2026 kavli prize in Astrophysics
2026 Kavli
Prize in
Astrophysics
The Norwegian Academy of Science and Letters has decided to award the 2026 Kavli Prize in Astrophysics to
for uncovering the fossil evidence of past mergers proving that the Milky Way galaxy was built through hierarchical accretion
Committee Members
- Per Barth Lilje (chair), University of Oslo, Norway
- Beatriz Barbuy, University of Sao Paolo, Brazil
- Martha Haynes, Cornell University, USA
- Thomas Henning, Max Planck Institute for Astronomy, Germany
- Didier Queloz, ETH Zürich, Switzerland
Citation from The Kavli Prize Committee in Astrophysics
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, and that their signatures remain visible today, billions of years later in our Milky Way galaxy.
Enormous datasets from surveys like the Sloan Digital Sky Survey and the Gaia space mission have delivered 3-dimensional positions, 3-dimensional velocities, and chemical abundances for billions of stars. By combining these datasets with innovative analytical methods and simulations, the three laureates have revealed our Galaxy as the outcome of repeated, dramatic, and violent cosmic collisions. They discovered numerous individual stellar streams and halo substructures that can be traced back to specific major accretion events, revealing a complex assembly history that goes far beyond a simple scenario of smooth, continuous accretion.
Vasily Belokurov demonstrated the importance of the ultra-faint dwarf galaxies for the accretion history of the Milky Way. He presented a comprehensive outline of the accretion history of the Galaxy (the so-called “field of streams”) with profound implications for the dark matter distribution. Belokurov and Helmi, independently of each other, identified the remnant – the Gaia-Enceladus Sausage – of the last major merger with a dwarf galaxy. Belokurov highlighted its strongly radial orbits and dominant role in building the local halo.
Amina Helmi identified stellar streams, remnants of ancient, accreted dwarf galaxies, demonstrating that old mergers can still be traced in local halo stars. She showed that the Gaia-Enceladus Sausage accretion event contributed a large fraction of inner-halo stars and likely heated the early disk 8–11 billion years ago. Helmi applies chemo-dynamics, combining detailed chemical abundances with kinematics, to distinguish in-situ formed stars from accreted ones.
Rodrigo Ibata’s discovery of the Sagittarius Dwarf Spheroidal Galaxy provided the first clear observational evidence that the Milky Way is still accreting satellite galaxies. His later mapping of the Sagittarius tidal stream and his identification of the rich substructure and asymmetry of the Andromeda galaxy’s halo have been central to visualizing the complex debris fields around the two galaxies which even today challenge our understanding of structure formation on galactic scales.
Vasily Belokurov
In his own words:
– Gaia is my demanding muse
Amina Helmi
In her own words:
We all share the same sky
Rodrigo Ibata
In his own words:
- If stars are like diamonds, why don’t they fall down?
Charting galactic growth through hidden mergers
By Edwin Cartlidge, science writer
Gaia in its final phase of construction. Credit: Montserrat Boix
Just as archaeologists use fossils to reconstruct the history of life on Earth, so astronomers look for evidence buried within the night sky to understand how the Milky Way galaxy has evolved over time. The three winners of this year's Kavli Prize in Astrophysics – Vasily Belokurov, Amina Helmi and Rodrigo Ibata – have used very precise measurements of stars' positions, velocities and chemical compositions, together with computer modelling, to establish that the Milky Way grew over the course of its roughly 13bn-year existence by merging with and gobbling up matter from surrounding smaller galaxies.
This is an artist's impression of the Milky Way, based on data from the European Space Agency’s Gaia telescope. Credit: European Space Agency
An increasingly sophisticated series of telescopes on the ground and in space has allowed astronomers to measure the positions of stars on the sky with ever greater precision. By making such observations at different times they can work out stars' velocities across the sky, and then combining these results with measurements of the objects' red- or blue -shifts – how much their light is stretched as they move away from or towards the Earth – researchers can plot complete trajectories in three dimensions and so identify groups of stars that appear to be moving in unison with one another.
An increasingly sophisticated series of telescopes on the ground and in space has allowed astronomers to measure the positions of stars on the sky with ever greater precision. By making such observations at different times they can work out stars' velocities across the sky, and then combining these results with measurements of the objects' red- or blue -shifts – how much their light is stretched as they move away from or towards the Earth – researchers can plot complete trajectories in three dimensions and so identify groups of stars that appear to be moving in unison with one another.
