Dark Matter Mysteries Deepen: Three New Studies Paint a Contradictory Picture
A series of recent papers present divergent findings regarding dark matter, ranging from a potential revision of its properties to the discovery of galaxies that either lack it or are overwhelmingly dominated by it.
The studies, published in The Astrophysical Journal Letters and The Astrophysical Journal, examine different scales and types of astronomical objects. While not directly contradictory, together they reveal a far more complex picture of dark matter than previously understood.
Study 1: Galaxy Cluster Data May Prompt Revision of Cold Dark Matter Model
A study led by astrophysicist Priyamvada Natarajan of Yale University, with co-authors Barry T. Chiang and Isaque Dutra, analyzed three distant galaxy clusters—MACS J0416, MACS J1206, and MACS J1149—using gravitational lensing data from the Hubble Space Telescope and the James Webb Space Telescope.
The standard cold dark matter (CDM) model accurately describes the universe's large-scale structure, such as the cosmic web and galaxy clustering. However, the distribution of dark matter sub-halos (smaller clumps of dark matter that host galaxies) within these clusters does not match model predictions. Observed sub-halos are more concentrated toward cluster centers than simulations suggest.
Galaxy-galaxy strong lensing events occur almost ten times more frequently than the standard CDM model predicts.
The excess lensing suggests sub-halo centers are denser than expected. This could be explained if dark matter particles self-interact and undergo extreme collapse, potentially indicating a second type of self-interacting dark matter particle or an entirely new particle.
The study does not invalidate the overall CDM model but identifies a scale-dependent anomaly. Natarajan stated that the results provide observational hints about how dark matter behaves, not just where it is located.
Study 2: Third "Dark Matter-Deficient" Galaxy Found in Same Chain
A study led by Michael Keim and Pieter van Dokkum of Yale University reports the discovery of NGC 1052-DF9, the third galaxy in a linear chain whose stellar motions can be explained without requiring dark matter.
NGC 1052-DF9 follows DF2 (identified in 2018) and DF4 (identified in 2019). All three galaxies belong to a chain of about a dozen galaxies located 67 million light-years away. The galaxies in the chain move through space in a similar manner, suggesting a common origin.
The researchers propose a "bullet dwarf collision" scenario, where two dwarf galaxies collide head-on.
In such a collision, stars and dark matter pass through each other, while gas clouds collide, stall, and remain behind. This process could form star systems with little to no dark matter.
Michael Keim stated the findings provide some of the clearest evidence that these galaxies formed together in a violent event that separated ordinary matter from dark matter. Pieter van Dokkum noted that the finding provides evidence that dark matter behaves as a physical substance rather than an effect of an alternative theory of gravity.
Study 3: Discovery of CDG-2, a Galaxy Heavily Dominated by Dark Matter
Astronomers have identified CDG-2 (Candidate Dark Galaxy-2), a low-surface-brightness galaxy located approximately 300 million light-years away in the Perseus galaxy cluster. The discovery was led by David Li of the University of Toronto.
Researchers used a novel method of searching for tight groupings of globular clusters. Hubble Space Telescope data revealed a close grouping of four globular clusters. Subsequent data from the Euclid space observatory and the Subaru Telescope detected a faint, diffuse glow surrounding these clusters, indicating an underlying galaxy.
This is the first galaxy detected solely through its globular cluster population.
CDG-2 has a luminosity equivalent to approximately 6 million Sun-like stars. The four globular clusters contribute about 16% of its visible light. Dark matter is estimated to constitute between 99% and 99.9% of CDG-2's total mass.
Researchers hypothesize that much of the galaxy's normal matter (hydrogen gas) was stripped away by gravitational interactions with other galaxies in the Perseus cluster. The dense, gravitationally bound globular clusters resisted this disruption and remain observable.
Neal Dalal, a researcher not involved in the study, noted that faint galaxies like CDG-2 provide a less complicated environment for studying dark matter, as the minimal presence of stars and gas offers a "much cleaner probe of dark matter physics." Yao-Yuan Mao described the find as significant, noting that the faint light in Hubble images supports the idea of CDG-2 as a single, cohesive object.