Dark Matter in NGC 1052-DF2: A Matter of Debate

Does the galaxy NGC 1052-DF2 really lack dark matter, or is this ultra-faint dwarf just misunderstood? A recently published paper calls into question this recent, surprising discovery 65 million light-years from our home.

Scientific Process at Work

Scientists don’t always agree — and that’s a good thing! Believe it or not, what makes the process of science work is precisely this: researchers challenging each other and questioning their data, analysis, or results. As scientists work to defend and adapt their research, they improve the robustness of their models and outcomes, and the research field gradually progresses as a whole.

This past month, astronomy has had a healthy dose of scientific disagreement after an article was published in Nature suggesting the discovery of a galaxy lacking dark matter. In a recently published response, a team of scientists led by Nicolas Martin (University of Strasbourg, France; Max Planck Institute for Astronomy, Germany) suggests that uncertainties and small-number statistics have led to NGC 1052-DF2 being misunderstood.

A Quick Recap

globular clusters of NGC 1052-DF2

The 11 globular clusters associated with NGC 1052-DF2 (imaged by HST), and their spectra (taken with Keck/LRIS). [van Dokkum et al. 2018]

Last month, Pieter van Dokkum (Yale University) and collaborators announced new observations of the dwarf galaxy NGC 1052-DF2, a very low-surface-brightness satellite orbiting the elliptical galaxy NGC 1052. Van Dokkum et al. identified 10 objects that are likely globular clusters in NGC 1052-DF2 and measured their line-of-sight velocities.

Here’s where it gets tricky: the authors used the measured velocities of these 10 tracers to estimate a velocity dispersion for the sample. They then used this as a measure of the dynamical mass of the galaxy. Van Dokkum and collaborators reached the conclusion that the galaxy’s dynamical mass is very nearly the same as its stellar mass, suggesting that NGC 1052-DF2 has a very low mass-to-light ratio and is severely lacking in dark matter.

Scrutinizing with Statistics

So what’s the catch? Statistics — it’s a dangerous game to make strong statements from the dynamics of only 10 globular clusters. With small numbers such as these, the statistical result depends critically on the model assumptions and how the data is analyzed.

mass-to-light ratio limits

Inferred mass-to-light ratio for multiple models. Martin et al.’s inferred upper limits (black and red arrows) are much less restrictive than van Dokkum et al.’s (grey arrow). [Martin et al. 2018]

In their recent ApJL publication, Martin and collaborators demonstrate that the use of several different statistical models and analysis procedures produce widely different results for NGC 1052-DF2’s mass-to-light ratio relative to van Dokkum et al.’s analysis — underscoring the difficulty in extracting information from small data sets with large uncertainties.

Martin and collaborators argue that the constraints on the dwarf galaxy’s dark-matter content aren’t as strong as van Dokkum et al. suggest. Martin et al. find only weak constraints on the system’s mass-to-light radius — the authors agree that NCG 1052-DF2 is not massively dominated by dark matter, but Martin and collaborators find that its properties could still be consistent with other dwarf galaxies found in the Local Group.

Thus, instead of a definitive answer about NGC 1052-DF2’s dark-matter content, we are left with a tale of caution and a hope of future work on the subject. Frustrating? Perhaps — but that’s science! The dialog begun between these authors is likely only the beginning of an ongoing conversation about NGC 1052-DF2, dark matter, statistics, and analysis of observations — and it’s exciting to watch in real time as the field progresses.

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Note: See Mia de los Reyes’s post over at Astrobites for some excellent additional coverage of Martin et al.’s article and more on the topic of NGC 1052-DF2.

Citation

Nicolas F. Martin et al 2018 ApJL 859 L5. doi:10.3847/2041-8213/aac216

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