A team of astronomers led by Yale's Pieter van Dokkum stunned the scientific community three years ago when they discovered NGC 1052-DF2, a far-off galaxy with little or no dark matter.
Yale News said that the discovery, made with NASA's Hubble Space Telescope, can challenge long-held beliefs about how galaxies form and evolve. This is because dark matter - the unseen substructure that makes up the majority of the universe's mass — is thought to be crucial in the formation and structuring of galaxies.
But how could there be a galaxy with so little dark matter? Some astronomers believe the result is wrong. They questioned the precision of distance estimations from Earth to NGC 1052-DF2.
Van Dokkum and Yale graduate student Zili Shen have now pinpointed the distance to DF2 and corroborated the original findings in a new study published in The Astrophysical Journal Letters. The research is titled A Tip of the Red Giant Branch Distance of 22.1 ± 1.2 Mpc to the Dark Matter Deficient Galaxy NGC 1052-DF2 from 40 Orbits of Hubble Space Telescope Imaging.
"We went out on a limb with our initial Hubble observations of this galaxy in 2018," van Dokkum, Yale's Sol Goldman Family Professor of Astronomy, said per Phys.org. "I think people were right to question it because it is such an unusual result. It would be nice if there were a simple explanation, like a wrong distance. But I think it is more fun and more interesting if it actually is a weird galaxy," he added.
It is vital to know how far away DF2 is to figure out how much dark matter it contains. Researchers calculated the total mass using the motions of the stars inside the galaxy, whose velocities are influenced by gravity's pull. According to the astronomers, the observed stellar mass, which varies with distance, matches the galaxy's overall mass, leaving little room for dark matter.
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A Better Yardstick
Astronomers say that if DF2 were closer to Earth, it would be fundamentally fainter and less massive. As a result, the galaxy would contain more dark matter to account for the entire mass' observed effects.
Van Dokkum's team determined that DF2 was 65 million light-years away in the first investigation. Other independent research teams calculated the distance to be 42 million light-years.
Van Dokkum and Shen used Hubble's Advanced Camera for surveys to capture long-exposure photos of DF2 for their revised distance determination. They used the brightness of aged red massive stars on the fringes of DF2 to calculate the distance from Earth.
The brightest red giants, according to Shen, are a well-established distance indicator for surrounding galaxies.
They calculated the distance to be 72 million light-years, virtually validating the previous discovery.
"For almost every galaxy we look at, we say that we cannot see most of the mass because it is dark matter. What you see is only the tip of the iceberg," van Dokkum said per NASA. According to van Dokkum, Hubble shows the complete picture — not just the top of the iceberg but also the entire iceberg.
The mystery of how a galaxy with essentially no dark matter evolved remains unsolved.
Former Yale astronomer Shany Danieli of the Institute for Advanced Study discovered a second galaxy - NGC 1052-DF4 — nearly devoid of dark matter after van Dokkum's original study in 2018. Danieli is one of the study's co-authors. In 2020, a separate research team discovered 19 dwarf galaxies that may also be devoid of dark matter.
According to Shen, extraordinary claims necessitate extraordinary evidence. The latest study, per Science Daily, strongly supports the previous finding that DF2 lacks dark matter. Now is the time to look beyond the question of distance and consider how such galaxies came to be.
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