On July 11, the world was left in awe by the release of the deepest astronomical image ever obtained, captured by the James Webb Space Telescope (JWST), NASA’s newest “flagship” observatory. On the background of a galaxy cluster named SMACS 0723, seen as it appeared 4.6 billion years ago, myriad galaxies of different shapes and sizes appear like bright gems in the darkness of the cosmos. Some of these faraway lighthouses were already shining when the universe was just a few hundred million years old. To understand how we reached this remarkable achievement—how astronomers have sailed to galactic islands so remote from us in space and time, collecting photons whose journey started breathtakingly close to the big bang—it helps to know how deep-field observations came to be.
The origin of Webb’s first deep field is best traced to the early 1990s, with the launch of JWST’s predecessor, the Hubble Space Telescope. The concept of deep-field observations was still in its infancy back then. Hubble was primarily designed for targeted observations—astronomers would point the telescope to a source at a specific spot in the sky and expose or “integrate” as needed, depending on the source’s brightness. But Hubble could also be used for deep-field imaging, which prescribes the opposite: astronomers would point the telescope to a sky region devoid of any visible source and use a very long exposure time to reach “deep” into the cosmos to observe as many faint sources of light as possible. From its perch in low-Earth orbit above our planet’s starlight-scattering ocean of air, Hubble was, at the time, the best platform for deep-field imaging astronomers had ever known.
Not everyone thought the approach would prove revolutionary, however. In a famous article published in Science in 1990, the Institute for Advanced Studies’ John Bahcall and colleagues argued that a deep-field image from Hubble would not reveal significantly more galaxies than ground telescopes. Bahcall, a giant in astrophysics, was widely known for his solution to the problem of solar neutrinos and his calculations of the distribution of stars around a massive black hole. He contributed fundamentally to the development of the Hubble Space Telescope from its original concept in the 1970s to its launch. Despite believing that the Hubble deep field would not reveal new populations of galaxies, he anticipated that such images could support the study of the morphology and size of faint galaxies and the demography of quasars, a rather old-fashioned word for accreting supermassive black holes.
Such tepid expectations tamped down any urgency to try deep-field imaging with Hubble. The first attempt did not occur until around the winter holidays of 1995, after a much-needed optics repair. The telescope spent 10 days of exposure time staring at a tiny patch of the sky in the Ursa Major constellation, just one-thirteenth of the moon’s angular diameter. But when weeks later astronomers saw its resulting image, known as the Deep Field North, they immediately realized it was a Christmas gift for the ages. The Milky Way’s stars are very sparse in the target region, allowing Hubble to probe the cosmic abyss like a viewer peering through a peephole. The telescope saw almost 3,000 faint galaxies of different shapes and sizes —many more than expected, some of them as far as 12 billion light-years away. Hubble was not only exploring space, but was also probing time, gathering starlight that had been emitted eons ago during earlier epochs of the universe. The image quickly became iconic.
A crucial question arose: was the galaxy-rich region revealed by the Deep Field North the norm? Or had astronomers been lucky (or unlucky, depending on the perspective) to have by chance pointed the telescope towards a Pantagruelian crowding of galaxies? In 1998, Hubble obtained the Deep Field South. The exposure was similar, but the telescope pointed towards the southern celestial hemisphere, as far as possible from the first spot. This new image confirmed that the universe contained many more galaxies than previously thought, especially at vast distances. In addition to their prominent scientific and inspirational values, the Hubble deep fields also represented a technical challenge, containing over 10,000 galaxies that constituted one of the first “big data” challenges astronomers ever faced.
Deep-field imaging is not restricted to the visible realm of the spectrum. At the turn of the millennium, the world was ready for the first high-energy deep field, obtained with the Chandra X-ray observatory, a revolutionary NASA mission launched in July 1999 and still active today. The Chandra Deep Field South was obtained by integrating for about one million seconds over a piece of the sky in the Lockman Hole, a window devoid of hydrogen clouds and dust from the Milky Way. The Chandra Deep Field South uncovered the extreme universe; hundreds of black holes, some very remote, would appear in an image not as visually spectacular as the Hubble photographs but dense with science. This field was imaged again with Chandra, with a total exposure of about seven million seconds, making it the deepest field ever obtained in x-ray. In 2003, the Chandra Deep Field North was released, with data from more than 500 x-ray sources.
With the addition to Hubble’s arsenal of the Advanced Camera for Surveys, the Hubble Ultra Deep Field was released in 2006. This history-making shot contained thousands of galaxies, some subsequently shown to have been shining when the universe was less than one billion years old. The Ultra Deep Field showed in unprecedented detail the history of galaxy formation; distant galaxies conclusively appeared to be smaller and more irregular in shape than closer ones, providing substantial evidence to support galaxy evolution theories.
The Ultra Deep Field is essentially the deepest image that can be obtained in optical wavelengths. If a galaxy is too far out, its optical light is shifted outside the visible range and into the infrared regime; this is a consequence of the cosmological redshift, in which the expansion of the universe stretches out the wavelengths of light traveling through enormous expanses of intergalactic space. An infrared camera was needed to look farther out in space and time. With the addition of the near-infrared camera to Hubble, an infrared Ultra Deep Field was obtained in 2009, revealing galaxies shining only 600 million years after the big bang. A decade later, in 2019, a deep field produced with NASA’s Spitzer infrared space telescope was released. Both these images are fertile ground for detecting galaxies at the cosmic dawn.
Finally, Hubble’s Frontier Fields campaign represented the cutting edge of deep-field imaging, and a prologue to the Webb’s first deep image. During this observational campaign, completed in 2017, Hubble was pointed toward six large concentrations of galaxies. The presence of a substantial density of mass along the line of sight, according to Einstein’s theory of general relativity, can bend and thus amplify the light incoming from a background source, with an effect named gravitational lensing. These galaxy clusters were therefore used as a magnifying glass to see even farther away. Besides being chock-full of a cluster’s swarming galaxies, the Frontier Fields images are adorned with strange arcs of light, representing the stretched and amplified images of background galaxies much more distant than the cluster and possibly too faint to be observed by direct imaging with Hubble. These shots revealed some of the most distant galaxies and the first gravitationally lensed supernova.
It’s been almost 200 years since the advent of photography—when humanity first managed to directly capture and record photons to make images. Today, highly complex cameras aboard a space telescope one million miles away are shaking our knowledge of the universe, opening new windows onto space and time. A relatively short time separates these two events, but they are linked by the same goal: achieving a deeper understanding of nature by looking at what our eyes cannot see.
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