I haven’t posted much recently about the European Space Agency’s Euclid Mission but I’ve got an excuse to remedy that today as I’ve just seen that the Special Issue of Astronomy & Astrophysics called Euclid on Sky has at last been published (with a date of 30th April 2025). This contains the main mission and instrument overview papers as well as scientific papers relating to the Early Release Observations. All the individual papers have been on arXiv for some time already.
The main mission overview paper has 1139 authors (including yours truly); that’s definitely the longest author list I’ve ever been on! The arXiv version has been available for almost a year and has already got 254 citations. Here is the abstract:
The current standard model of cosmology successfully describes a variety of measurements, but the nature of its main ingredients, dark matter and dark energy, remains unknown. Euclid is a medium-class mission in the Cosmic Vision 2015-2025 programme of the European Space Agency (ESA) that will provide high-resolution optical imaging, as well as near-infrared imaging and spectroscopy, over about 14,000 deg^2 of extragalactic sky. In addition to accurate weak lensing and clustering measurements that probe structure formation over half of the age of the Universe, its primary probes for cosmology, these exquisite data will enable a wide range of science. This paper provides a high-level overview of the mission, summarising the survey characteristics, the various data-processing steps, and data products. We also highlight the main science objectives and expected performance.
Today is Q1 Day! This means the first public release of data from the full Euclid Survey. It’s only a very small portion (0.4%) of the survey – just 63 square degrees on the sky, while the full survey will be over 14,000 square degrees – but in contrast to earlier data releases, this has been passed through the full Euclid Ground Segment so it represents the true quality of the data we can expect for the rest of the mission. There are no actual cosmology results yet – there isn’t enough data to address the key science goals of Euclid – but there are some great illustrations of the many byproducts of a survey of this type.
Update: here’s one of the Cosmology Talks video by Shaun Hotchkiss with two members of the Euclid Consortium commenting on today’s data release:
As well as the splash of press coverage likely to follow the lifting of today’s embargo, there will be a deluge of Q1-related papers hit the arXiv on 20th March. You can find details here.
Here’s a gallery of pretty pictures released today. These are low resolution versions; try opening the image in a new tab to see it without the caption. You can find and explore higher resolution images on ESASky (see below). Picture credits are: ESA/Euclid/Euclid Consortium/NASA, image processing by J.-C. Cuillandre, E. Bertin, G. Anselmi for the first six images, then ESA/Euclid/Euclid Consortium/NASA, image processing by M. Walmsley, M. Huertas-Company, J.-C. Cuillandre for the next two (bottom row); and ESA/Euclid/Euclid Consortium/NASA; ESA/Gaia/DPAC; ESA/Planck Collaboration for the last one.
This is Euclid’s Deep Field Fornax. After only one observation, the space telescope already spotted 4.5 million galaxies in this field. In the coming years, Euclid will make 52 observations of this field to reach its full depth. This is a zoom-in of Euclid’s Deep Field North, showing the Cat’s Eye Nebula in the centre of the image, around 3000 light-years away. Also known as NGC 6543, this nebula is a visual ‘fossil record’ of the dynamics and late evolution of a dying star. This dying star is shedding its outer colourful shells. This is Euclid’s Deep Field North. After only one observation, the space telescope has already spotted more than ten million galaxies in this field. It is also very rich in Milky Way stars, as it is close to the Galactic plane. In the coming years, Euclid will make 32 observations of this field to reach its full depth. This is Euclid’s Deep Field South. After only one observation, the space telescope already spotted more than 11 million galaxies in this field. In the coming years, Euclid will make more observations of this field to reach its full depth. This image shows an area of Euclid’s Deep Field South. The area is zoomed in 16 times compared to the large mosaic.This image shows an area of Euclid’s Deep Field South. The area is zoomed in 70 times compared to the large mosaic.This image shows examples of galaxies in different shapes, all captured by Euclid during its first observations of the Deep Field areas. This image shows examples of gravitational lenses that Euclid captured in its first observations of the Deep Field areas. This graphic shows the location of the Euclid Deep Fields (yellow). This all-sky view is an overlay of ESA Gaia’s star map from its second data release in 2018 and ESA Planck’s dust map from 2014.
