780,000 galaxies revealed in JWST’s largest science operation

Back in 2021, before the James Webb Space Telescope (JWST) was launched, an incredible suite of decisions were made: about which observing projects and programs would be granted observing time, and how much time they would be granted. Some of the decisions granted are known as GTO, or Guaranteed Time Observations, which went to members of the teams that put painstaking efforts into developing the software and hardware JWST would require in order to function and conduct science operations properly. But the rest of the decisions were part of the GO, or General Observers, program, which provide the worldwide astronomical community the opportunity to apply for JWST observing time.

Of all the GO programs that were selected before JWST ever launched, the COSMOS-Web program, led by Dr. Jeyhan Kartaltepe and Dr. Caitlin Casey, was the largest and most comprehensive. All told, the COSMOS-Web team:

• was composed of 50 researchers,
• was granted 208.6 hours (almost 9 days) of JWST observing time,
• was designed to map 0.54 square degrees of sky in the near-infrared (with NIRCam),
• as well as a third of that area (0.18 square degrees) with MIRI, in the mid-infrared.

After a long wait that included the release of many preliminary papers and results, the full field of COSMOS-Web is finally out and so are the various data catalogues — released across five separate papers — open to everyone throughout entire world. Here, without further ado, is the full field image, along with the highlights of what’s inside!

This view shows the (rotated) full field of the COSMOS-Web survey. Spanning 0.54 square degrees on the sky in NIRCam imaging, it represents the largest first-cycle general observer program of JWST observing programs awarded telescope time, at 255 hours: 209 with NIRCam and 46 with MIRI.

Above is the full-field image of the COSMOS-Web survey with NIRCam: all 0.54 square degrees of it. Sure, even with more than 200 hours (208.6 to be precise) of observing time, it would take a whopping seventy-five-thousand comparable fields to map the entire sky: an endeavor that would take JWST close to 2000 years to complete. While we can’t practically accomplish that, we can make one simple assumption about the COSMOS-Web field: that it’s a representative, fair sample of what the majority of the Universe looks like. Therefore, if we examine this large-field region in detail, we can learn something meaningful about what the entire observable Universe looks like to JWST’s eyes.

The first, and perhaps simplest, thing you might think to ask is, “well, how many galaxies can JWST identify in this region of space?” And with NIRCam data alone, the answer seems to be right around 780,000 of them: an impressively large number. However, if we multiply the number of galaxies found in this region of space by the number of regions that it would take to cover the entire sky, we don’t get a very good estimate for the total number of galaxies in the Universe! That number — of 780,000 multiplied by 75,000 — only comes out to 58.5 billion, or just a third of the estimate that we’d arrive at by applying the same method to the Hubble eXtreme Deep Field.

As deep as COSMOS-Web has gone, it’s ultimate strength is breadth and its wide field of view, not its depth.

This area of the COSMOS-Web field shows off a huge array of interesting features, from elliptical and disk galaxies at all cosmic distances to clusters of galaxies to tiny, faint, distant objects that come from the very earliest stages of cosmic history. The COSMOS-Web field, all told, reveals over 780,000 galaxies.

The second thing you might think to ask, then, is that if JWST is so good at looking back into the ultra-distant Universe, and particularly at finding bright, star-forming, and active galaxies from the earliest stages in cosmic history, how many ultra-distant galaxies (or galaxy candidates) has it found?

This is still a work in progress for the full COSMOS-Web field, but there are a few papers on the most distant galaxy candidates from portions of the COSMOS-Web field. For instance, in this paper, which focused on just 77 square arc-minutes (or just 4%) of the total COSMOS-Web field, they identified an impressive 15 galaxy candidates at a redshift of z ≥ 9, or coming from the first 500 million years of cosmic history.

In an independent study that used about half of the full COSMOS-Web field, the researchers found a separate selection of 15 remarkably bright galaxy candidates from the first 400 million years of cosmic history (at redshifts between 10 and 14) that are exceptionally luminous, allowing us to make estimates about the exceptionally bright, bursty galaxies that might contribute largely to the first fully reionized pockets of space appearing in cosmic history.

However, all galaxy candidates can only remain “candidates” until they have spectroscopic follow-up performed on them, confirming their ultra-distant nature. A couple of those follow-up proposals have already been approved, and by the end of next year, we should have spectroscopic data for these and other COSMOS-Web galaxies.

This close-up image, in false color, shows the strong gravitational lens known as the COSMOS-Web ring. This system represents one of the most distant gravitational lens systems of all-time, for both the lensing source (at a redshift greater than two) and for the background lensed object (making an Einstein ring at a redshift of greater than 5). Two redder-colored clumps, annotated as CW and CE, are identified.

One of the major findings of COSMOS-Web has to do with the stellar mass function, or how much mass is present in the form of stars over cosmic history. COSMOS-Web, leveraging JWST NIRCam and MIRI data, is sensitive to galaxies from all across cosmic history: from when the Universe was just ~2% of its present age all the way to today, where it’s 100% of its present age.

