“The big surprise is that with JWST, we see roughly 10 times more galaxies than expected at these incredible distances.” These are the words of Caitlin Casey, co-leader of the COSMOS-Web collaboration, which have reverberated throughout the world of astronomy since the release of the most extensive map of the universe ever created a feat not merely breaking records, but also undermining accepted assumptions regarding cosmic history.
The magnitude is breathtaking. The COSMOS-Web map contains almost 800,000 galaxies, spread over 0.54 square degrees of sky around three times the whole moon as viewed from Earth. But within this small region is a cross-section of nearly the entire history of the universe, going back 13.5 billion years, or 98% of cosmic time. The map, constructed from 255 hours of James Webb Space Telescope (JWST) observations, constitutes a mural-sized step over the legendary Hubble Ultra Deep Field, which itself once amazed the world with its glimpse of almost 10,000 galaxies.
How is JWST’s feat so groundbreaking? The reason lies in its infrared instrumentation. As the universe grew, light from the farthest objects was stretched redshifted into the infrared, making it invisible to optical telescopes such as Hubble. JWST’s 6.5-meter mirror and collection of sensitive infrared detectors, including NIRCam and MIRI, are equipped to detect these distant, ancient signals. As Jeyhan Kartaltepe, COSMOS-Web co-lead, explained, “The sensitivity of JWST lets us see much fainter and more distant galaxies than ever before, so we’re able to find galaxies in the very early universe and study their properties in detail.”
This technological leap has led to unexpected discoveries. Not only are there many more galaxies from the early universe than anticipated, but JWST has also unveiled a population of supermassive black holes that Hubble couldn’t see. Some of those black holes, like the one in galaxy CEERS 1019, are “lightweights” by universal standards only 9 million solar masses yet they existed fewer than 600 million years after the Big Bang. Rebecca Larson, the lead on CEERS 1019, said, “Looking at this distant object with this telescope is a lot like looking at data from black holes that exist in galaxies near our own.” There are so many spectral lines to analyze!
The two-year effort of the COSMOS-Web collaboration to create an interactive public map from raw data from JWST is a victory of engineering and open science. The catalog contains photometry, redshifts, and physical parameters on every galaxy and is available for viewing using an online viewer, calling amateur astronomers and students to discover the structure of the universe in person.
In Casey’s words, “A big part of this project is the democratization of science and making tools and data from the best telescopes accessible to the broader community.” But the map is more than an exhibition of cosmic sights it is a challenge to cosmological theory. Before JWST, conventional wisdom, based on half a century of theory and Hubble observations, was that galaxies in the first 500 million years after the Big Bang would be extremely uncommon. The reasoning was straightforward: following the Big Bang, it must take some time for matter to collapse, for the stars to light up, and for the galaxies to form. But JWST’s deep field has thrown all this into doubt, revealing a universe that was making too much light too soon. We just do not know how to make that happen, Casey conceded.
The excess of the first, large galaxies and black holes compels astronomers to re-examine the physics of galaxy and star formation, and even the character of dark matter and the early universe. The scientific explanation of the galaxy number count shape, as unveiled by JWST, is based both on cosmology and galaxy evolution. Recent modeling utilizing the GALFORM semi-analytical model has demonstrated that the break seen in the number counts an inflection from a steep to a shallower slope at some apparent magnitude is not necessarily a remnant of cosmic structure, but an observational phenomenon caused by the interplay of redshift, the intrinsic luminosity function of galaxies, and the so-called k-correction. The latter explains how the galaxy’s spectral energy distribution changes when its light is redshifted by cosmic expansion.
The break in the counts, most sharply visible at longer infrared wavelengths, signals the joining of populations dominated by luminous, bulge-dominated (elliptical) galaxies and less luminous, disk-dominated (spiral) galaxies. Even with the record-breaking depth of JWST’s images, the galaxies contributing most of the number counts at the detection limit of the survey (magnitude ~28) are primarily at redshifts less than 2. As a result, for the time being, the most distant and faint galaxies those that would be able to unveil the earliest stages of structure formation are yet at the margin of detectability.
Nonetheless, JWST’s power to advance the observational frontier is already paying dividends in the form of discoveries like the galaxy MoM-z14, which lived a mere 280 million years after the Big Bang and has elements such as carbon and nitrogen, pointing toward even older generations of stars. The import permeates astrophysics. The fact that there are so many large, luminous galaxies and black holes so early after the Big Bang provokes questions as to how these objects came together so rapidly.
Some black holes appear to be “feeding” at rates far above theoretical limits, as in the case of LID-568, which is accreting matter at 40 times its Eddington limit a finding that may help explain the rapid growth of supermassive black holes in the early universe using NASA telescopes. Not only is the new COSMOS-Web map a scientific achievement, it is also a tribute to the synergy of cutting-edge engineering, global cooperation, and an open-data spirit. As lead author Maximilien Franco explained, “We combined more than 10,000 images of the sky together to form the largest contiguous image available from JWST.” To do this, we needed to ensure that all the images were properly aligned with existing data, and also to correct for any observational biases.
With additional data collection in progress and follow-up spectroscopy in the pipeline to validate the distances and chemical makeup of the oldest galaxies, the COSMOS-Web collaboration is still pushing the limits of cosmic mapping. The map is open now for the whole world to peruse, a living history of a cosmos that, thanks to JWST, is far richer and more surprising than anyone ever dreamed.
