How do you spot 2,500 hidden galaxies within an area of sky already thoroughly searched by Hubble for more than a decade? The James Webb Space Telescope’s new MIRI Deep Imaging Survey has precisely that effect, finding objects so distant and faint that some of them were alive fewer than a billion years after the Big Bang.
The survey aimed at the same part of the sky as the famous Hubble Ultra Deep Field, but Webb’s Mid-Infrared Instrument (MIRI) and Near-Infrared Camera (NIRCam) offered a whole new toolset. For almost 100 hours of observation with a single 41-hour exposure through one MIRI filter, the longest extragalactic field observation Webb has made to date the telescope took the deepest-ever mid-infrared view of this sky. The outcome is a catalog that not only contributes thousands of previously unknown galaxies to the count but also untangles their structures in invisible wavelengths.
Mid-infrared astronomy is particularly well adapted to exploring the early universe. As the light from distant, ancient galaxies travels through expanding space, it is distorted or redshifted from visible into infrared wavelengths. MIRI’s sensitivity to wavelengths between 5 and 28 microns allows it to detect the glow of dust-enshrouded star-forming regions and the cooler emission from older stellar populations. In the MIDIS image, galaxies rich in dust and intense star formation appear in orange and red, extremely distant compact galaxies with high redshift are rendered greenish-white, and near-infrared-bright systems stand out in blue and cyan. This chromatic mapping is not superficial; astrophysical properties are encoded directly in the visual information.
Processing images like these is a careful, human-led process. Raw data from Webb detectors come in as binary code, translated into black-and-white FITS files that need to be stretched to bring out faint detail hidden in the noise. Imaging specialists register exposures with several filters, eliminate hits by cosmic rays and detector artifacts, and map visible colors onto infrared wavelengths in strict order of color. In the case of MIDIS, merging the shorter-wavelength NIRCam data with the longer-wavelength views from MIRI necessitated careful filter weaving to maintain both resolution and scientific integrity. The final composite is as much an engineering tour de force in data management as an astronomical instrumentation triumph.
Cosmological stakes are high. Seeing galaxies less than a billion years after the Big Bang is a direct window into the era of reionization, when the first stars and galaxies ionized the neutral hydrogen that permeated the universe. Webb has already established galaxies such as JADES-GS-z13-1 at redshift 13.0 only 330 million years after the Big Bang with surprisingly robust Lyman-alpha emission. “This result was totally unexpected by theories of early galaxy formation and has caught astronomers by surprise,” comments Roberto Maiolino from the University of Cambridge. Such results indicate that parts of the early universe cleared their hydrogen haze much sooner than simulated by models, maybe because of exceptionally massive, extremely hot stars or by the energetic radiation of new-born supermassive black holes.
The MIDIS region also has hundreds of very red galaxies, possibly massive, dusty systems or evolved galaxies with old stars produced surprisingly earlier. Their mid-infrared brightness defies current models of star formation rates in the young universe. As Haojing Yan of the University of Missouri wrote in an independent study of luminous early-universe prospects, “If even a few of these objects turn out to be what we think they are, our discovery could challenge current ideas about how galaxies formed in the early universe.”
By coming back to an heirloom science first made famous by Hubble, Webb has taken the deep-field science into novel spectral ground. Its bigger mirror, higher resolution, and extension into longer wavelengths have turned what was once the deepest view of the universe into just the beginning. The MIDIS image isn’t just a deeper view; it is a profoundly different one, illuminating the dusty, dynamic, and surprisingly mature structures that filled the universe’s first billion years.
