Seeing the invisible is not magic it’s engineering, one astronomer said when the James Webb Space Telescope’s Mid-Infrared Instrument, or MIRI, presented its deepest-ever gaze at the Hubble Ultra Deep Field. That engineering has now lifted the curtain on galaxies whose light has taken over 13 billion years to reach us, showing a population of ancient, dust-shrouded systems and presenting a clearer view of the early years of the universe.
The MIRI Deep Imaging Survey (MIDIS) integrated almost 100 hours of exposure in the 5.6 micron filter, the longest single-filter extragalactic observation to date by JWST. The outcome is an unprecedented mosaic of high depth, finding about 2,500 objects within a region of the sky previously believed to be almost empty. “In the images, we can see the most distant galaxies known to us,” said Stockholm University professor of astronomy Göran Östlin. Most of these galaxies released their light less than a billion years into the universe’s existence since the Big Bang.
The benefit of MIRI is that it can record mid-infrared wavelengths from 5 to 28 microns light that the Hubble Space Telescope and the human eye cannot observe. At these wavelengths, astronomers are able to see through interstellar dust that scatters visible and ultraviolet light, uncovering the glow of older, cooler stars and the thermal emanations of dust warmed by vigorous star formation or active galactic nuclei. As Stockholm University’s Jens Melinder described it, “MIRI allows us to see through the veil of dust and observe what lies behind.”
The survey’s technical realization was as careful as its scientific objectives. Observations were divided into several spacecraft visits, with 50 different dither positions and fractional-pixel offsets for maximum resolution. The final image achieves a 5σ point-source sensitivity of 28.65 AB magnitudes approximately 12.6 nanoJanskys better than preflight predictions. With a point spread function full width at half maximum of only 0.2 arcseconds, MIRI resolves down to 1.4 kiloparsecs at redshift 4, allowing for high-resolution investigations of stellar mass distribution in galaxies from cosmic noon to the reionization era.
Such capabilities are already providing new insights into early heavy element assembly and supermassive black hole growth. Dust-enriched galaxies in the sample, some with black holes embedded in hot dust tori, provide a laboratory in which to quantify how rapidly metals produced in the early universe condensed. Because MIRI can observe rest-frame near-infrared emission from galaxies at z > 9, it sees light from stars that were formed just a few hundred million years after the Big Bang stars not seen by shorter-wavelength instruments.
The MIDIS data are supplemented by other JWST programs, including the Cosmic Evolution Early Release Science Survey, which has revealed small, low-mass galaxies whose high ultraviolet luminosity could have sparked cosmic reionization. Concurrently, simultaneous JWST and ALMA spectroscopy has validated galaxies such as GHZ2 at redshift 12.33, where metallicities are only one-tenth solar and stellar population densities compete with those of globular clusters. All these discoveries highlight the heterogeneity of conditions in the universe’s first few hundred million years.
Processing the mid-infrared imagery involved converting invisible wavelengths into a color-coded composite. In the publicly released HUDF mosaic, dusty star-forming galaxies are orange and red, very distant compact galaxies are greenish, and near-infrared-bright systems appear blue or cyan. The public release not only contains the images but photometric catalogs as well, allowing researchers everywhere to harvest the data for research into galaxy evolution, star formation history, and black hole accretion.
The value of this survey goes beyond its immediate findings. In creating a rich mid-infrared baseline in one of the most heavily mapped areas of the sky, MIDIS lays the groundwork for future JWST programs aimed at longer MIRI wavelengths. As Östlin noted, “We have contributed brand new data that will be used in future by researchers studying galaxy evolution and the formation of the first galaxies.” For astronomers probing the cosmic dawn, MIRI’s view is not just deeper it is clearer, richer, and more revealing than any before it.