I’m taking the liberty to append the official ESA Press Release, which follows:
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On 19 March 2025, the European Space Agency’s Euclid mission released its first batch of survey data, including a preview of its deep fields. Here, hundreds of thousands of galaxies in different shapes and sizes take centre stage and show a glimpse of their large-scale organisation in the cosmic web.
Covering a huge area of the sky in three mosaics, the data release also includes numerous galaxy clusters, active galactic nuclei and transient phenomena, as well as the first classification survey of more than 380,000 galaxies and 500 gravitational lens candidates compiled through combined artificial intelligence and citizen science efforts. All of this sets the scene for the broad range of topics that the dark Universe detective Euclid is set to address with its rich dataset.
“Euclid shows itself once again to be the ultimate discovery machine. It is surveying galaxies on the grandest scale, enabling us to explore our cosmic history and the invisible forces shaping our Universe,” says ESA’s Director of Science, Prof. Carole Mundell.
“With the release of the first data from Euclid’s survey, we are unlocking a treasure trove of information for scientists to dive into and tackle some of the most intriguing questions in modern science. With this, ESA is delivering on its commitment to enable scientific progress for generations to come.”
Tracing out the cosmic web in Euclid’s deep fields
Euclid has scouted out the three areas in the sky where it will eventually provide the deepest observations of its mission. In just one week of observations, with one scan of each region so far, Euclid already spotted 26 million galaxies. The farthest of those are up to 10.5 billion light-years away. The fields also contain a small population of bright quasars that can be seen much farther away. In the coming years, Euclid will pass over these three regions tens of times, capturing many more faraway galaxies, making these fields truly ‘deep’ by the end of the nominal mission in 2030.
But the first glimpse of 63 square degrees of the sky, the equivalent area of more than 300 times the full Moon, already gives an impressive preview of the scale of Euclid’s grand cosmic atlas when the mission is complete. This atlas will cover one-third of the entire sky – 14 000 square degrees – in this high-quality detail.
“It’s impressive how one observation of the deep field areas has already given us a wealth of data that can be used for a variety of purposes in astronomy: from galaxy shapes, to strong lenses, clusters, and star formation, among others,” says Valeria Pettorino, ESA’s Euclid project scientist. “We will observe each deep field between 30 and 52 times over Euclid’s six year mission, each time improving the resolution of how we see those areas, and the number of objects we manage to observe. Just think of the discoveries that await us.”
“The full potential of Euclid to learn more about dark matter and dark energy from the large-scale structure of the cosmic web will be reached only when it has completed its entire survey. Yet the volume of this first data release already offers us a unique first glance at the large-scale organisation of galaxies, which we can use to learn more about galaxy formation over time,” says Clotilde Laigle, Euclid Consortium scientist and data processing expert based at the Institut d’Astrophysique de Paris, France.
Humans and AI classify more than 380 000 galaxies
Euclid is expected to capture images of more than 1.5 billion galaxies over six years, sending back around 100 GB of data every day. Such an impressively large dataset creates incredible discovery opportunities, but huge challenges when it comes to searching for, analysing and cataloguing galaxies. The advancement of artificial intelligence (AI) algorithms, in combination with thousands of human citizen science volunteers and experts, is playing a critical role.
“We’re at a pivotal moment in terms of how we tackle large-scale surveys in astronomy. AI is a fundamental and necessary part of our process in order to fully exploit Euclid’s vast dataset,” says Mike Walmsley, Euclid Consortium scientist based at the University of Toronto, Canada, who has been heavily involved in astronomical deep learning algorithms for the last decade.
“We’re building the tools as well as providing the measurements. In this way we can deliver cutting-edge science in a matter of weeks, compared with the years-long process of analysing big surveys like these in the past,” he adds.
