Nov 30 2020
A distant galaxy with virtually no dark matter has questioned the standard theory of galaxy formation. Fresh evidence indicates the galaxy is not an anomaly, but a victim of theft.
A ground-based view of the sky around the galaxies NGC1052-DF4 and NGC1052-DF2—another galaxy with little to no dark matter. NGC1052-DF2 is next on Dr Montes’ list of galactic mysteries to solve. Image Credit: ESA/Hubble, NASA, Digitized Sky Survey 2 (Davide de Martin).
Dark matter—an invisible substance as mysterious as its name indicates—is the main ingredient that promotes the formation and survival of galaxies. It generates the strong gravity required to trigger galaxy formation and retain the present galaxies structurally undamaged.
However, astronomers have been puzzled by the 2019 discovery of “NGC1052-DF4,” a firm and long-lived galaxy with virtually no dark matter. How is it possible for the galaxy to exist without this essential ingredient? Are the theories regarding galaxy formation incorrect?
Currently, an international study under the guidance of the University of New South Wales (UNSW) Sydney indicates the dark matter existed already to start with—it has just been robbed by a greedy neighbor.
The dark matter isn’t there because it’s already been removed. We found that the gravitational pull from the nearby massive galaxy NGC1035 is removing its stars—and dark matter.
Dr Mireia Montes, Study Lead Author and Astronomical Researcher, Space Telescope Science Institute and UNSW Science
The study was recently published in The Astrophysical Journal and explains why so much dark matter has been missing from the galaxy without conflicting the present knowledge of galaxy formation.
When two galaxies are passing close to each other, they suffer from each other’s gravitational pull. Our very deep imaging found faint stars being pulled away by the larger galaxy—an interaction called ‘tidal disruption’.
Dr Mireia Montes, Study Lead Author and Astronomical Researcher, Space Telescope Science Institute and UNSW Science
Moreover, the same phenomenon can be identified on Earth: in this case, the Moon’s gravitational pull impacts Earth’s ocean tides. However, tidal disruption can result in the bending of galaxies—which are not as solid as the Earth or the Moon—and loss of their shape.
If the theory of tidal disruption is right, the smaller galaxy NGC1052-DF4 will soon begin to exhibit more signs of deterioration. It could ultimately disintegrate completely.
“Tidal stripping would remove a significant percentage of dark matter before affecting the stars,” stated Dr Montes. “If the stars are starting to be disrupted now, most of the dark matter has already escaped.”
According to Dr Ignacio Trujillo, study co-author and researcher at the Instituto de Astrofísica de Canarias (IAC), “With time, the galaxy will end being cannibalised by the large system around it (NGC1035), with at least some of their stars free floating in the deep space”.
A Powerful Magnifying Glass
Dr Montes and her collaborators employed robust telescopes and deep imaging methods—such as long-exposure photography of up to 60 hours—to determine the vague clues in the galaxy’s outer edges.
Such methods have the ability to illuminate very dim galaxies and stars, or what astronomers refer to as the “low surface brightness” of the universe.
Initial papers showed that the galaxy has a very ‘relaxed’ symmetrical shape, suggesting that no outside forces were perturbing it.
Dr Mireia Montes, Study Lead Author and Astronomical Researcher, Space Telescope Science Institute and UNSW Science
“But our deep images show that this galaxy is in fact being affected by its neighbour galaxy—it’s just caught in the beginning of the interaction. The inner part of the galaxy keeps its shape, but the outer, fainter parts are where you see these ‘tidal tails’: stars that have been already separated from the galaxy,” added Montes.
The images were captured with the help of the Gran Telescopio Canarias, the IAC80 Telescope, and the Hubble Space Telescope—one of the most enormous telescopes in space.
Dark matter is considered to be an invisible force, so it can only be observed by the interaction of stellar objects—such as galaxies and stars—with the space present around them.
“Ultra-deep imaging is hard not only because of the huge amount of time you need to reach such depths, but the extremely careful processing of the data needed to preserve the faintest structures,” stated Mr Raúl Infante-Sainz, PhD candidate at IAC and second author of this study.
“We needed to look for features that are 1000 times fainter than the darkest sky visible on Earth,” stated Dr Montes.
Solving New Galactic Mysteries
The Vera C. Rubin Observatory, a most advanced optical facility under construction in Chile, will soon improve deep imaging abilities to completely new levels.
The major project of the observatory will be the Legacy Survey for Space and Time (LSST)—a 10-year imaging survey that will offer the deepest images of the Southern Hemisphere’s night sky.
Sarah Brough, a professor and an astronomer at UNSW Science, heads Australia’s involvement in this project.
“LSST will revolutionise low surface brightness astronomy, transforming our understanding of galaxy evolution,” she noted. “It will provide extremely deep imaging data over the entire Southern Sky, which will be vital to the success of future Australian astronomical surveys and science.”
The LSST camera, which will be approximately the size of a small car, will allow scientists to detect galaxies with low surface brightness. Moreover, it will be able to determine faint features near and inside galaxy clusters.
Although the 10-year survey cannot commence until 2023, researchers are looking ahead to the chances for galactic exploration.
“This work is an example of how important it is to have deep images to understand apparently weird things in the Universe,” concluded Dr Montes. “Deep imaging can help explain mysteries that might otherwise remain unsolved.”
Journal Reference:
Montes, M., et al. (2020) The Galaxy “Missing Dark Matter” NGC 1052-DF4 is Undergoing Tidal Disruption. The Astrophysical Journal. doi.org/10.3847/1538-4357/abc340.