We're only able to see about half of our Universe with the naked eye. The other half is hidden, obscured by intervening dusk that partially or completely blocks our view. To fully understand our Universe and its evolution, astronomers must observe the sky at various wavelengths, or colours, of light, so they can see as much as possible.
A team led by RAL Space and Imperial College London have developed the deepest-ever image of the Universe at long far-infrared wavelengths, using data from the Herschel Space Observatory, a European Space Agency mission which ran from 2009 to 2013.
Dr Chris Pearson, Astrophysics Programme Lead at RAL Space and lead author on one of two papers published today in Monthly Notices of the Royal Astronomical Society, said:
“This work has pushed the science with Herschel to its absolute limit, probing far below what we can normally discernibly see and potentially revealing a completely new population of galaxies that are contributing to the very faintest light we can observe in the Universe."
The final SPIRE Dark Field image map was created by combining the Blue (250 micrometres), Green (350 micrometres) and Red (500 micrometres) SPIRE camera channels together, each channel stacking a total of 141 images on top of each other. The blobs on the image are all individual groups of galaxies. However, the image is so crowded that there is almost no empty space with the faintest galaxies merging into the background light in the map. Credit: Pearson et al. 2025
Delving deeper into the Universe
The team created the “Herschel-SPIRE Dark Field" by stacking 141 individual observations taken throughout the mission's lifetime.
The resulting image is the deepest ever image of the far-infrared sky – five times deeper than the previous single deepest Herschel observation and at least twice as deep as any other area on the sky observed by the telescope.
Placing the images on top of each other allowed astronomers to see the dustiest galaxies, where most new stars are formed in the cosmos. However, the image was so deep and detected so many galaxies that individual objects began to merge, becoming indistinguishable from each other.
This made extracting information challenging, according to Thomas Varnish, a PhD student at Massachusetts Institute of Technology and lead author on the second paper.
"The team employed novel statistical techniques to get around this overcrowding, analysing the blurriest parts of the image to probe and model the underlying distribution of galaxies not individually discernible in the original image," he said.
Challenging current models
The analysis revealed possible evidence of a previously undiscovered population of faint galaxies hidden in the image blur, too faint to be detected by conventional methods. This unexpected increase in the number of galaxies is not predicted by current models.
If proven to exist, this new population may provide the missing piece to account for all remaining infrared energy observed in the Universe.
The researchers are now hoping to confirm the existence of the potential new group of galaxies using telescopes at other wavelengths.
“These results show just how valuable the Herschel archive is," says Dr David Clements, astrophysicist at Imperial College. “We're still getting great new results more than 10 years after the satellite stopped operating.
“What we can't get, though, is more data at these wavelengths to follow up these fascinating new results. For that we need the next generation far-IR mission, PRIMA, currently being proposed to NASA."
The Probe far-Infrared Mission for Astrophysics (PRIMA) is being supported by a UK consortium including RAL Space, the University of Sussex, Imperial College London and Cardiff University.
It would involve the use of a 1.8-metre telescope optimised for far-infrared imaging and spectroscopy, bridging the gap between existing observatories such as the James Webb Space Telescope and radio telescopes.
PRIMA is one of two proposals shortlisted for NASA's next $1 billion (£772 million) probe mission. The US space agency will confirm its final mission selection in 2026.
RAL Space's instrumental role
Herschel's SPIRE instrument was led by Cardiff University with involvement from other institutions across the UK.
RAL Space were responsible for:
- Instrument assembly and test before launch
- Project management
- Hosting the instrument control centre during mission operations
Support from early career scientists
A considerable amount of work going into this study was accomplished by early career colleagues supported by RAL Space.
Matthew Pearson undertook much of the initial work on the Herschel-SPIRE Dark Field Image during a Summer Work Experience Placement. Matthew helped filter the list of relevant observations to create the maps and developed the initial test images of the field.
Thomas Varnish, lead author of the second paper, completed the final statistical analysis of the Dark Field image whilst working at RAL Space as a Summer Graduate Placement.