A major milestone in this effort is the first detailed catalogue of more than 380 000 galaxies, which have been classified according to features such as spiral arms, central bars, and tidal tails that infer merging galaxies. The catalogue is created by the ‘Zoobot’ AI algorithm. During an intensive one-month campaign on Galaxy Zoo last year, 9976 human volunteers worked together to teach Zoobot to recognise galaxy features by classifying Euclid images.
This first catalogue released today represents just 0.4% of the total number of galaxies of similar resolution expected to be imaged over Euclid’s lifetime. The final catalogue will present the detailed morphology of at least an order of magnitude more galaxies than ever measured before, helping scientists answer questions like how spiral arms form and how supermassive black holes grow.
“We’re looking at galaxies from inside to out, from how their internal structures govern their evolution to how the external environment shapes their transformation over time,” adds Clotilde.
“Euclid is a goldmine of data and its impact will be far-reaching, from galaxy evolution to the bigger-picture cosmology goals of the mission.”
Gravitational lensing discovery engine Light travelling towards us from distant galaxies is bent and distorted by normal and dark matter in the foreground. This effect is called gravitational lensing and it is one of the tools that Euclid uses to reveal how dark matter is distributed through the Universe.
When the distortions are very apparent, it is known as ‘strong lensing’, which can result in features such as Einstein rings, arcs, and multiple imaged lenses.
With the help of these models, Euclid will capture some 7000 candidates in the major cosmology data release planned for the end of 2026, and in the order of 100 000 galaxy-galaxy strong lenses by the end of the mission, around 100 times more than currently known.
Euclid will also be able to measure ‘weak’ lensing, when the distortions of background sources are much smaller. Such subtle distortions can only be detected by analysing large numbers of galaxies in a statistical way. In the coming years, Euclid will measure the distorted shapes of billions of galaxies over 10 billion years of cosmic history, thus providing a 3D view of the distribution of dark matter in our Universe.
“Euclid is very quickly covering larger and larger areas of the sky thanks to its unprecedented surveying capabilities,” says Pierre Ferruit, ESA’s Euclid mission manager, who is based at ESA’s European Space Astronomy Centre (ESAC) in Spain, home of the Astronomy Science Archive where Euclid’s data will be made available.
“This data release highlights the incredible potential we have by combining the strengths of Euclid, AI, citizen science and experts into a single discovery engine that will be essential in tackling the vast volume of data returned by Euclid.”
Notes to editors
As of 19 March 2025, Euclid has observed about 2000 square degrees, approximately 14% of the total survey area (14 000 square degrees). The three deep fields together comprise 63.1 square degrees.
Euclid ‘quick’ releases, such as the one of 19 March, are of selected areas, intended to demonstrate the data products to be expected in the major data releases that follow, and to allow scientists to sharpen their data analysis tools in preparation. The mission’s first cosmology data will be released to the community in October 2026. Data accumulated over additional, multiple passes of the deep field locations will be included in the 2026 release.
The three deep field previews can now be explored in ESASky from 19 March 12:00 CET onwards:
Euclid was launched in July 2023 and started its routine science observations on 14 February 2024. In November 2023 and May 2024, the world got its first glimpses of the quality of Euclid’s images, and in October 2024 the first piece of its great map of the Universe was released.
Euclid is a European mission, built and operated by ESA, with contributions from its Member States and NASA. The Euclid Consortium – consisting of more than 2000 scientists from 300 institutes in 15 European countries, the USA, Canada and Japan – is responsible for providing the scientific instruments and scientific data analysis. ESA selected Thales Alenia Space as prime contractor for the construction of the satellite and its service module, with Airbus Defence and Space chosen to develop the payload module, including the telescope. NASA provided the detectors of the Near-Infrared Spectrometer and Photometer, NISP. Euclid is a medium-class mission in ESA’s Cosmic Vision Programme.
Usually I disapprove of using a wine glass for any purpose other than drinking wine, but here’s a very neat short video by Phil Marshall explaining how you can use a one to simulate a strong gravitational lens such as the system that produced the wonderful Einstein ring recently discovered by Euclid. More specifically it shows how perfect alignment leads to a ring whereas other configurations can produce multiple images or arcs.