We’ve known for several years that star-formation is only a trickle today, and that if you go back earlier and earlier in cosmic history — back to a time when the Universe was only about ~3 billion years old, corresponding to a redshift of z ~ 2 — star-formation was stronger and stronger. Today, star-formation is just ~3% of what it was at its peak.

But what happened at even earlier times? This was a big question in the pre-JWST era, as models and simulations gave a specific answer that seemed reasonable, but that weren’t observationally validated. Then, when JWST arrived, those expectations appeared to be in great conflict with the early galaxies JWST was seeing. Indeed, that’s what COSMOS-Web confirmed, showing that the star-formation rate early on, particularly from an age of 1-to-3 billion years, must have been greater than we previously expected in order to create the populations of stars the COSMOS-Web field uncovered. It’s a fascinating find.

This region of the COSMOS-Web field is incredibly rich in galaxies, although it isn’t just one grand galaxy cluster shown here, but rather a variety of galaxy groups and clusters from across cosmic time. The yellowish giant ellipticals, at bottom, are in the foreground, with the streaks and arcs characteristic of strong gravitational lenses visible even to the unaided human eye.

COSMOS-Web, with its large area coverage and deep sensitivity, was also able to uncover a number of important gravitational alignments, resulting in the optical phenomenon of strong gravitational lensing. In strong lensing systems, the large mass of a foreground objects bends the spacetime in its vicinity, causing the light from background objects to get:

• stretched,
• distorted,
• magnified,
• and often sent in many different directions,

where the light from background objects can then wind up appearing as multiple images, arcs, or even rings.

In fact, one of the feathers-in-the-cap of COSMOS-Web is the highest-redshift (i.e., most distant) gravitational lens spotted to date. (And yes, that one has been spectroscopically confirmed!) With so many galaxies — and so many gravitational lenses — we’ve been able to learn an enormous amount about the Universe, from the abundance of strong lenses to a large number of extremely spectacular lenses with different source colors, all the way back deep into the epoch of reionization. Interestingly, the COSMOS-Web data has probed the relationship between a galaxy’s size and mass throughout history, all the way back to when the Universe was just 3% of its current age.

This large, impressive, extended spiral galaxy is a striking feature in the COSMOS-Web field, but isn’t the only interesting thing to look at. Many interacting galaxies, as well as galactic groups, can be spotted in the background in this COSMOS-Web image.

In fact, there’s so much data about galaxies from COSMOS-Web, from all throughout cosmic history, that future studies (forthcoming) are going to be able to focus on how the populations of galaxies see their morphologies (i.e., their shapes, whether spiral, elliptical, ring, irregular, along with various sub-types of those galaxies) evolve over cosmic history. When did the first spiral shapes appear? When did spirals with bars appear? Today, grand design spirals make up about 10% of all spiral galaxies; how abundant were they early on? When did the first elliptical, red-and-dead galaxies appear? COSMOS-Web is well-equipped, arguably better equipped than any other survey, to answer those questions and more.

Another remarkable capability of large-angle surveys with JWST, such as from COSMOS-Web, is their ability to identify ultra-distant galaxy clusters and even galactic protoclusters. In fact, JWST has already discovered the earliest galaxy protocluster known, and there are many early galaxy cluster and protocluster candidates that have been found in the COSMOS-Web field. With follow-up observations and spectroscopic confirmation, many of these are likely to turn out to be bona fide early galaxy protoclusters, possibly even challenging the current record for earliest and most distant. Work is still needed to confirm them, but this is a promising avenue of research that is presently underway.

This portion of the full COSMOS-Web field highlights several giant elliptical galaxies as well as many large, nearby massive spirals. Their large, massive, but low-surface-brightness extended halos are easily revealed by JWST, allowing astronomers to better determine how the stellar mass of galaxies evolves with time. Many candidate galaxy clusters and protoclusters, along with several gravitational lenses, can be seen in the background.

Something that’s fascinating about JWST images is that they aren’t just sensitive to the brightest components of galaxies, but are capable of measuring huge contrasts: between the extremely bright parts of galaxies and the must fainter components, like the outer stellar disks and halos of galaxies. Above, a very bright collection of giant elliptical galaxies are shown — part of the COSMOS-Web field, like all images shown in this article — with many of these galaxies not just showing a bright central concentration of stars, but an outer, diffuse halo of stars around them. Even some of the larger, brighter spiral galaxies have these halos.