It is by picking out these stars from among the billions of others in our galaxy that this year's laureates have confirmed the hierarchical framework of galaxy formation – the process by which massive galaxies start off quite small and grow progressively larger by consuming or merging with other galaxies in their vicinity. The stars in question might either make up such a galaxy or instead constitute "streams" across the sky that serve as relics of past merging – having been spewed out as their former host galaxy was swallowed by the Milky Way.
In 1994, Rodrigo Ibata and his collaborators obtained the first clear evidence that such mergers continue today when he discovered a large dwarf galaxy in the constellation of Sagittarius being consumed by the Milky Way. This extended group of co-moving stars was closer to our galaxy than any satellite previously discovered but being located on the other side of the galactic centre had been obscured by the vast number of stars in-between it and us.
In the years that followed, Ibata went on to identify the expected tidal stream being generated as the Sagittarius dwarf galaxy is torn apart by the Milky Way over the course of billions of years. He found this stream in data from the Sloan Digital Sky Survey (SDSS), in the form of a gigantic band of carbon-rich stars that wraps around the Milky Way on an almost polar trajectory. The band follows Sagittarius's predicted orbit through the galactic halo – the enormous, low-density spheroid that surrounds the galaxy's spiral-shaped disc.
The SDSS telescope camera. Credit: Sloan Digital Sky Survey
The building housing the SDSS telescope. Credit: Resonance cascade
As more and more data were released from the SDSS survey, astronomers were able to refine their view of the streams from Sagittarius and other dwarf galaxies as well as those from globular clusters. In 2006, Vasily Belokurov and his colleagues revealed that the Sagittarius stream was more complex than previously thought, showing that its giant arc was made up of two if not three different branches. He also realised that the newly mapped web of star trails, named the "Field of Streams", had implications for our understanding of dark matter's role in galaxy formation.
This is an artist's impression of ESA's Gaia satellite observing the Milky Way. The background image of the sky is compiled from data from more than 1.8 billion stars. Credit: European Space Agency
Dark matter is a still unknown substance that scientists theorise formed the gravitational nuclei around which gas condensed to create the kernel of galaxies. Belokurov and co-workers showed that the Sagittarius stream posed a serious challenge to simple models of the Milky Way’s dark-matter halo. More recent data from ESA’s Gaia spacecraft have helped resolve this puzzle by tying the stream’s unusual behaviour to the massive satellite galaxy known as the Large Magellanic Cloud.
This is an image of the Large Magellanic Cloud taken by the European Southern Observatory's VISTA telescope. Credit: ESO/VMC Survey
More than a decade later, Belokurov analysed observations of the inner halo by SDSS and Gaia to find evidence for the biggest merger in the history of the Milky Way – the amusingly named Gaia-Enceladus Sausage. "Sausage" refers to the shape of a graph created by plotting the azimuthal velocity of more than 2,000 metal-rich stars against their radial velocity. Belokurov realised that the sausage-shaped distribution in velocity space – implying that many halo stars move on very stretched-out, needle-like orbits – could not easily be produced by several smaller mergers. Considering also the stars' metal-richness, he concluded that the observations instead pointed to a single, massive ancient collision.
This shows a simulation of the merger between the Milky Way and the Gaia-Enceladus galaxy. Yellow arrows represent the positions and motions of stars originating from Gaia-Enceladus. Credit: ESA (artist’s impression and composition); Koppelman, Villalobos and Helmi (simulation)
Independently, Amina Helmi and her collaborators, using data from Gaia's second release, found that many stars in the nearby halo have opposite trajectories to those in the disk. They also saw that these stars form a distinct population in age and metallicity. Having in addition analysed data from the SDSS' APOGEE spectroscopic survey and found that the retrograde stars have distinctive chemical properties, they concluded that these stars – making up a significant fraction of the halo – must have formed in an external galaxy.
This dish shows Enceladus (right) fighting Athena, his traditional adversary in the mythological war between giants and gods. Credit: Gift of Nicolas Koutoulakis, 1958
Helmi and her team named the colliding galaxy Gaia–Enceladus after Enceladus, the giant of Greek mythology who was son of the Earth (Gaia) and sky and who was said to cause earthquakes. Using the chemical data, she estimated that at the time of the merger – about 10 billion years ago – the total mass of Gaia–Enceladus would have been about a quarter of that of the Milky Way. She also realised, thanks to previous work from herself and others, that the violent clash between these galaxies would not only have created the inner halo but also shaken up the precursor of what we see today as the galaxy's thick disc.