If you’re planning to try this at home, please remember to empty your glass beforehand.
What better way to start a cold February morning than with a lovely image from Euclid? The picture above on the left shows an image of the galaxy NGC 6505 and on the right a closer view of the central portion that reveals a near perfect Einstein Ring. This phenomenon is caused by gravitational lensing and is quite a rare occurrence because it requires a perfect alignment between a background source, a concentration of mass that acts as a lens, and the observer (in this case the Euclid telescope):
This find is all the more extraordinary because it was made using observations made during Euclid’s commissioning phase when the telescope was not yet fully focussed. The first release of (a small sample) of full-quality data from Euclid – the so-called Q1 release – will actually be announced next month.
The published paper by O’Riordan et al is available here, from which I have taken this image showing the two relationship between the two images above:
There has already been quite a lot of media coverage of this discovery (even in Ireland). Here is the Press Release from the European Space Agency explaining the background and some comments from people involved in the work:
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Euclid blasted off on its six-year mission to explore the dark Universe on 1 July 2023. Before the spacecraft could begin its survey, the team of scientists and engineers on Earth had to make sure everything was working properly. During this early testing phase, in September 2023, Euclid sent some images back to Earth. They were deliberately out of focus, but in one fuzzy image Euclid Archive Scientist Bruno Altieri saw a hint of a very special phenomenon and decided to take a closer look.
“I look at the data from Euclid as it comes in,” explains Bruno. “Even from that first observation, I could see it, but after Euclid made more observations of the area, we could see a perfect Einstein ring. For me, with a lifelong interest in gravitational lensing, that was amazing.”
The Einstein Ring, an extremely rare phenomenon, turned out to be hiding in plain sight in a galaxy not far away. The galaxy, called NGC 6505, is around 590 million light-years from Earth, a stone’s throw away in cosmic terms. But this is the first time that the ring of light surrounding its centre is detected, thanks to Euclid’s high-resolution instruments.
The ring around the foreground galaxy is made up of light from a farther out bright galaxy. This background galaxy is 4.42 billion light-years away, and its light has been distorted by gravity on its way to us. The far-away galaxy hasn’t been observed before and doesn’t yet have a name.
“An Einstein ring is an example of strong gravitational lensing,” explains Conor O’Riordan, of the Max Planck Institute for Astrophysics, Germany, and lead author of the first scientific paper analysing the ring. “All strong lenses are special, because they’re so rare, and they’re incredibly useful scientifically. This one is particularly special, because it’s so close to Earth and the alignment makes it very beautiful.”
Albert Einstein’s general theory of relativity predicts that light will bend around objects in space, so that they focus the light like a giant lens. This gravitational lensing effect is bigger for more massive objects – galaxies and clusters of galaxies. It means we can sometimes see the light from distant galaxies that would otherwise be hidden.
If the alignment is just right, the light from the distant source galaxy bends to form a spectacular ring around the foreground object. These Einstein rings are a rich laboratory for scientists. Studying their gravitational effects can help us learn about the expansion of the Universe, detect the effects of invisible dark matter and dark energy, and investigate the background source whose light is bent by dark matter in between us and the source.
“I find it very intriguing that this ring was observed within a well-known galaxy, which was first discovered in 1884,” says Valeria Pettorino, ESA Euclid Project Scientist. “The galaxy has been known to astronomers for a very long time. And yet this ring was never observed before. This demonstrates how powerful Euclid is, finding new things even in places we thought we knew well. This discovery is very encouraging for the future of the Euclid mission and demonstrates its fantastic capabilities.
By exploring how the Universe has expanded and formed over its cosmic history, Euclid will reveal more about the role of gravity and the nature of dark energy and dark matter. The space telescope will map more than a third of the sky, observing billions of galaxies out to 10 billion light-years. It is expected to find around 100 000 strong lenses, but to find one that’s so spectacular – and so close to home – is astonishing. Until now, less than 1000 strong lenses were known, and even fewer were imaged at high resolution.