These are crucially important for understanding how the Universe grows up! An earlier study of galaxies imaged with Hubble at similar depths revealed that ignoring the outer stellar halos, which can be an easy mistake to make if you use an overly aggressive calibration and field-flattening algorithm, resulted in the loss of light from perhaps up to a trillion stars, with around two-to-three times the stellar mass of the modern Milky Way, just from ignoring these outer stellar halos. Meanwhile, with JWST in the COSMOS-Web survey, these low surface brightness features are real. In some cases, one can even make out what appear to be surrounding these galaxies, highlighting just how sensitive JWST can be.

Although the most visually spectacular regions of the COSMOS-Web field are rich in galaxies and other luminous objects, there’s a quiet beauty (and many interesting lessons to be learned) within the sparser regions of space: along those lines-of-sight with practically no galaxies or galaxy clusters intervening. This provides a pristine, clear path that can expose the ultra-distant Universe to our most powerful telescope’s views.

While the most spectacular portions of the COSMOS-Web survey are certainly the areas with lots of luminous features — big, massive, nearby galaxies, groups and clusters of distant galaxies, and galaxies in the process of interacting with one another — there are also extremely sparse regions of space that are a part of the survey. The Universe, in many ways, is often described as a cosmic web, with interconnected filaments creating a network of structure separated by vast cosmic voids. In fact, it’s the word “web” from the cosmic web that inspired the name of the survey: COSMOS-Web, not COSMOS-Webb, as it was named after the Universe, not the telescope observing it.

Even though the largest cosmic web features, like baryon acoustic oscillations and the largest cosmic voids, are far too grand in scale and angular size for a narrow-angle survey like COSMOS-Web to image, the appearance of filaments and voids on these smaller scales is still very real. Rather than a web, it turns out that the Universe’s structure is more like a block of Swiss cheese: with holes of various sizes distributed all throughout it, representing initially underdense regions that gave up their matter, over time, to their denser, galaxy-rich surroundings. In these void regions, there are often the clearest paths, or line-of-sights, to some of the most distant, faint objects in all the Universe. What appear as mere tiny, red dots against a backdrop of darkness may turn out to be some of the most important galaxies in understanding how our Universe grew up.

This portion of the COSMOS-Web field is centered on the single largest galaxy, in terms of angular size, across the entire 0.54 square degree area of the COSMOS-Web survey. Alongside that galaxy are many other features, including several stars from within our own Milky Way with prominent diffraction spikes emanating from them.

There are also, all throughout the COSMOS-Web field, a number of very bright spike-filled sources: practically all of them representing foreground stars contained within our own Milky Way. Interestingly, the area around the brightest of these stars (like the lower-left star in the image above) often overlaps with the position of one or more background galaxies; the Universe is a very rich place, and cares nothing for the fact that objects within our own galaxy are in the way when we attempt to observe it.

Yet, these galaxies are not included at all when we tally up the total number of galaxies within the COSMOS-Web field of view, as these stars — as well as the unavoidable diffraction spikes they possess owing to JWST’s optical design — interfere with the ability of scientists to perform photometry and detect-and-identify individual sources. The figure I reported for you earlier, that there are 780,000 galaxies detected in the COSMOS-Web field over an area of just 0.54 square degrees, does not include the areas around very bright stars. Even discounting the fact that there are certainly fainter, less-luminous galaxies out there that haven’t been revealed by COSMOS-Web just yet, the removal of the areas around bright stars has likely resulted in undercounting the number of galaxies in this field: by thousands or maybe even tens of thousands.

A small section of the full COSMOS-Web field, blown up in detail, shows a variety of spectacular features, from interacting spiral and elliptical galaxies to star-forming face-on spirals to galaxies with distended spiral arms and stellar streams, plus many other remarkable features. The entire Universe, with enough data, wavelength coverage, and gathered light, can be revealed with cutting-edge astronomical tools.

But what’s most remarkable, to me at least, about COSMOS-Web is the now-realized potential that only comes with pointing a new, more-powerful-than-ever telescope at the Universe: the potential to discover rare events, objects, and phenomena. Galaxies with active supermassive black holes at their centers, galaxies in the process of merging and cannibalizing other galaxies, galaxies with stellar streams being ripped out of them, galaxies twisted by gravity into strange and irregular shapes, and galaxies being bent into arcs and other distended shapes, for example, can all be seen in just one tiny region of the COSMOS-Web field, highlighted above.

There’s a whole Universe out there to explore, and JWST is a remarkable tool in the arsenal of astronomers to help us do it. However, it’s important to remember that what you learn, ultimately, isn’t strictly determined solely by the tools you have, but also by how you use them. Narrow, deep JWST surveys like JADES, UNCOVER, GLASS, and CEERS have all given us a remarkable tour of the Universe, but there’s a tremendous amount to be learned — independently and complementarily — from wider-field surveys like COSMOS-Web. Now that the initial survey is complete, all the world, from the greater astronomical community to interested laypersons and amateurs, can enjoy and dive into what it found. With follow-ups coming on many of the most exciting and interesting objects, the cosmic lessons that we’ll glean are true gems just waiting to be polished.