It started as a side note in a high-resolution ALMA observation. Elena Shablovinskaia and her team weren’t hunting for the weird. They were mapping the bright, active heart of NGC 4945 — a nearby starburst galaxy 11 million light-years away — when they spotted a faint blip about 60 parsecs off-center.
At 50% ± 14% polarization in millimeter wavelengths, the source — now nicknamed “Punctum” from the Latin for “point” or “dot” — has one of the highest polarization degrees ever seen in an extragalactic compact object. Even stranger, it’s invisible in optical, infrared, X-ray, and lower-frequency radio data. The only window into its existence is ALMA’s millimeter eye.
Astronomers now claim Punctum could represent an entirely new astrophysical object.
“Outside of the realm of supermassive black holes, Punctum is genuinely powerful,” Shablovinskaia told Space.com.
A Source That Refuses to Fit In
Punctum isn’t just bright for its category — it may be its own category. Shablovinskaia’s team measured its millimeter luminosity at roughly 2 × 10³⁵ erg/s, making it 10,000 to 100,000 times more luminous than typical magnetars, 100 times brighter than microquasars, and rivaled only by the Crab Nebula among known stellar remnants.
But size-wise, it’s tiny in astronomical terms — less than 2 parsecs (about 6.5 light-years) across, unresolved even with ALMA’s long baselines. For comparison, the Crab Nebula is 10-15 light-years across. Its brightness has held steady in every high-resolution ALMA dataset from 2019 to 2023, ruling out flaring events or short-lived transients like gamma-ray bursts.
The leading suspect is synchrotron radiation from an object with a highly ordered magnetic field. That’s the same basic physics behind pulsars, magnetars, supernova remnants, and non-thermal filaments in the Milky Way. But Punctum’s combination of intensity and polarization doesn’t match any of these.
Magnetars can reach very high polarizations, even near 100% in rare cases, but they fall short by several orders of magnitude in brightness. Supernova remnants can match the brightness but usually have messy magnetic fields that dilute polarization. Non-thermal filaments can polarize strongly but are far too faint at millimeter wavelengths. Microquasars? Their jets don’t seem capable of producing polarization anywhere near Punctum’s.
Polarization is basically light with a sense of direction — instead of its electric fields wobbling every which way, they’re all lined up, like cosmic hairs brushed neatly by a magnetic field. In space, that often happens when charged particles spiral around magnetic field lines at near-light speeds, producing synchrotron radiation. The stronger and more orderly the magnetic field, the more coordinated the light’s “dance” becomes — which is why Punctum’s 50% polarization is such a smoking gun for something with an unusually well-organized magnetic environment.
Clues, caveats, and cosmic context
In archival Chandra data, there’s no X-ray counterpart — just an upper limit of about 1 × 10³⁷ erg/s. Radio surveys that looked in longer wavelengths also drew a blank. The James Webb Space Telescope hasn’t yet looked directly at the spot, leaving the deep infrared spectrum unexplored.
The simplest explanation — that Punctum is an extreme example of something familiar — hasn’t been ruled out. A magnetar in an unusual environment, a supernova remnant tangled with dense material, or even a background blazar would technically fit some of the data. But the odds are slim.
And so, the tantalizing possibility remains: Punctum could be the first detected member of an entirely new class of astrophysical object, one that hides from most of the electromagnetic spectrum and only reveals itself in polarized millimeter light.
“Punctum is showing us that there is still a lot to discover in the millimeter sky,” Shablovinskaia said.
The next step is clear. ALMA follow-ups targeted directly at Punctum could probe deeper, while JWST’s infrared view might help unravel whether dust, emission lines, or pure synchrotron power its glow. For now, it sits as a quiet point in the sky — dazzlingly bright in one narrow band, and nowhere to be found in any other.
If the universe has a habit of surprising us, Punctum might be its latest calling card.
The study is about to be published in the journal Astronomy & Astrophysics. It’s currently available on the preprint server arXiv.