“Euclid is going to revolutionise the field, with all this data we’ve never had before,” adds Conor.
Although this Einstein ring is stunning, Euclid’s main job is searching for the more subtle effects of weak gravitational lensing, where background galaxies appear only mildly stretched or displaced. To detect this effect, scientists will need to analyse billions of galaxies. Euclid began its detailed survey of the sky on 14 February 2024 and is gradually creating the most extensive 3D map of the Universe yet. Such an amazing find, so early in its mission, means Euclid is on course to uncover many more hidden secrets.
Here’s an intriguing experiment in sonification of an image from Euclid.
And here’s the official blurb about it:
An ethereal dance of misty clouds of interstellar dust with a myriad of distant stars and galaxies speckled like paint drops over a black canvas. This is a sonification of a breathtaking image taken by ESA’s Euclid space telescope of the young star-forming region Messier 78.
The sonification offers a different representation of the data collected by Euclid, and lets us explore the stellar nurseries in M78 through sound. Close your eyes and listen to let the cosmic image be drawn by your mind’s eye, or watch as the traceback line in this video follows the sounds to colour the image from left to right.
The twinkling sounds of various pitches and volumes represent the galaxies and stars in the frame. The pitch of the sound points towards where we see the dot of light in the image. Higher pitches tell us that a star or galaxy appears further at the top in the image along the traceback line.
The brightness of these objects in and around M78 are represented by the volume of the twinkles. Whenever we hear a particularly loud clink, the star or galaxy that Euclid observed appears particularly bright in the image.
Underlying these jingling sounds, we can hear a steady undertone, made up of two chords which represent different regions in Messier 78. This sound intensifies as the traceback line approaches first the brightest, and later the densest regions in the nebula.
The first two deeper crescendos in this undertone indicate two patches in the image where the most intense colour is blue/purple. These appear as two ‘cavities’ in M78, where newly forming stars carve out and illuminate the dust and gas in which they were born.
The chords intensify a third time at a slightly higher pitch corresponding to the red-orange colours in the image, as the sound draws over the densest star-forming region of the frame. This stellar nursery is hidden by a layer of dust and gas that is so thick that it obscures almost all the light of the young stars within it.
As the sound traces over the entire Euclid image, these different tones together form a cosmic symphony that represents the image of Messier 78, and the stars and galaxies that lie behind and within it. You can read more about this image that was first revealed to the eyes of the world earlier this year here: https://www.esa.int/Science_Explorati…
Many thanks to Klaus Nielsen (DTU Space / Maple Pools) for making the sonification in this video. If you would like to hear more sonifications and music by this artist, please visit: https://linktr.ee/maplepools
P.S. The first sentence of the Wikipedia page on sonification uses the word “perceptualize”. Ugh!
A few months ago I posted about a joint initiative between Euclid and Galaxy Zoo that involved engaging members of the public in a project involving galaxy morphology. Well, a new “collab” (as you young people call such things) has just been announced on social media, and I encourage you to investigate further
The new venture is called `Space Warps – ESA Euclid’, and its aim is to find strong gravitational lenses in Euclid survey images. You can find out more about this project in this blog post by Knud Jahnke and you can find instructions and sign up for the project here.
The announcement of this initiative gives me an opportunity to pass on a little update on progress with the Euclid survey. The first `Quick’ Data Release (known to its friends as Q1) was made available to Euclid Consortium members just a few weeks ago. This will be made available to the general public next March, around the same time as the joint ESLAB and Euclid Consortium meeting in Leiden next year.
The Euclid survey is constructed as a set of contiguous `tiles’ covering the survey region, which will ultimately be about 15,000 square degrees (about one-third of the sky), with most of the region scanned by the satellite many times. The Q1 data will just be a taster of this. The main component of the Q1 data relates to a single visit (at the depth of the Euclid Wide Survey) over the Euclid Deep Fields (EDFs): 20 deg2 of the EDF North, 10 deg2 of EDF Fornax, and 23 deg2 of the EDF South. The deep fields will subsequently be visited multiple times during the mission. The Q1 release will be of Level 2 data, i.e. data at the level of individual tiles.
