In a groundbreaking two-part study, astronomers have leveraged the Herschel Space Observatory’s SPIRE instrument to produce the deepest-ever image of the sky in the far-infrared to submillimetre range. This investigation, focused on the so-called SPIRE Dark Field near the North Ecliptic Pole, not only mapped an exceptionally quiet patch of sky but also pushed the boundaries of our understanding of the hidden universe.
A Treasure Hidden in Calibration Data
The Dark Field wasn’t initially designed for scientific discovery, it was a calibration region observed 141 times throughout the Herschel mission. However, when combined, these observations revealed a region of sky far deeper than any previous far-infrared survey, with uniform coverage in the central 12-arcminute area, dubbed the Deep Region.
In the first paper, the team applied traditional and advanced source extraction techniques to identify nearly 1,800 galaxies at 250, 350, and 500 microns. While standard methods were limited by “confusion noise”, the blurring together of faint, overlapping sources, the researchers cleverly used prior observations from the Spitzer Space Telescope to guide their detections deeper than Herschel could manage alone.
The results confirmed existing models of galaxy evolution at brighter flux levels, but hinted at discrepancies in the counts of fainter galaxies, a mystery that requires deeper analysis.
Statistical Sleuthing: Part II
The second paper turned to a powerful statistical technique known as P(D) analysis (probability of deflection). Rather than trying to detect individual galaxies, this method analyses fluctuations in the pixel brightness across the image. These subtle variations arise from countless faint sources too small to be seen individually, allowing astronomers to probe the structure of the universe at flux levels down to a few tenths of a milliJansky — well below the confusion limit.
The findings were striking: at faint fluxes, the galaxy number counts deviated from every existing theoretical model. The team identified a “bump-and-turnover” pattern in the data, a steep rise in the number of faint galaxies, peaking and then rapidly falling off, that wasn’t predicted by any published model. This was observed most clearly at 250 and 350 microns, with a more gradual effect at 500 microns.
Implications for Cosmic Evolution
These results suggest the presence of a previously unaccounted-for component in the universe’s history. Whether this is a new population of faint, dusty star-forming galaxies, or a shortcoming in how current models handle galaxy evolution, remains unclear. What’s certain is that the conventional models are missing something vital at the faint end of the submillimetre spectrum.
Moreover, the authors found that the peak of this unexpected “bump” shifts to lower fluxes as the wavelength increases — a clue that the responsible galaxies might emit more strongly at shorter wavelengths, possibly indicating different dust temperatures or star-formation characteristics.
These discoveries have profound implications for our understanding of galaxy formation and evolution, especially during the early epochs of the universe when dust-enshrouded starbursts were common.
Completing the Cosmic Infrared Puzzle
By integrating the best-fitting source count distributions derived from their P(D) analysis, the researchers estimated the contribution of these faint galaxies to the cosmic infrared background (CIB) — the diffuse glow from all star-forming galaxies over cosmic history. Their results agree well with previous measurements from the Planck satellite, especially in the 350-micron band, where the match was nearly perfect.
This level of agreement suggests that the Herschel-SPIRE Deep Field has likely captured the entire population of galaxies responsible for the 350-micron CIB in that region. At 250 and 500 microns, the match was still within the expected uncertainty range, though some tension at 500 microns hints at the limits of current methods and models.
The Road Ahead
The SPIRE Dark Field represents the deepest far-infrared view of the sky ever achieved. Until a new generation of space-based telescopes with higher resolution and sensitivity is launched, this dataset will remain unparalleled. Upcoming missions may help to clarify the nature of the mysterious sub-mJy population revealed in this study.
In the meantime, these findings push astronomers to rethink their models of how galaxies evolve and contribute to the infrared cosmos. They also reaffirm the value of long-duration, high-quality data, even when originally collected for calibration, and the power of clever statistical methods to unveil the universe’s faintest secrets.
References and further Reading.
Chris Pearson et al, The Herschel-SPIRE Dark Field I: The deepest Herschel image of the submillimetre Universe, Monthly Notices of the Royal Astronomical Society (2025). DOI: 10.1093/mnras/staf335
Thomas Varnish et al, The Herschel-SPIRE Dark Field II: A P(D) Fluctuation Analysis of the Deepest Herschel Image of the Submillimetre Universe, Monthly Notices of the Royal Astronomical Society (2025). DOI: 10.1093/mnras/staf318