Euclid telescope captured galaxy NGC 646 as a festive space garland
A new image of galaxy NGC 646, released by the European Space Agency (ESA) in collaboration with the Euclid mission consortium, has transformed a distant corner of the universe into a kind of holiday postcard. The image features a large barred spiral galaxy whose bright spiral arms overflow like a decorative garland, while stars and more distant galaxies flicker in the background. Behind the aesthetic impression lies an ambitious scientific story: the Euclid team observes this and many other galaxies to build the most detailed three-dimensional map of the universe and better understand the role of dark matter and dark energy.
A holiday postcard from the constellation Hydrus
NGC 646 is located in the southern constellation Hydrus, invisible to the naked eye from most of Europe but well accessible to telescopes in the southern hemisphere. It is a large barred spiral galaxy – its central part consists not only of a compact glowing "bulge," but an elongated stellar "bar" emerges from it, from which the spiral arms then develop. Such a structure is typical for many mature spiral galaxies in today's universe.
The galaxy was first recorded by British astronomer John Herschel in 1834 during an extensive campaign to observe the southern sky. Today, nearly two centuries later, Euclid observes it with sophisticated instruments in the visible and near-infrared range, revealing details inaccessible to classical ground-based telescopes.
Redshift measurements show that NGC 646 is moving away from us at a speed of approximately 8,145 kilometers per second, which corresponds to a distance of about 120 megaparsecs – or approximately 392 million light-years. The light we see now started its journey toward us when the era of dinosaurs was just beginning in popular culture, and human civilization did not exist even in traces. Yet, on cosmological scales, NGC 646 still belongs to the "neighborhood" compared to the most distant galaxies Euclid plans to study.
In Euclid's image, NGC 646 looks like a bright central core from which two clearly defined spiral arms emerge. Their bluish tint betrays the presence of young, hot stars, while dark strands of dust outline the structure of the disk. This contrast makes the galaxy truly resemble a space garland stretched across the black background of the universe, which prompted the comparison to holiday decorations.
"Neighbor" PGC 6014: close in the image, far in reality
In the left part of the image, a smaller, rounder galaxy labeled PGC 6014 is visible. At first glance, it appears as if NGC 646 has a close companion "partner" with which it exchanges gravitational force and gases. However, distance measurements show that PGC 6014 lies at approximately 106 megaparsecs, or about 347 million light-years from us. This means the gap between the two galaxies is about 45 million light-years – much more than their apparent proximity in the sky suggests.
Such a relationship is typical for so-called apparent galaxy pairs. They are projected next to each other in the sky but are actually located on different "floors" of the universe. Unlike truly close, gravitationally bound systems where strong tidal forces can distort disks, trigger star formation waves, or even merge galaxies into one, in the case of NGC 646 and PGC 6014, any gravitational interaction would be very weak and short-lived. Euclid's sharpness helps astronomers distinguish apparent from real close neighbors and better understand how the environment affects galaxy evolution.
Euclid: Europe's "detective of the dark"
The image of NGC 646 is part of a much broader story about the Euclid mission, one of Europe's most ambitious astronomical projects in recent decades. The telescope was launched in July 2023 and placed in a stable orbit around the Sun–Earth Lagrange point L2, about one and a half million kilometers from our planet. From that position, it can continuously observe a large part of the sky, far from atmospheric disturbances and thermal noise that plague ground-based instruments.
Euclid is designed to image approximately one-third of the sky during its six-year nominal mission with an unprecedented combination of width and sharpness. Two instruments are used: the VIS camera for exceptionally sharp images in the visible range and the NISP instrument for near-infrared observation and spectroscopy. Together, they allow the creation of a three-dimensional map of galaxy distribution in time and space, up to distances of ten billion light-years.
The main scientific goal of the mission is to investigate the nature of dark matter and dark energy – components that make up about 95 percent of the total energy density of the universe, but do not emit light and cannot be seen directly. Instead, Euclid measures how the gravity of these invisible ingredients affects the shape, distribution, and motion of visible matter, primarily galaxies. Through precise tracking of weak gravitational lensing – the subtle distortion of galaxy shapes under the influence of mass on the light's path – scientists aim to reconstruct the "skeleton" of dark matter on cosmic scales.
