The COSMOS scientific collaboration has released the largest map of the Universe ever created. It contains almost 800,000 galaxies, some from the Universe’s earliest times. The map challenges some of our ideas about the early universe.
Many of us remember the Hubble Deep Field and the Ultra Deep Field. Those groundbreaking images showed us that what appears as a small square of empty space in the sky is actually filled with countless galaxies. This was not only a scientific revelation but a social one as more people realized the reality of our place in the Universe.
The JWST has captured its own deep fields, with the first one being captured in 2022. It shows the galaxy cluster SMACS 0723 in extreme galactic detail. While the Hubble needed hundreds of hours to do the job, the JWST only needed 12.5 hours, showcasing the telescope’s power. Now the JWST has outdone itself with the COSMOS-Web.
COSMOS-Web is based on 255 hours of JWST observations using its NIRCam and MIRI instruments. It’s driven by three primary science goals. The first is to detect thousands of galaxies from the Epoch of Reionization and understand more about how the EOR played out. The second is to identify hundreds of rare quiescent galaxies at z > 4, which will place constraints on how the Universe’s most massive galaxies formed and evolved. The third is to measure the evolution of the stellar-mass-to-halo-mass relation in galaxies, thereby linking visible matter to dark matter.
In short, the COSMOS-Web is more than just a map of galaxies; it tries to put them in a broader context of our outstanding questions about the Universe.
The sheer scale of the map is its most pertinent feature.
“Our goal was to construct this deep field of space on a physical scale that far exceeded anything that had been done before,” said UC Santa Barbara physics professor Caitlin Casey, who co-leads the COSMOS collaboration with Jeyhan Kartaltepe of the Rochester Institute of Technology. “If you had a printout of the Hubble Ultra Deep Field on a standard piece of paper,” she said, referring to the iconic view of nearly 10,000 galaxies released by NASA in 2004, “our image would be slightly larger than a 13-foot by 13-foot-wide mural, at the same depth. So it’s really strikingly large.”
COSMOS-Web is an interactive map that allows users to zoom in and explore the region of the sky.
Zooming in on any region of COSMOS-Web reveals a proliferation of galaxies. Image Credit: COSMOS Collaboration.
Deep fields aren’t as stunning as when the first ones arrived on our desktops. Now they’re captured with a scientific purpose, not just curiosity. COSMOS-Web is more than just galaxies; it’s an attempt to understand their surroundings, including cosmic voids.
“The cosmos is organized in dense regions and voids,” Casey explained in a press release. “And we wanted to go beyond finding the most distant galaxies; we wanted to get that broader context of where they lived.”
One of the things the JWST has revealed since it began operations is how many large, bright galaxies there were in the early Universe. Prior to its observations, the Hubble Space Telescope suggested that these early galaxies would be rare. “It makes sense — the Big Bang happens and things take time to gravitationally collapse and form, and for stars to turn on. There’s a timescale associated with that,” Casey explained.
Before the JWST was operational, astronomers wondered how many more of these early galaxies it would reveal with its much larger mirror and more advanced technology.
“And the big surprise is that with JWST, we see roughly 10 times more galaxies than expected at these incredible distances. We’re also seeing supermassive black holes that are not even visible with Hubble.” The raw numbers of observed galaxies aren’t the only surprise. They’re also finding different types of galaxies and black holes. In a paper describing the COSMOS-Web, the authors think they may be able to use it to identify the very first direct collapse black hole candidates.
This figure from the paper shows how different multi-wavelength surveys of the same region work together to paint a more complete picture. Image Credit: Casey et al. 2023. The Astrophysical Journal
Like other grand observations, COSMOS-Web both answers existing questions and poses new ones. The JWST has already forced us to reconsider our understanding of the early Universe by finding so many massive galaxies so soon after the Big Bang. COSMOS-Web reinforces that questioning, but it may also hold an answer.
“Since the telescope turned on we’ve been wondering ‘Are these JWST datasets breaking the cosmological model? Because the universe was producing too much light too early; it had only about 400 million years to form something like a billion solar masses of stars. We just do not know how to make that happen,” Casey said. “So, lots of details to unpack, and lots of unanswered questions.”
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In their paper, Casey and her colleagues write that the COSMOS-Web can help answer a long list of questions. “The breadth of scientific studies that COSMOS-Web may advance is extraordinary and impossible to anticipate in full,” they write. They emphasize that though its contribution will be powerful, it’s not in isolation, and progress relies on contributions from other observatories.
The COSMOS Collaboration released the data to the public for astronomers around the world to make use of. They call this the ‘democratization of science’, and they hope that by making this data from the world’s best space telescope widely available, it could prompt new discoveries, new understandings, and new questions. Initially, the data was in a format available only to those with specialized knowledge and access to supercomputers. It took two years for the COSMOS collaboration to take the JWST’s raw observational data and make it widely available.
“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,” Casey said in the press release. “Because the best science is really done when everyone thinks about the same data set differently. It’s not just for one group of people to figure out the mysteries.”
“We hope the value of this survey continues to grow with time, as have many other deep-field observations before COSMOS-Web and JWST,” the authors conclude.
