Yale Astronomers Discover a Third Galaxy Missing Its Dark Matter, Strengthening the Case for Violent Cosmic Collisions
NGC 1052-DF9 is the third ultra-diffuse galaxy found lacking dark matter in a linear trail near NGC 1052, lending significant support to the Bullet Dwarf collision theory and challenging alternative gravity models.
Overview
A team of Yale astronomers has identified a third galaxy in the NGC 1052 field that appears to contain little or no dark matter, adding to a growing body of evidence that supports a dramatic theory about how some galaxies form. The discovery of NGC 1052-DF9, detailed in a preprint published on arXiv by Michael A. Keim, Pieter van Dokkum, Zili Shen, Shany Danieli, and Imad Pasha, strengthens the so-called Bullet Dwarf collision scenario and poses a direct challenge to Modified Newtonian Dynamics, the leading alternative to dark matter theory.
The finding is the latest chapter in a line of research that began in 2018 when van Dokkum’s team first reported that NGC 1052-DF2 appeared to lack dark matter entirely — a claim that upended assumptions about galaxy formation, since dark matter is generally understood to provide the gravitational scaffolding around which visible matter coalesces.
The Bullet Dwarf Collision Theory
Most dwarf galaxies are strongly dominated by dark matter. DF2 and DF4, two earlier discoveries in the same region of space, were therefore deeply puzzling. Both appeared to have stellar motions consistent with the gravitational pull of their visible matter alone, with no need to invoke an invisible dark matter halo.
The Bullet Dwarf theory, first proposed in 2019, offers an explanation. It suggests that when two gas-rich dwarf galaxies collide at extreme velocities, their dark matter halos — which interact only through gravity — pass straight through each other. The normal matter, however, physically collides, separating gas from dark matter in a process analogous to the well-documented Bullet Cluster observed at a much larger scale. The violent impact triggers an intense burst of star formation, producing a trail of galaxies stripped of their dark matter.
Critically, DF2, DF4, and several other low-luminosity galaxies in the NGC 1052 field were found to follow a linear spatial alignment and to exhibit a correlation between their positions along the trail and their radial velocities — exactly what the collision model predicts.
How DF9 Was Measured
To determine whether DF9 also lacks dark matter, the researchers turned to the W.M. Keck Observatory in Hawaii, using the Keck Cosmic Web Imager (KCWI) to perform absorption line spectroscopy. This technique measures the velocity dispersion of stars within the galaxy — essentially how fast the stars are moving relative to one another — which in turn constrains the total mass holding the galaxy together.
The team measured DF9’s stellar velocity dispersion at approximately 6.4 kilometers per second, a value consistent with the gravitational influence of its visible stellar mass alone, estimated at roughly 140 million solar masses. In a galaxy with a typical dark matter halo, the velocity dispersion would be significantly higher. The result indicates that DF9, like DF2 and DF4, does not require dark matter to explain its internal dynamics.
Implications for Dark Matter and MOND
The discovery carries weight beyond confirming a single theory of galaxy formation. Dark-matter-free galaxies are paradoxically among the strongest pieces of evidence that dark matter exists as a distinct physical substance. If dark matter were not a separate component but instead an artifact of modified gravity, as proponents of Modified Newtonian Dynamics (MOND) argue, then every galaxy should exhibit its effects uniformly. As the researchers note, under MOND a galaxy cannot simply opt out of the laws of gravity.
The fact that DF2, DF4, and now DF9 show normal stellar velocities — velocities that match predictions based solely on visible matter — suggests that dark matter is something that can be physically separated from baryonic matter through energetic processes. This is consistent with the standard Lambda-CDM cosmological model, in which dark matter is a particle-based substance that interacts gravitationally but not electromagnetically.
What Remains Unknown
While the Bullet Dwarf theory now has its strongest supporting evidence yet, several open questions remain. The exact progenitor galaxies that collided to produce the trail have not been conclusively identified. The frequency of such collisions in the broader universe is also uncertain, though some estimates suggest dark-matter-deficient galaxies may occur approximately eight times in every 65 million square light-years.
Further spectroscopic observations of the remaining galaxies along the NGC 1052 trail could determine whether the entire string shares the same dark-matter-free property, which would provide even more definitive confirmation of the collision scenario. For now, the identification of DF9 marks a significant step in understanding how extreme astrophysical events can reshape the composition of galaxies.