First datasets: millions of galaxies and an avalanche of discoveries
Already during its first years of operation, Euclid began delivering impressive results. In May 2024, ESA and the Euclid consortium published the first "Early Release Observations" – a demonstration set of images and data intended for the public and the scientific community. They featured diverse objects: nearby spiral galaxies, globular clusters, nebulae, galactic groups, and rich clusters of galaxies. These materials served as a kind of catalog of the mission's capabilities, but also as a testing ground for the development of scientific methods and software.
In March 2025, the first major dataset collected during regular sky mapping was published. It contains more than 26 million galaxies imaged in a small but representative part of the sky, representing only a fraction of the planned final coverage. Even this early sample allowed the creation of a catalog of hundreds of thousands of galaxies with measured properties – from morphology and color to indications of star formation history.
Euclid also identified hundreds of strong gravitational lenses in these data, including spectacular so-called Einstein rings, in which a massive galaxy or cluster of galaxies bends the light of a distant object into an almost perfect circular arc. Such systems are extraordinarily useful for precisely measuring mass distribution, especially dark matter, in the lenses, but also for studying very distant, otherwise too faint galaxies. Scientists described the series of discoveries as an "avalanche of new insights," emphasizing that the mission's true potential is only beginning to open up.
Why barred spiral galaxies are important for cosmology
NGC 646 stands out not only for its beauty; it is also interesting as an example of a barred spiral galaxy. In such systems, the central stellar "bar" acts like a gravitational transporter that directs gas from the outer parts of the disk toward the core. This can trigger more intense star formation in the inner regions, but also feed the central supermassive black hole, if present. As the bar evolves, the distribution of stars and gas changes, so the galaxy's structure is directly linked to its history.
Observations of nearby galaxies show that bar structures are very common today – estimates suggest they are present in at least one-third of visible spiral galaxies, and perhaps more. In the earlier universe, however, barred galaxies were rarer. This suggests that bars form and strengthen as galaxies age, as their disks stabilize, and as the gravitational distribution of mass becomes more favorable for the formation of such structures.
This is precisely where Euclid can offer a new level of understanding. Its ability to image vast areas of the sky in a short time means that scientists will obtain a statistically extremely large sample of galaxies of different shapes, sizes, and distances. By comparing galaxies like NGC 646, which we see relatively close, with those whose light we observe from a much earlier phase of the universe, it is possible to track how the frequency of bars changes through cosmic time. This provides insight into how disks mature, how dark matter is distributed in the halo around galaxies, and how gas gradually transitions into stars.
NGC 646 is additionally interesting because it is classified as a low surface brightness galaxy – its disk is relatively diffuse and shines faintly per unit area. Such galaxies were long a challenge to observe because their light was easily "swallowed" by background luminescence and noise. Euclid's instruments, optimized for the detection of very faint structures, make precisely such objects a natural target, opening a window into the previously poorly explored segment of the "dark edge" of galactic disks.
A universe that is slowly cooling
The broader scientific picture into which galaxies like NGC 646 fit tells us that the universe is in a later, calmer phase of its development. Analyses of Euclid data combined with measurements from the Herschel space telescope suggest that the star formation rate in the universe already reached its peak more than ten billion years ago. Since then, the average rate of new star formation has been gradually decreasing, while galaxies slowly "cool" and exhaust their gas supplies.
Although stars will continue to form for a very long time – in some estimates trillions of years – current research confirms that the era of the most intense star formation is behind us. In this context, Euclid's observations of today's galaxies serve as a kind of inventory of the universe's late phase: they show the distribution of stars, gas, and dust after the "golden age" of star formation. In galaxies like NGC 646, new stars are still being born, especially in the spiral arms, but the global trend is toward a gradual calming.
For cosmology, it is crucial to understand how this long-term evolution relates to the action of dark energy, which accelerates the expansion of the universe. As space expands faster and faster, galaxies move away from each other, and gravitational attraction finds it harder to oppose this trend. Euclid's large-scale mapping will allow testing of different dark energy models and an accurate description of how the structures of the universe evolve under its influence.
