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Monday, 21 June 2021

Astronomers Confirm Existence of Dark-Matter-Deficient Galaxies


In a new study, astronomers used data from the NASA/ESA Hubble Space Telescope to calculate a more accurate distance to an ultra-diffuse galaxy called NGC 1052-DF2 (DF2 for short). The new measurement solidifies previous claims that DF2 lacks dark matter, the invisible glue that makes up the bulk of the Universe’s contents.

DF2 lies in the constellation of Cetus and belongs to the NGC 1052 group of galaxies.

This ultra-diffuse galaxy is roughly the size of our Milky Way Galaxy, but hosts only 1/200 the number of stars.

In 2018, a team of astronomers from the United States and Canada discovered that DF2 contains virtually no dark matter.

A year later, a second galaxy in this class, NGC 1052-DF4, was discovered in the NGC 1052 group.

To bolster their original finding, the astronomers followed up their initial studies with a more robust Hubble look at DF2.

“We went out on a limb with our initial Hubble observations of this galaxy in 2018,” said Dr. Pieter van Dokkum, an astronomer in the Astronomy Department at Yale University.

“I think people were right to question it because it’s such an unusual result. It would be nice if there were a simple explanation, like a wrong distance. But I think it’s more fun and more interesting if it actually is a weird galaxy.”

The astronomers focused on aging red giant stars on the outskirts of the DF2 galaxy that all reach the same peak brightness in their evolution.

“Studying the brightest red giants is a well-established distance indicator for nearby galaxies,” said Zili Shen, a graduate student in the Astronomy Department at Yale University.

“The new Hubble observations help us confirm that DF2 is not only farther from Earth than some astronomers suggest, but also slightly more distant than our original estimates.”

The new distance estimate is that DF2 is 72 million light-years away as opposed to 42 million light-years, as reported by other teams.

This places the galaxy farther than the original Hubble 2018 estimate of 65 million light-years distance.

“The more accurate measurements solidify the initial conclusion of a galaxy deficient in dark matter,” the astronomers said.

“So the mystery of why DF2 is missing most of its dark matter still persists.”

The team’s paper was published in the Astrophysical Journal Letters.

Reference:

Zili Shen et al. 2021. 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. ApJL 914, L12; doi: 10.3847/2041-8213/ac0335

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