new suspect found in the galactic mystery

A distant galaxy with almost no dark matter threatened to break our theory of galaxy formation. New evidence suggests the galaxy is not an anomaly, but a victim of a theft.

Dark matter, an enigmatic invisible substance as its name suggests, is a key ingredient in helping galaxies form and stay alive.

It creates the strong gravity needed to trigger galaxy formation and keep existing galaxies structurally intact.

But astronomers were puzzled by last year’s discovery of “NGC1052-DF4,” a stable, long-lived galaxy with almost no dark matter. How can the galaxy exist without this important ingredient? Are our theories of galaxy formation wrong?

Today, an international study conducted by UNSW Sydney suggests that dark matter was there to begin: it was just stolen from a greedy neighbor.

“The dark matter is not there because it has already been removed,” says Dr Mireia Montes, lead author of the study and an astronomical researcher at UNSW Science and the Space Telescope Science Institute.

“We found that the gravitational pull from the nearby massive galaxy NGC1035 is removing its stars and dark matter.”

The research, published today in The Astrophysical Journal, provides an explanation of why so much dark matter is missing from the galaxy without contradicting our current understanding of galactic formation.

“When two galaxies pass close to each other, they suffer from each other’s gravitational pull,” says Dr. Montes. “Our very deep image detected faint stars being dragged away from the larger galaxy, an interaction called ‘tidal disruption’.”

The same phenomenon can also be found on Earth: in our case, the gravitational pull of the Moon affects the Earth’s ocean tides. But the interruption of the tides can cause galaxies, which are not as solid as the Earth or the Moon, to bend and lose their shape.

If the tidal interruption theory is correct, the smaller galaxy NGC1052-DF4 will soon begin to show more signs of deterioration. Eventually it may completely disintegrate.

“Tidal stripping will remove a significant percentage of dark matter before it hits the stars,” says Dr Montes.

“If the stars are starting to be destroyed now, most of the dark matter has already escaped.”

Dr Ignacio Trujillo, co-author of the article and researcher at the Instituto de Astrofísica de Canarias (IAC), says: “Over time, the galaxy will eventually be cannibalized by the great surrounding system (NGC1035), with at least some of their stars float free in deep space “.

A powerful magnifying glass

Dr. Montes and her colleagues used powerful telescopes and deep imaging techniques, including long exposure photography of up to 60 hours, to find faint clues in the outer edges of the galaxy.

These techniques are capable of illuminating very faint stars and galaxies, or what astronomers call the “low surface brightness” of the universe.

“Initial documents showed that the galaxy has a very ‘relaxed’ symmetrical shape, suggesting that no external forces were disturbing it,” says Dr Montes.

“But our deep images show that this galaxy is indeed affected by the neighboring galaxy: it was only captured at the beginning of the interaction.

“The inner part of the galaxy retains its shape, but the outer and fainter parts are where you see these ‘tidal tails’: stars that have already been separated from the galaxy.”

The observations were made using the IAC80 telescope, the Gran Telescopio Canarias and the Hubble Space Telescope, one of the largest telescopes in space.

Since dark matter is an invisible force, it can only be seen from the way stellar objects, such as stars and galaxies, interact with their surrounding space.

“Ultra-deep imaging is difficult not only because of the enormous amount of time it takes to reach such depths, but also because of the extremely careful processing of the data needed to preserve the weakest structures,” says Raúl Infante-Sainz, Ph D. IAC candidate and second author of this study.

“We needed to look for features that were 1000 times fainter than the darkest sky visible on Earth,” says Dr. Montes.

Solve new galactic mysteries

The Vera C. Rubin Observatory, a state-of-the-art optical facility currently under construction in Chile, will soon take deep imaging capabilities to new levels.

The observatory’s main project will be the Legacy Survey for Space and Time (LSST): a ten-year survey of images that will provide the deepest images ever seen of the Southern Hemisphere’s night sky.

UNSW Science astronomer Professor Sarah Brough leads Australia’s involvement in this project.

“LSST will revolutionize low surface luminosity astronomy, transforming our understanding of the evolution of the galaxy,” he says.

“It will provide extremely deep imaging data across the entire Southern Sky, which will be vital to the success of future Australian astronomical and scientific investigations.”

The LSST camera, which will be about the size of a small car, will allow researchers to detect galaxies with low surface brightness. It will also be able to identify faint features around and within galaxy clusters.

Although the 10-year investigation won’t start until 2023, scientists are looking forward to seizing opportunities for galactic exploration.

“This work is an example of how important it is to have deep images to understand seemingly strange things in the Universe,” says Dr. Montes.

“Deep imaging can help explain mysteries that might otherwise remain unsolved.”