Euclid Sheds Light on How Galaxies Form and Transform

ESA’s space telescope captures the astonishing diversity of galaxies – and MPE scientists trace how mergers shape their cores

ESA’s Euclid space telescope is revealing the patterns of galaxy evolution, capturing the shapes, sizes, and structures of millions of galaxies across cosmic time. Scientists from the Max Planck Institute for Extraterrestrial Physics (MPE) are using these data to trace how galaxies grow, merge, and transform, including identifying hundreds of systems with secondary nuclei that hint at the formation channels of supermassive black hole binaries. Euclid also uncovers rare systems with highly ionized emission lines and thousands of previously hidden dwarf galaxies, providing key insights into the building blocks of larger systems like the Milky Way. Together, these observations offer a comprehensive view of how galaxies and their central black holes coevolve across the universe.

Summary:

Euclid Telescope: ESA's Euclid space telescope captures diverse galaxy forms and structures, enhancing understanding of galaxy evolution and mergers.

Galaxy Evolution: Researchers from the Max Planck Institute for Extraterrestrial Physics (MPE) study how galaxies grow and merge, identifying systems with secondary nuclei that may host supermassive black hole binaries.

Data Insights: The first data release includes millions of galaxies, allowing astronomers to investigate connections between galaxy morphology and environmental influences.

Research Breakthroughs: Euclid’s sharp, wide-field images enable the systematic study of the central structures of galaxies and the identification of rare phenomena—including highly ionized emission lines and previously hidden dwarf galaxies—providing crucial insights into galaxy formation.

Comprehensive View: The findings illustrate the relationship between galaxy structure, star formation history, and cosmic environment, offering a holistic view of galactic evolution.

After just one year of observations, ESA’s space telescope Euclid is shedding new light on one of astronomy’s oldest questions: why does the universe contain such a stunning variety of galaxies? Just like flowers, galaxies come in a large variety of different colours, sizes, and shapes — all encapsulated in the term: morphology.

Are these different morphologies linked? How is the evolution of blue spiral galaxies related to that of giant elliptical galaxies? And how much does a galaxy’s environment — whether it lives in crowded clusters or cosmic solitude — influence its shape and fate? With millions of galaxies now catalogued in Euclid’s first data release (Q1, March 2025, ESA), astronomers are gaining access to a new treasure trove of data to address these questions. 

Euclid’s sharp, wide-field view marks a breakthrough in extragalactic astronomy. Its images combine exceptional depth and resolution, allowing scientists to study more than 1.2 million large galaxies in its first year alone—and tens of millions over its six-year mission. 

We understand today that the diversity of galaxies — from majestic grand-design spirals like our own Milky Way to giant ellipticals such as the mighty Messier 87 — is a consequence of their evolutionary paths. Galaxies begin their lives on the right side of the Hubble diagram (see Figure above) as disky, blue, star-forming systems. They move to the left in the diagram as they grow, gradually exhaust their gas supplies, and merge with other systems, eventually forming large elliptical galaxies.

Tracing the hidden growth of galaxies and their black holes

“Euclid offers an unprecedented combination of sharpness and sky coverage — it will map the entire extragalactic sky,” says Maximilian Fabricius, scientist at the Max Planck Institute for Extraterrestrial Physics (MPE), which built significant parts of the telescope’s optics. “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.”

With their team, Maximilian Fabricius and Roberto Saglia have identified hundreds of candidate systems showing potential secondary nuclei in early-type galaxies — on their way to becoming future supermassive black hole binaries. These discoveries provide crucial insights into how galaxies coalesce and how their central structures become depleted or reshaped during mergers.

“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,” explains Fabricius. “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.”

A comprehensive view of cosmic evolution

Euclid’s Q1 release covers 63 square degrees of the extragalactic sky — only about 0.5% of the total dataset the mission will ultimately deliver. Yet, even this small fraction already enables a remarkable range of high-impact studies across all areas of extragalactic astronomy, demonstrating one of Euclid’s key strengths: its ability to efficiently survey vast regions of the sky and reveal rare astronomical phenomena.

Another example is the study by Daniela Vergani et al., co-led by Christoph Saulder (MPE), which identifies a rare population of 65 galaxies exhibiting highly ionised emission lines — signatures of extreme astrophysical phenomena such as active galactic nuclei, shock fronts, or Wolf–Rayet stars — offering a new window into the energetic feedback mechanisms shaping galaxy evolution.

With its remarkable sensitivity, Euclid also reveals that the most common galaxies in the Universe are not the majestic spirals but tiny dwarf galaxies—faint, low–surface-brightness systems that were once too elusive to study in detail. Among the 2,674 dwarf galaxies identified so far, about 58% are dwarf ellipticals and 42% are dwarf irregulars, some containing compact blue cores or globular clusters. These dwarfs are thought to be the building blocks of larger systems like our own Milky Way, offering vital clues to cosmic assembly on the smallest scales.

These studies — from tiny dwarfs to giant ellipticals — demonstrate Euclid’s extraordinary ability to provide a complete, multi-scale view of galaxy formation and evolution. Its data reveal the physical links between a galaxy’s structure, its star-formation history, and its cosmic environment, connecting all phases of galactic life into a single, coherent picture.
Euclid is transforming our understanding of the Universe’s “tuning fork,” showing how galaxies light up with star formation, collide, and fade — and how, at their hearts, black holes and stellar cores evolve together.