galaxy NGC 1277 devoid of dark matter.
© NASA / ESA / M. Beasley (IAC)The galaxy NGC 1277, which is believed to be devoid of dark matter.
The galaxy, called NGC 177, is located near the center of the Perseus cluster, 240 million light-years from Earth.

One of the most baffling discoveries of 2023 was that of the galaxy NGC 1277.

In July, an international team of astrophysicists led by the Instituto de Astrofísica de Canarias (IAC) and the University of La Laguna (ULL) announced that they had discovered a massive galaxy with no dark matter.

The discovery, though still not fully understood, challenged the standard model of cosmology, which states that all massive galaxies are primarily composed of dark matter.

In that case, what are we to make of a dark matter-deficient galaxy?

In an interview with IE, study lead Sebastién Comerón explained that he had previously doubted the dark matter hypothesis. Still, the lack of dark matter in NGC 1277 actually strengthened his belief in the existence of the mysterious invisible matter.

Detecting the absence of invisible matter

Scientists believe that dark matter and dark energy account for approximately 95 percent of the universe. Still, they know very little about the mysterious phenomena, first discovered by Swiss astronomer Fritz Zwicky.

That's because the quest to directly detect dark matter and dark energy particles - in the form of Weakly Interacting Massive Particles (WIMPS) and other candidates - is an ongoing one that has so far produced little in the way of credible leads.

So, how exactly do you detect the absence of something undetectable to the naked eye? The IAC and ULL researchers, who published their findings in a paper in the journal Astronomy and Astrophysics, measured the total mass of NGC 1277 and used their measurement to infer the absence of dark matter.

"Dark matter is invisible, but its gravity is as real and detectable as that of "regular" matter such as stars or gas clouds," Comerón explained. "By studying the motion of stars in the outskirts of galaxies around the center of the galaxy, we can infer how much mass is found within their orbits."

"The motions of stars can be obtained from the reddening and blueing of the spectral lines caused by the Doppler effect," he continued. "In the case of NGC 1277, we found that the mass enclosed within the orbits of stars at the edges of the galaxy corresponds to that of the stars that we see. That is, there is not much room for a massive dark matter component."

In other words, this galaxy does not feature the same discrepancy usually observed within a galaxy of its size. Namely, the gravitational force exerted on individual star systems is larger than the force we expect from all visible matter in the galaxy. There is no "invisible" force, no dark matter.

In search of ancient relic galaxies

Interestingly, Comerón and his team didn't set out to study dark matter, let alone an incredibly rare dark matter-deficient galaxy. He explained that the discovery was "completely serendipitous," and he and his colleagues "were observing NGC 1277 for other reasons, namely that it is a relic galaxy."

Relic galaxies are a rare subset of galaxies that are believed to be "untouched" since the Big Bang first formed them. In other words, they have never merged with another galaxy and are supposed to have retained their structures and properties from the early universe.

"Most galaxies have undergone a complicated evolution since they were born," Comerón said. "Galaxies interact with other galaxies, they cannibalize smaller bodies. But NGC 1277 seems to be one of a handful of galaxies in the local universe that have remained virtually untouched since its formation, some 10 or 12 billion years ago."

"If this is true, studying NGC 1277 allows us to have a glimpse of what galaxies looked like when they were formed, without the need to resort to the always technically difficult study of very distant objects through facilities such as the James Webb Space Telescope."

A renewed belief in the dark matter hypothesis

Thanks to their interest in relic galaxies, Comerón and his team stumbled upon one of the strangest cosmic discoveries of 2023. And they still don't fully understand why NGC 1277 shows no signs of dark matter.

"We do not know for sure why NGC 1277 lacks dark matter," Comerón explained. "What we know is that it did lose its dark matter very early on. Indeed, extended matter distributions (such as a dark matter halo) have the power of dragging neighboring galaxies, which favors mergers, through a process called dynamical friction."

"The fact that NGC 1277 has not suffered mergers (hence its relic condition) implies that dynamical friction has been turned off, which clearly indicates the lack of a massive extended dark matter halo," he continued, adding that "We have postulated a couple of possibilities to explain the lack of dark matter, but their plausibility has to be tested through dedicated numerical simulations."

The first possibility considered by Comerón's team is that "most of the dark matter was removed by the environment once NGC 1277 fell into the rich galaxy cluster to which it belongs," he said. The second is that "NGC 1277 was born without dark matter at all."

"This could happen if NGC 1277 were born out of the collision of several gas-rich fragments with high relative speeds (a few 1000 km/s). In that case, it is conceivable that the gas would have collided and started an intense burst of star formation leading to the growth of NGC 1277, whereas the collisionless dark matter would have escaped the system due to its large speed."

Either way, the discovery of NGC 1277 calls the standard model of cosmology into question, as it states that dark matter is abundant throughout the universe.

Surprisingly, though, Comerón said he was more of a doubter of the dark matter hypothesis before discovering and investigating NGC 1277. The scientist also stressed that these are his views. They don't necessarily reflect those of his co-authors.

"Until very recently, I was not completely sure about the validity of the dark matter hypothesis," he explained, "and I considered other possibilities, such as alternative gravities, to be a plausible alternative explanation to the apparent lack of matter in galaxies and galaxy clusters."

"I think that our discovery is hard to reconcile with a modified gravity scenario," he continued. "It might seem a paradox that a galaxy lacking dark matter can be used as evidence for dark matter, but let me try to explain why it is so. If all galaxies display a behavior compatible with the existence of a dark matter halo, you can invoke alternative gravities as an alternative explanation."

"But if a few galaxies are found not to have a dark matter halo, then alternative gravities are in trouble because any law of nature should be universal and should hold for all galaxies."

In other words, "it seems much more natural to assume that dark matter can be stripped for a few galaxies than to hypothesize that alternative gravities can be turned on and off depending on the galaxy we are looking at."

Of course, Comerón and his team won't be satisfied with assumptions. Next, they will conduct follow-up research to understand NGC 1277 better. They aim to set tighter constraints on the dark matter percentage outlined in their initial study.

"Our ability to measure a dark matter fraction depends on how far out in the disc of the galaxy we are measuring the movement of the stars," Comerón explained. "The further out, the better the constraints. Currently, we have taken measurements within six kpcs (kiloparsecs) of the galaxy's center, showing there is less than 5 percent dark matter - though I would like to stress that this 5 percent is an upper boundary set by the quality of our data and that the actual number might be zero."

The team will use data from the galaxy's outer edges, captured via large telescopes and long exposures, to set tighter constraints on this dark matter percentage.

According to Comerón, they will use the new WEAVE (WHT Enhanced Area Velocity Explorer) instrument at the Observatorio del Roque de los Muchachos on the Island of La Palma, Spain, to expand their measurements to 10 kpc from the center.

"With these data, we could lower that 5 percent by a factor of two or three if NGC 1277 is, indeed, completely devoid of dark matter," Comerón explained.