After only one year of operations, the European Space Agency’s Euclid mission has begun to unravel the mystery of why galaxies take on different shapes and how these different shapes relate to each other. Answering this question involves tracking how galaxies and their central supermassive black holes grow together over time.
Having only launched in July of 2023, the Euclid space telescope has used its extraordinary field of view to observe a staggering 1.2 million galaxies. These galactic subjects are cataloged in the spacecraft’s first data release, which dropped in March of 2025. It is estimated that, by the end of its 6-year primary mission, Euclid will have studied tens of millions of galaxies. It is therefore little wonder that astronomers are expecting it to make major waves in our understanding of how galaxies evolve.
“Euclid offers an unprecedented combination of sharpness and sky coverage — it will map the entire extragalactic sky,” Maximilian Fabricius, scientist at the Max Planck Institute for Extraterrestrial Physics (MPE), said in a statement. “For the first time, we can systematically study how the shapes and central structures of galaxies relate to their formation history on truly cosmic scales.”
Scientists are aware that the distinct morphology of galaxies, ranging from vast spirals like the Milky Way to featureless ellipticals like Messier 87, results from the course of their evolution. Euclid data has been used to create a “galactic tuning fork” diagram that shows blue star-forming galaxies on the right, moving to the left as they grow and exhaust their star-birthing gas and dust, merge with other galaxies, and eventually form vast elliptical galaxies.
Galaxies grow with their black holes
Fabricius and colleagues began their research by diving into Euclid data and identified galaxies that show potential “secondary nuclei.” These have the potential to join with the existing nuclei to create a supermassive black hole binary. This is a vital stage in the merging of galaxies and helps dictate how the central regions of these galaxies are reshaped during these events.
The identified nuclei both host a supermassive black hole with a mass millions or even billions of times the mass of the sun, which are brought together via the merger between their host galaxies. These black holes initially form a binary system, swirling around each other. But as they orbit each other, this system emits ripples in spacetime called “gravitational waves,” which carry angular momentum away from the system.
This causes the black holes to spiral together until they collide and merge, creating an even more massive supermassive black hole. That means black hole growth via merger is an inevitable outcome of the merger of galaxies that gives rise to huge elliptical galaxies. But before that comes a relatively short “double nuclei” period.
“The most massive black holes lie at the centres of giant elliptical galaxies and are thought to grow primarily through mergers with other supermassive black holes,” Fabricius said. “By detecting and analysing secondary nuclei, Euclid enables us to explore how these enormous black holes continue to grow — and how their growth influences the galaxies that host them.”
The first data release from Euclid only covers around 0.5% of the dataset that the mission will ultimately deliver — but the space telescope has already enabled other forms of research.
The sensitivity of Euclid has already revealed that the most common galaxies in the cosmos are not spiral galaxies like the Milky Way, but rather small and faint dwarf galaxies, which have been too dim to observe in detail previously.
Thus far, Euclid has identified 2,674 dwarf galaxies, some of which contain compact blue cores or globular clusters. This is significant to the evolution of galaxies because it is these dwarf galaxies that are thought to be the building blocks of larger galaxies like the Milky Way.
Thanks to Euclid, our view of the galactic tuning fork is changing and becoming far more detailed, leading to a better understanding of galactic structure and evolution.