Third Dark-Matter-Free Galaxy Found in Same Cluster, Validating "Bullet Dwarf" Collision Theory

Astronomers at Yale University have identified a third galaxy in the same region of sky that appears to contain no dark matter at all, providing the strongest statistical evidence yet for the radical "Bullet Dwarf" theory — which proposes that a catastrophic collision between two ancient dwarf galaxies could shear normal matter away from its invisible gravitational scaffold. The galaxy, known as NGC 1052-DF9, lies along a linear trail connecting the two previously known dark-matter-free objects in the NGC 1052 galaxy group, making a coincidental alignment increasingly implausible and lending credence to the idea that all three were born from a single violent event.

Michael Keim and Pieter van Dokkum of Yale led the study, which used Keck Observatory's KCWI spectrograph to measure the velocities of stars inside NGC 1052-DF9. In ordinary galaxies, stars orbiting farther from the center move faster than classical physics would predict — the difference is accounted for by the gravitational pull of dark matter halos that make up the bulk of a galaxy's mass. In DF9, the stellar velocity dispersion matched precisely what would be expected from the visible stellar mass alone, with no dark matter required to explain the kinematics. "Dark matter is the invisible scaffolding that holds galaxies together," the researchers wrote; without it, rotational forces should cause most galaxies to "simply fly apart."

The three galaxies — NGC 1052-DF2, DF4, and now DF9 — are arrayed along what appears to be a collision debris trail. The Bullet Dwarf scenario, first proposed after the discovery of DF2 in 2018, posits that two gas-rich dwarf galaxies collided at extreme velocities billions of years ago. In such a collision, dark matter halos pass through each other largely unimpeded by gravity while the denser gas clouds of normal matter slam together and are slowed. Star formation is then triggered along the compressed gas trail — explaining why DF2, DF4, and DF9 all share similar ages and compositions — while the dark matter halos continue along separate trajectories, leaving the new galaxies stripped bare.

The discovery has important implications for Modified Newtonian Dynamics, or MOND, an alternative theory of gravity that some physicists have proposed as a replacement for dark matter. MOND predicts that at very low accelerations, gravity should be stronger than Newton's law predicts — yet the stars in NGC 1052-DF2 move at speeds entirely consistent with ordinary Newtonian physics, not the MOND enhancement. A third galaxy now showing the same behavior makes the MOND explanation for these objects increasingly difficult to sustain. Keim and van Dokkum plan to measure the kinematics of additional galaxies along the collision trail; at least two more candidates have been identified, though they are faint enough that observations will require extremely long telescope exposures.

The research was posted to the arXiv preprint server and has been submitted for peer review. Astronomers not involved in the study described the work as a significant advance. The NGC 1052 galaxy group, located roughly 65 million light-years away in the constellation Cetus, has become one of the most closely watched fields in extragalactic astronomy since the first dark-matter-deficient galaxy was reported — and it may yet yield further surprises as the new generation of large ground-based and space telescopes comes online.