The discovery of two ghostly galaxies DF2 and DF4, devoid of dark matter, has sparked a cosmic debate within the astronomical world. While the answer is still in the air, this cosmic-stir ending in an affirmation of the question can invalidate our decades-long understanding of how galaxies form and evolve.

An image of NGC1052-DF2 captured using the Advanced Camera for Surveys on the Hubble Space Telescope. Image courtesy:  NASA/ESA/P. van Dokkum (Yale University)

2018, Pieter van Dokkum’s Research on NGC 1052-DF2

Some 64 million lightyears away, near the local galactic neighborhood of our planet, is a galaxy named NGC 1052-DF2. Classified as an ultradiffuse galaxy (UGD), NGC 1052-DF2 is home to fewer stars (around 1/200th of Milky Way) than a traditional galaxy. To probe the properties of this galaxy, Pieter van Dokkum of Yale University collaborated with researchers and led a research group. In February 2018, the group identified ten globular clusters of stars and observed their brightness. However, it wasn’t this extraordinary brightness of the celestial bodies that caught the eyes of researchers. Instead, it was the apparent scarcity of dark matter which surprised them. The researchers observed how quickly the clusters completed their orbits. The results were a big surprise as the globular clusters moved way slower than what had ever been seen in other galaxies before. With further research, the conclusion was that DF2 has little to no dark matter because if it did, the clusters would’ve been moving much faster

Van Dokkum’s team analyzed the absorption lines of spectra taken at Keck Observatory to figure out the velocity of globular clusters in DF2. The determined velocities later on helped determine the overall galaxy’s mass. Image courtesy: Gemini Observatory/NSF/AURA/W.M. Keck Observatory/Jen Miller/Joy Pollard

As the discovery of a galaxy without dark matter would’ve nullified our decades-long research and understanding of the formation and evolution of galaxies, its news spread at the speed of light throughout the astronomical community. However, not everyone agreed with van Dokkum’s conclusion, especially the research group led by Ignacio Trujilo based in the Canary Islands.

The NGC 1052-DF2  Distance Feud

According to Trujillo, DF2 was anomalous for having not only an apparent scarcity of dark matter but also an unusual abundance of bright globular clusters of stars. “I remember thinking,” says Trujillo, “Two anomalies at the same time looks really odd.”

This conflict of thought led him to consider the distance of DF2. According to Trujillo, if DF2 was as distant as van Dokkum suspects, then the galaxy would potentially be the first one without dark matter known to humans. However, if closer, then the properties observed were no anomaly and fell roughly in line with what astronomers expect from an average galaxy with dark matter.

To test his theory, Trujillo and his team used five different methods to measure the distance of DF2. For the first two of their attempts, his group analyzed the brightness and sizes of the globular clusters in DF2 respectively. In the third attempt, he compared the properties of DD04, a galaxy similar to DF4, to obtain a more reliable distance estimate. His fourth method revolved around recalculation of the distance using the original methodology. Finally, his team analyzed the Tip of the Red Giant Branch (TRGB) — a primary distance indicator of any galaxy that uses the luminosity of the brightest giant red branch stars as a standard using a color-magnitude diagram (CMD) of DF2. 

“This is by far the most accurate way of measuring the distance to the galaxy if the data has good quality,” quoted Trujillo. Based on all five methods, Trujillo and his team challenged the speculation of van Dokkum, citing that DF2 is about 42 million light-years away only.

But the academic debate didn’t end here. Van Dokkum published yet another research paper, wherein DF2 is said to be around 61 million lightyears away, concluding that DF2 probably has a scarcity of dark matter.

The Discovery of DF4 — Room For More Disagreement?

Amid this DF2 Distance debate, van Dokkum found yet another galaxy with an apparent lack of dark matter. Known as DF4, the team approximated its distance to be about 65 million lightyears.

An image with NGC 1042(the main spiral), NGC 1052 (left), and NGC 1048 (two interacting galaxies). Image courtesy: Schulman Telescope

Using the TRGB method, Trujillo and his team published another analysis of DF4’s distance where they claimed it to be about 44 million lightyears. The proposition that both [DF2 and DF4] are “missing dark matter” is still far from being placed on sure footing”, they concluded.

Van Dokkum’s research team then secured a chunk of observing time with the Hubble Space Telescope for more accurate pinning. In summer 2019, van Dokkum using the new data released another research paper with the findings of this telescope. According to his team, Trujjilo’s research group misidentified the brightest red giant stars leading to a closer derived distance. The paper is still under review.

The Verdict

While the questions of DF2 and DF4 having no dark matter remain unanswered, in November 2019 another research paper surfaced in Nature Astronomy citing the discovery of 19 dwarf galaxies with an apparent dearth of dark matter.

So do galaxies without dark matter really exist? Or is it a mere coincidence along with some errors in the modern scientific processes in action? Unfortunately, there is no definite answer for now. It could be a yes, or a no, or perhaps both maybe.

The astronomical world now awaits new and extraordinary data, as extraordinary claims require extraordinary evidence. And when they do, we may have the answers to our questions. One thing that’s for sure, is that this cosmic-stir ending in an affirmation will truly change our understandings of how galaxies form and evolve.

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