'Hidden galaxies': Key to unlocking some of universe's secrets

If their existence is confirmed it would "effectively break current models of galaxy numbers and evolution."

The possible galaxies may also provide the missing piece of the puzzle for the energy generation in the universe in infrared light.

That's because their combined light would be enough to top-up the energy budget of the universe to the maximum we observe, effectively accounting for all remaining energy emission at these long wavelengths.

Possible evidence of the galaxies' existence was detected on the deepest ever image of the universe at long far-infrared wavelengths, which features almost 2,000 distant galaxies and was created by a team of researchers led by STFC RAL Space and Imperial College London.

Dr Chris Pearson, from STFC RAL Space, is lead author on one of two papers published today in Monthly Notices of the Royal Astronomical Society.

He 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 team behind the research created their deep view of the universe by stacking 141 images on top of each other using data from the SPIRE instrument on the Herschel Space Observatory, a European Space Agency mission which ran from 2009 to 2013.

The resulting Herschel-SPIRE Dark Field 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.

It also enabled them to track how the number of galaxies changes with brightness and to measure the contribution each one makes to the total energy budget of the universe.

However, the image was so deep and detected so many galaxies that the individual objects began to merge and become indistinguishable from each other.

This made extracting information challenging, according to Thomas Varnish, a PhD student at the Massachusetts Institute of Technology (MIT) and lead author on the second paper.

"We employed 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," said Mr Varnish, who carried out most of his research as a summer intern at Imperial College London and RAL Space.

"What we found was possible evidence of a completely new, undiscovered population of faint galaxies hidden in the blur of the image, too faint to be detected by conventional methods in the original analysis.

"If confirmed, this new population would effectively break all of our current models of galaxy numbers and evolution."

The researchers are now hoping to confirm the existence of the potential new group of galaxies using telescopes at other wavelengths.

Their aim is to decipher the nature of these faint, dusty objects and their importance in the grand scheme of the evolution of our universe.

Dr Pearson said: "When we look at starlight through normal telescopes, we are only able to read half of the story of our universe, the other half is hidden, obscured by the intervening dust.

"In fact, roughly half of the energy output of the universe is from starlight that has been absorbed by dust and re-emitted as cooler infrared radiation. To fully understand the evolution of our universe we need to observe the sky in both optical and longer wavelength infrared light."

The Herschel Space Observatory was tasked with observing the universe in the infrared, with its SPIRE instrument covering the very longest wavelengths.

Like any scientific instrument in space, the SPIRE instrument also required regular observations for calibration and routinely stared at a single patch of 'dark sky' every month or so, over the duration of its four-year mission.

Herschel held the record for the largest ever infrared space telescope, until it was eclipsed by the James Webb Space Telescope in 2021.

Imperial College London astrophysicist Dr David Clements, who was also involved in the research, added: "These results show just how valuable the Herschel archive is.

"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.