new suspect found in galactic mystery

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

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

It creates the strong gravity necessary to trigger the formation of galaxies and keep existing galaxies structurally intact.

But astronomers have been puzzled since the discovery last year 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 about the formation of galaxies wrong?

Today, an international study by UNSW Sydney suggests dark matter was here to begin – it has just been stolen by a greedy neighbor.

“Dark matter isn’t there because it’s already been eliminated,” says Dr. Mireia Montes, lead author of the study and astronomical researcher at UNSW Science and the Space Telescope Science Institute.

“We have discovered that the gravitational pull of the nearby massive galaxy NGC1035 is removing its stars and dark matter.

The research, published today in The Astrophysics Journal, explains why so much dark matter is missing from the galaxy without contradicting our current understanding of galaxy formation.

“When two galaxies pass close to each other, they both suffer from gravitational pull,” says Dr Montes. “Our very deep imagery revealed that faint stars were being moved away by the larger galaxy – an interaction called ‘tidal disturbance.’

The same phenomenon is also found on Earth: in our case, the gravitational pull of the Moon influences the oceanic tides of the Earth. But tidal disturbances can cause galaxies – which aren’t as strong as the Earth or the Moon – to bend and lose their shape.

If the tidal disturbance theory is correct, the small galaxy NGC1052-DF4 will soon begin to show more signs of deterioration. It could eventually disintegrate completely.

“Tidal stripping would remove a significant percentage of dark matter before it affects the stars,” explains Dr Montes.

“If the stars start to be disturbed 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: “In time, the galaxy will eventually be cannibalized by the large system that surrounds it (NGC1035) , with at least some of their stars floating freely in deep space “.

A powerful magnifying glass

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

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

“The first papers showed that the galaxy had a very ‘relaxed’ symmetrical shape, which suggests that no outside force was disturbing it,” says Dr Montes.

“But our deep images show that this galaxy is in fact affected by its neighboring galaxy – it’s just taken at the start of the interaction.

“The inner part of the galaxy keeps its shape, but the paler outer 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 observed by the way stellar objects – like stars and galaxies – interact with the space around them.

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

“We had to look for features that are 1000 times paler 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 Legacy Survey for Space and Time (LSST): a ten-year imagery survey that will provide the deepest images ever seen of the southern hemisphere’s night sky.

Professor Sarah Brough, astronomer at UNSW Science, is leading Australia’s participation in this project.

“LSST will revolutionize low surface light astronomy, transforming our understanding of the evolution of galaxies,” she says.

“It will provide extremely deep imagery data across the entire southern sky, which will be critical to the success of future Australian astronomical and scientific studies.”

The LSST camera, which will be roughly the size of a small car, will allow researchers to detect galaxies with low surface light. It will also be able to identify weak features around and in galaxy clusters.

Although the decennial survey does not begin until 2023, scientists are eagerly awaiting opportunities for galactic exploration.

“This work is an example of the importance of having deep images to understand seemingly strange things in the Universe,” says Dr Montes.

“In-depth imagery can help explain mysteries that might otherwise go unresolved.”