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As NASA released the first set of images and data obtained by the James Webb Space Telescope in July 2022, we were all reminded of how little we truly know about the cosmos. The state-of-the-art, USD 10 billion telescope now allows scientists to observe galaxies further than ever before. And as they do, they will also be able to compare these faintest and earliest galaxies to today’s giant ellipticals – which still hold many secrets as well.
Sune Toft, professor of Cosmology and Extragalactic Astrophysics at the Niels Bohr Institute, knows all too well the need to better understand elliptical galaxies by looking at their origins in the early universe. With his ConTExt project, he has been investigating the origin of massive ellipticals to clarify whether the most extreme galactic phenomena scientists have observed are in fact different phases of a massive galaxy’s evolution.
Unlike the Milky Way, which is continuously forming new stars, elliptical galaxies are dead, and stopped forming stars billions of years ago. “Elliptical galaxies are the most massive, contain the most stars and reside in galaxy clusters where the largest concentrations of dark matter in the universe keep hundreds of them gravitationally bound together,” Toft explains. “Despite their dominance, we still don’t fully comprehend their cosmic origin. This has been a major challenge for astronomy.”
To test his theory, Toft decided to question the common view that many different types of extreme galaxies were in fact early-phase elliptical galaxies. He says: “I wanted to establish, with solid statistics and accurately measured physical properties, whether dusty starburst galaxies were the direct progenitors of some of the earliest elliptical galaxies we know.”
Starburst galaxies are peculiar in that they have an exceptionally high rate of star formation. Over five years, Toft conducted observations of these galaxies at different wavelengths, ranging from radio, submillimetre and infrared to optical. He analysed statistical studies of millions of galaxies along with in-depth studies of individual galaxies. And his findings have a huge impact on the scientific community.
Thanks to ConTExt, starbursts are now known to be at the origin of most massive galaxies, but they are in fact not triggered by major mergers like the ones scientists could observe in the local universe. Instead, they involve minor merging and regular gas disks. Toft and his team also found examples of quiescent galaxies at z=2 with fast spinning disks, which is consistent with his model.
Inspiring new research
ConTExt is now a widely accepted model of the evolution of massive galaxies, as many different groups of scientists have been reaching similar results to Toft’s. New generations of astronomical surveys using ALMA – the largest telescope in the world – and the James Webb Space Telescope are aiming to further explore the model.
“My project shows that the largest elliptical galaxies can be traced all the way back to less than a billion years after the Big Bang, where they formed in enigmatic starbursts. This was established with detailed studies of the structure, morphology, dynamics, star formation properties, gas contents and other characteristics of different galaxy populations,” Toft adds.
The ConTExt project has also paved the way to a major centre of excellence called the Cosmic Dawn Center at the Niels Bohr Institute and DTU Space in Denmark. As Toft notes: “The main mission of the Center is to push studies of galaxies all the way back to the Cosmic Dawn Epoch, when the very first stars, galaxies and black holes formed. The ConTExt model might still be relevant here, as the most massive galaxies form in the highest densities of underlying dark matter distributions, which are the first to collapse.”