Astronomers have unveiled a new cosmic mystery following the detection of a faint signal from a nearby galaxy, which they claim is unlike anything previously observed in our universe.
The highly polarized signal, only detectable in millimeter radio wavelengths, has been dubbed “Punctum,” a nickname derived from the Latin word for “point” or “dot,” by its discoverers in a team led by Elena Shablovinskaia of the Institute of Astrophysical Studies, Diego Portales University in Chile.
The discovery was revealed in a newly posted paper on the preprint server arXiv.org, based on observations made by the team using the Atacama Large Millimeter/submillimeter Array (ALMA) in northern Chile.
Introducing Punctum
The mystery celestial object discovered by Shablovinskaia and her team lies within nearby NGC 4945, a starburst Seyfert 2 galaxy located approximately 13 million light-years from Earth.
Although several other compact millimeter sources exist in the galaxy’s central region, the mysterious Punctum stood out among its faint and very polarized peers, displaying a significantly greater degree of polarization—an estimated 50% ± 14%—a signature Shablovinskaia’s team believes is likely evidence of non-thermal synchrotron radiation.
Punctum was detected during a pair of observation periods separated by two weeks, and in both cases, the signal remained faint, but unambiguous, showing no signs of variability across days or weeks, as well as in comparison to earlier archival observations.
The object’s spectral index, which denotes a measure of how an object’s brightness varies with its frequency, also remained stable within large uncertainties of about -1.8 ± 2.5 and -1.3 ± 2.5, according to the team’s new paper.
A Celestial Object Unlike Any Other
One of the most puzzling things about Punctum is its absence of any matching signals in other parts of the electromagnetic spectrum. Despite exhaustive searches through archival data obtained by the Chandra X-ray Observatory, no viable counterparts could be found.
Currently, upper limits suggest Punctum has an X-ray output weaker than that of some recognized bright X-ray-emitting sources, although it still displays remarkable brightness.
“It is important to note that this represents an upper limit, as contributions from surrounding star-forming material were not accounted for,” the team writes. “No distinct X-ray source was identified as a counterpart to the polarized mm emission.”
Additional data from the Australia Telescope Compact Array similarly confirmed that no corresponding radio sources could be found that matched its possible identity.
Meanwhile, comparisons with known astrophysical objects, such as accreting neutron stars, supernova remnants, or galactic filaments, also revealed no clear match. While Punctum’s strong polarization and synchrotron emission seem to suggest that an energetic process is at work, no currently known astrophysical processes are a good match for it.
In short, Punctum’s unusual nature remains a mystery, at least for now.
Clues Toward Solving a Cosmic Mystery
“The exact nature of this highly polarized source remains undetermined,” the authors note in their study, leaving open an intriguing possibility: that the faint detection revealing Punctum’s presence could represent an entirely new class of astrophysical object.
However, there are at least a few things about the object that the team says could offer clues toward solving this celestial mystery.
Specifically, known sources of synchrotron emission appear to be consistent with the high degree of linear polarization exhibited by Punctum. Of the known phenomena capable of producing comparable emissions, magnetars are perhaps the best-known candidate, despite generally possessing lower luminosities.
Other possibilities may include supernova remnants, as well as galactic non-thermal filaments, both of which possess some of the characteristics associated with Punctum, although neither are a perfect match.
“Thus, Punctum cannot currently be associated with any known class of astrophysical source,” the authors conclude. Going forward, additional observations may help Shablovinskaia and her team introduce even better constraints on the object’s spectral index, allowing them to investigate the object’s unusual polarization in greater detail.
“High-resolution, multi-wavelength follow-up observations would also be invaluable for further characterization,” the team writes.
The recent paper, “ALMA discovery of Punctum – a highly polarized mm source in nuclear starburst galaxy NGC 4945,” was uploaded to the preprint server arXiv.org on July 17, 2025.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.