The European Space Agency's Euclid mission-designed to map the geometry of the dark Universe with unprecedented precision-continues to deliver its first scientific insights. The Euclid Consortium has published a fresh set of seven scientific papers based on data from the Euclid Quick Data Release and also unveiled a new Euclid-derived visual collage illustrating the classical 'Tuning Fork' of galaxy morphologies.
In this interview, Francine Marleau of the Department of Astro- and Particle Physics summarizes the results.
The Euclid telescope has already provided groundbreaking insights into galaxy formation and evolution. Can you elaborate on how the new results has advanced our understanding of the earliest galaxies?
Francine Marleau: Although Euclid is primarily designed to study dark energy and large-scale structure, its imaging and near-infrared capabilities are already making significant contributions to galaxy formation science. The telescope's first data release covers approximately 63 square degrees of the sky and includes over 20 million galaxies, probing cosmic look-back times of up to about 10.5 billion years. Euclid's unique combination of excellent image quality, near-infrared photometry, and wide-field coverage within a single pointing enables the identification and characterization of numerous faint, distant galaxies in ways that were not previously possible with other telescope facilities. For the earliest galaxies, this provides more robust statistics - rather than relying on small samples of sources - to measure how galaxy population properties (such as star formation rates, morphologies, and clustering) evolve at high redshift, allowing us to trace their mass assembly history.
The Euclid mission has identified thousands of dwarf galaxies, which are thought to play a key role in the formation of larger galaxies. What are the implications of these findings for our understanding of cosmic evolution?
Francine Marleau: The detection of large, well-characterized samples of dwarf galaxies represents a major milestone, as dwarfs play a crucial role in hierarchical structure formation and provide some of the strongest constraints on galaxy formation and dark matter models. Recent results report 2674 dwarf galaxy candidates from a small portion of the Euclid Quick Data Release (Q1), with the following morphological mix: 58% dwarf ellipticals, 42% dwarf irregulars, and a few systems hosting globular clusters or blue compact centers. In the Euclid Perseus cluster dataset, 1100 dwarf candidates have been identified, more than doubling previous dwarf counts in that environment. Euclid's depth and spatial resolution enable not only detection but also detailed structural characterization of dwarfs - including their sizes, surface brightnesses, nuclei, and globular cluster systems - across a range of environments. The abundance of dwarfs reinforces their role as the fundamental building blocks - through merging and accretion - of more massive galaxies. As Euclid's imaging surveys progressively expand sky coverage, the full scientific impact on dwarf galaxy studies will emerge from successive data releases sampling increasingly diverse environments. With these growing samples, it will become possible to test how many dwarfs remain isolated, how many merge, and how their properties (mass, metallicity, and star formation history) vary with environment. The spatial distribution of dwarfs will also probe the small-scale regime of structure formation and dark matter models.
The ARTEMIDE algorithm appears to be a significant development in identifying gravitational arcs and studying mass distributions in galaxies and clusters. How do you envision this tool shaping future research in astrophysics?
Francine Marleau: Strong gravitational lensing is one of the most powerful methods for measuring galaxy masses at cosmological distances. Euclid has already discovered over 500 lenses in its images, soon making it the largest lens sample from a single survey, with the total expected to reach around one hundred thousand by the end of the mission. With tools such as ARTEMIDE, it will be possible to construct one of the largest catalogues of gravitational arcs across a wide range of redshifts, masses, and environments. This will enable statistical studies of lens populations, mass-profile evolution, the baryon-dark matter connection, and sub-halo abundance through detailed modelling of arcs in galaxies and clusters.
The morphological classification of galaxies has been a cornerstone of astrophysics for decades. How has Euclid's imaging capabilities and data contributed to refining this classification, and what new insights have emerged about the relationship between galaxy morphology and environment?
Francine Marleau: Euclid's capabilities are significantly advancing our ability to study galaxy formation and evolution through galaxy morphology. The high quality of Euclid's data allows morphological features to be classified for very large samples of galaxies across a wide range of redshifts and sky areas, enabling statistically robust measurements of how morphological types evolve with cosmic time and environment, rather than relying on small fields. The morphology-environment relation is now being quantified with greater precision, with Euclid data showing that, at fixed stellar mass, environmental density influences both the fraction of quenched galaxies and the morphological mix up to a redshift of 0.75. This strengthens the view that environment plays an important role in driving morphological transformation, particularly in low-mass galaxies. Euclid's sensitivity to faint surface brightness levels allows for a more complete morphological classification of lower-mass systems and their outer regions. We are beginning to see how morphology correlates not only with local density but also with large-scale environment (clusters, filaments, voids), and how the outskirts - such as tidal features and stellar streams - contribute to morphological transformation.
