collisions or encounters would happen.
- So stellar streams are also useful in this
- context, and what people are finding
- at the moment is that very many of
- the streams
- actually show signs of perturbations,
- which is very interesting,
- except that for at the moment, it's hard
- to disentangle
- what are the sources of the perturbations.
- So we're really working out what other ways,
- could be,
- could actually cause,
- the the the features that we see in
- the streams. It might be these dark matter
- subhalos, but we're not certain yet.
- I see. And and so
- so when you look at a galaxy
- or at the Milky Way,
- your
- I suppose your interpretation
- of the structure would would lean towards,
- cold dark matter rather than, you know, for
- example, the theory of modified gravity,
- like MOND,
- the hierarchical
- accretion.
- Should it be interpreted as, well proof is
- probably a strong word for dark matter rather
- than evidence for,
- MOND? Or or are the things that you're
- looking at maybe too
- or the or the the the length scales
- too small to really tell you anything about
- something like MOND?
- So so in fact, I think MOND,
- can be tested. We've also tested our, MOND
- using stellar streams,
- and we find we find it's not a
- very good description
- actually of the motions of stars in stellar
- streams.
- But that doesn't rule out that there may
- be other theories, alternative theories of gravity that
- could explain what we see around us. I
- think
- if we have an alternative model, it probably
- will be very close to the Lambda CDM
- model because,
- you know, the it works. It seems to
- it's not proof, like you say, because that's
- too strong, but, certainly, there is consistency
- as far as we can tell.
- And,
- there must be lots that we don't know
- about the Milky Way.
- Amina,
- what's the big mystery for you? What's the
- mystery that you would like to solve,
- you know, if if you had the chance?
- I guess there are quite a few mysteries.
- I think I I can think of a
- couple. One is indeed the nature of dark
- matter. Right? So using streams to really constrain
- if,
- establish if there are these dark matter clumps
- orbiting in the halo of our galaxy.
- That is one really big question.
- But also going further back in time, we've
- been discussing,
- you know, the last merger that we know
- of, last big merger ten billion years ago.
- What happened before that? And that's actually rather
- hard,
- to figure out because
- at the early epochs, the universe was a
- lot denser, so mergers
- were happening a lot more often. And then
- you also get mergers of similar object
- in terms of their sizes or masses.
- So it becomes all rather,
- difficult to disentangle.
- But I think
- with the new,
- Gaia data releases,
- and also with new surveys giving us chemistry,
- I'm hopeful that we can push the boundary
- much further back in time.
- And and we've been talking about, I suppose,
- the history or the archaeology
- of the Milky Way.
- What what does
- your research and the research of your of
- your colleagues, the other laureates, what does it
- tell us about the fate of the Milky
- Way? I mean, do you think is it
- gonna keep going
- on accreting
- dwarf galaxies, or has it hoovered up all
- the dwarf galaxies that are available?
- And, you know, it's it it'll
- go into a different phase of its existence,
- or maybe it's in that
- sort of steady phase at the moment. What
- what what's going to happen to the Milky
- Way?
- So as as Per was saying,
- it's currently eating up Sagittarius.
- Right? So I think it will be one
- more passage of the dwarf and it will
- be gone. It will be completely dissolved.
- What we're seeing is it's actually interacting with
- the large and small Magellanic
- clouds.
- And these are more massive, so that also
- leaves an imprint on the on the Milky
- Way. We can actually see a response of
- the Milky Way to their present
- presence.
- So that is, the next
- merger.
- And then in the in the future,
- what we see is also,
- you know, our neighbor that also Rodrigo Ibotta
- has studied, the m 31 galaxy, the Andromeda
- galaxy.
- And it's very likely that the Milky Way
- and Andromeda will actually merge in the future.
- But the sky will look rather different,
- at the time because the moment you merge
- two systems of similar size, the discs are
- very perturbed,
- and so we may end up in an
- elliptical galaxy. But, you know, it's a long
- way down the road.
- I see. Okay. And and what about you,
- Per? What's,
- I mean, what would you like to see
- in terms of Milky Way
- research or, you know, research on galaxies in
- general? What what what what's your wish list?
- What would you like to see
- happen in the future of research?
- Well, as a cosmologist,
- I'm most intrigued really by,
- the the work which Amina and her colleagues
- are doing,
- on what what it can say about the
- nature of the dark matter. That is really,
- because
- there have been a number of of
- problems with the Lambda CDM model, which
- grew up in the nineteen nineties and really
- was
- and so is,
- to from the late nineteen nineties,
- have been the
- really leading theory.
- And I want to see that dead in
- some way, because it's been with us for
- so long now.
- No. It it gives a very, very good
- description of,
- of our observations,
- but I would hope to find something fishy
- there. And,
- what has been looked upon as a problem
- was what, Amina
- mentioned that,
- the the the theory predicts
- very many more dwarf galaxies
- than we see. And the way out of
- that is to say that, well, they are
- there in dark matter, but they are not
- there in stars. They they they don't have
- stars.
- And,
- if, I mean, I've, said we can study
- this further in the by the stellar streams.
- That is something I'm looking forward to see.
- Well, that's great. I mean, it sounds like,
- there's lots more to know about the Milky
- Way.
- Amina, congratulations
- again
- on,
- winning or sharing this year's,
- Kavli Prize in astrophysics.
- And, Per, thank you, and to Amina for
- coming on the podcast.
- Thank you.
- That was the astronomer
- Amina Helmi
- who shares the 2026
- Kavli Prize in Astrophysics
- with Vasili
- Balakarov
- and Rodrigo
- Ibadah.
- We were joined by Per Barth Lilia, who
- is chair of the twenty twenty six
- Kavli Prize Committee in Astrophysics.
- Congratulations
- to Amina, and thanks to both of my
- guests for their insights into the Milky Way.
- This episode is sponsored by the Kavli Prize,
- which honors scientists for breakthroughs in astrophysics,
- nanoscience,
- and neuroscience,
- transforming our understanding of the big, the small,
- and the complex.
- The vision for the Kavli Prize comes from
- Fred Kavli, a Norwegian
- American
- entrepreneur
- and philanthropist
- who turned his lifelong
- fascination with science
- into a lasting legacy for recognizing
- scientific breakthroughs
- and for supporting basic research.
- I'm afraid that's all the time we have
- for this week's podcast.
- I'm Hamish Johnston, and our producer is Fred
- Iles.
- The music in this episode was composed and
- performed
- by the physicist
- Philip Moriarty,
- and it's called
- one three seven.
- We'll be back again next week.