In October 2022, NASA’s James Webb Space Telescope observed the Hubble Ultra Deep Field for more than 20 hours, focusing on wavelengths between 2 and 4 microns. The research team, comprising Christina Williams, Sandro Tacchella and Michael Maseda, used Webb’s mid-band image filters to gather more precise data on star formation history and ionization properties. galaxies during the first billion years of the universe. This information will help them understand how galaxies have reionized the universe. The researchers were impressed with the resulting high-resolution images and elected to make the data immediately public to allow for more diverse analysis and help develop best practices for analyzing Webb data. Credit: NASA
On October 11, 2022, NASA’s James Webb Space Telescope spent over 20 hours observing the long-studied Hubble Ultra Deep Field for the first time. The general observer program (GO 1963) focused on analyzing the field in wavelengths between approximately 2 and 4 microns.
NASA spoke with Christina Williams (NSF’s 
This image of the Hubble Ultra Deep Field was taken by the Near-Infrared Camera on NASA’s James Webb Space Telescope. The Webb image observes the field at depths comparable to Hubble – revealing galaxies of similar faintness – in just one-tenth as much observing time. It includes 1.8-micron light shown in blue, 2.1-micron light shown in green, 4.3-micron light shown in yellow, 4.6-micron light shown in orange, and 4.8-micron light shown in red (filters F182M, F210M, F430M, F460M, and F480M). Credit: Science: NASA, ESA, CSA, STScI, Christina Williams (NSF’s NOIRLab), Image Processing: Joseph DePasquale (STScI)
What is important for people to know about these Webb observations?
Michael Maseda: The fact that we see hot, ionized gas is telling us exactly where stars are being born in these galaxies. Now we can separate those areas from where stars already existed. That piece of information is very important because, billions of years later, we don’t exactly know how galaxies became how they are today. It’s important to note that we still haven’t seen everything there is to see. Our whole program was ~24 hours, which isn’t that much time in the grand scheme of how much time other observatories have looked at it. But, even in this relatively short amount of time, we’re starting to put together a new picture of how galaxies are growing at this really interesting point in the history of the Universe.
What are you interested in learning by exploring the Hubble Ultra Deep Field with Webb?
Christina Williams: We proposed to image the Ultra Deep Field using some of Webb’s NIRCam’s medium-band image filters, which allowed us to take images of spectral features more accurately than we could with broadband filters because medium-band filters span a shorter wavelength range. This gives us more sensitivity in measuring colors, which helps us understand the history of star formation and ionization properties of galaxies during the first billion years of the universe, like in the Reionization Era. Measuring the energy that galaxies produced in that time will help us understand how galaxies reionized the universe, reverting it from being neutral gas to once again being an ionized 
(Click image for full Cosmic Reionization infographic. More than 13 billion years ago, during the Era of Reionization, the universe was a very different place. The gas between galaxies was largely opaque to energetic light, making it difficult to observe young galaxies. What allowed the universe to become completely ionized, or transparent, eventually leading to the “clear” conditions detected in much of the universe today? The James Webb Space Telescope will peer deep into space to gather more information about objects that existed during the Era of Reionization to help us understand this major transition in the history of the universe. Credit: NASA, ESA, Joyce Kang (STScI)