The first full data release (DR1) is due to be published in June 2026.
Today (15th October 2024) saw the release of a sneak preview of the main survey of the European Space Agency’s Euclid survey at the International Astronautical Congress in Milan. Here’s the key image.
This image is not at full science resolution of the Euclid survey and is meant primarily as an appetizer. The resolution is11Kx4K, and is processed by the same pipeline that produced the Euclid Early Release Observations featured here and here. You can find more detail about these images here and here. I have taken this from the latter article:
Euclid has been surveying the sky since 14 February 2024 and data processing is in full swing – the first public release of 53 deg² of science-grade Wide Survey data will take place in March next year. But how much data has Euclid already observed and how can we possibly visualize this? At a rate of 10 deg² per day, the Euclid Wide Survey has already surpassed 1000 deg², that is 5000x the apparent size of the Moon in the sky! Now ESA has put out a first set of images that allow to grasp how much data Euclid is and will be producing.
There’s also this explanatory video:
This is just taster. The main survey will take many years to complete. But it’s a start…
I had a very busy day yesterday culminating in the Space Week event I blogged about a few weeks ago. There was a good attendance – lots of young kids as well as adults – and the lecture room was very full. We could probably have filled a much bigger room, actually, but had been moved to a smaller venue and had to close registrations very early to avoid having too many people. I’d guess we had about 350. My talk was the last one, and didn’t finish until 8.30 by which time I was definitely ready for a pint.
You can find the slides I used for my presentation, The Universe according to Euclid, here.
There was an official photographer there who took quite a few pictures but I haven’t seen any of them yet. I’ll post a selection if and when I get them.
The Euclid Consortium is celebrating the first year of the journey of the European Space Agency’s Euclid Mission into space! Over the past 52 weeks, Euclid has been scanning the cosmos, uncovering new insights into dark matter, dark energy, and the structure of the universe. The Euclid Consortium has produced a slideshow, showcasing the key moments and discoveries from the first year in space.
The slideshow can be seen on YouTube here:
Here is a poster:
This can also found in interactive form here where you can click on each of the 52 images to see what it’s about.
P.S. The subtitle of the poster is “first year of a big journey to new physics”. There’s no guarantee that Euclid will find any new physics, rather than confirming our existing ideas, but it might.
I haven’t posted anything about the European Space Agency’s Euclid mission recently but I can remedy that today by sharing a new video that describes one of the methods that Euclid will use to map the dark Universe. Here’s the video:
ESA’s Euclid mission is surveying the sky to explore the composition and evolution of the dark Universe. But how can Euclid see the invisible? Watch this video to learn about the light-bending effect that enables scientists to trace how dark matter is distributed in the Universe.
By making use of Euclid’s flagship simulation, the video illustrates how dark-matter filaments subtly alter the shape of galaxies. Light travelling to us from vastly distant galaxies is bent and distorted by concentrations of matter along its way. The effect is called gravitational lensing because matter (both ‘normal’ and dark matter) acts as a kind of magnifying glass.
Scientists distinguish between strong and weak gravitational lensing. In strong gravitational lensing distortions of background galaxies or other light sources are very apparent, resulting in arcs, multiple images or so-called Einstein rings. In weak lensing, background sources appear only mildly stretched or displaced. This means we can only detect this effect by analysing large numbers of sources in a statistical way.
The further we look, the more prominent the distortions from weak gravitational lensing are, because there are more dark-matter structures acting as lenses between us and the light sources.
Euclid will measure the distorted shapes of billions of galaxies over 10 billion years of cosmic history, providing a 3D view of the dark matter distribution in our Universe. This will shed light on the nature of this mysterious component.
The map of the distribution of galaxies over cosmic time will also teach us about dark energy, which affects how quickly the Universe expands. By charting the Universe’s large-scale structure in unprecedented detail, Euclid will enable scientists to trace how the expansion has changed over time.
The views presented here are personal and not necessarily those of my employer (or anyone else for that matter).
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In the Dark 