The price of precision: from instrument to analysis
For images like this one of NGC 646 to be both beautiful visual materials and reliable scientific data, complex processing is required. Data from Euclid's detectors go through a series of steps: removal of instrumental artifacts, astrometric and photometric calibration, stacking of multiple exposures, and the creation of object catalogs. For early demonstration observations, specialized processing was developed to preserve the fine structure of the spiral arms, while ensuring that measurable parameters – such as brightness, color, and shape – can be used in serious statistical analyses.
Euclid is particularly sensitive to very faint, diffuse light, thereby setting new standards in the research of low surface brightness galaxies and extended stellar halos around galaxies. In the case of NGC 646, this means that astronomers can study not only the bright disk but also the subtle structures in the outer parts, where the "historical record" of mergers with smaller satellite galaxies and tidal interactions is often hidden.
A European project with global reach, with Croatian participation
Euclid is a typical example of a large international endeavor involving dozens of scientific institutes, industrial partners, and space agencies. Along with ESA and its member states, key roles are played by partner organizations outside Europe, including NASA, which contributed specific components and expert knowledge. The common goal is to create an open scientific resource from which thousands of researchers worldwide will draw for years.
The Republic of Croatia cooperates with ESA based on a cooperation agreement and "European Cooperating State" status, signed in 2023. This framework makes it easier for domestic institutions and companies to participate in projects like Euclid, whether through software development, data processing, or specialized technical services. Although direct contributions from individual countries are hard to distinguish in such large missions, the fact that Croatia participates in the European space ecosystem means that domestic scientists and engineers can also engage in using Euclid data.
For students of physics, computer science, and engineering, this opens up opportunities to work on cutting-edge problems – from machine learning algorithms for galaxy recognition to the development of visualizations that bring complex cosmological results closer to the general public. The image of NGC 646 is thus, in addition to being a "holiday greeting" from space, also a reminder that European and Croatian institutions participate in the global race to understand the universe.
A celestial garland as a preview of the great cosmological atlas
The image of NGC 646 is just one small segment of a massive mosaic that Euclid is assembling during its multi-year mission. By the end of 2026, the release of the first extensive dataset from one year of observation is planned, covering about 1,900 square degrees of the sky – approximately 14 percent of the total area planned for the entire project. Hundreds of thousands of galaxies will be recorded in these data in exceptional resolution, from simple, faint dwarf systems to massive clusters in which entire galactic cities collide.
For scientists, these images will represent an inexhaustible source of information on how galaxies form, how they merge, and how they are shaped by gravity, dark matter, and dark energy. For the public, photographs like this one of NGC 646 offer an opportunity to experience space research as part of everyday life – whether as inspiration during the holidays or as a reason for conversation about what it means to live on a small planet within one spiral galaxy, in a universe full of structures resembling glittering garlands.
In the coming years, Euclid will continue to send new images and measure data that will gradually fill its "cosmic atlas." Galaxy NGC 646 will remain remembered as one of the early postcards of this great venture – a visually impressive symbol of the fact that, despite the slow cooling of the universe, our curiosity does not cool down, but is only getting new reasons to look further.
Sources:
- Euclid Consortium – blog post "Happy Holidays 2025" with a description of galaxy NGC 646 and Euclid's holiday image (link)
- Copernical – article "Euclid’s galaxy garland" about the image of galaxy NGC 646 and the first years of the Euclid mission (link)
- COSMOS / ESA – official Euclid mission page with a description of goals, instruments, and the scope of the sky survey (link)
- ESA – "Euclid’s first images: the dazzling edge of darkness" – announcement about the first full-color images and telescope capabilities (link)
- Euclid ERO – official page for early public release of data and the first scientific results of the mission (link)
- NGC 646 – encyclopedic data on the galaxy, distance, recession speed, and status as a barred spiral galaxy of low surface brightness (link)
- Reuters – article on the first major release of Euclid data in 2025 and the creation of the cosmological atlas (link)
- The Guardian / AP – articles on Einstein rings and gravitational lensing observed by Euclid and the meaning for the study of dark matter (link)
- LiveScience – analysis of Euclid and Herschel data on the decline in star formation rates and the "cooling" of the universe (link)
- ESA / Ministry of Science, Education and Youth RH – information on the status of the Republic of Croatia as a European Cooperating State of the European Space Agency and the framework of cooperation (link)
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