Key Takeaways:

  1. Fraternali’s team unveils galaxies defying traditional dark matter models, challenging long-held astronomical beliefs.
  2. Unlike their counterparts in crowded environments, these galaxies appear isolated, posing intriguing questions about their formation sans dark matter.
  3. Ultra-diffuse galaxies’ near-transparency makes them elusive, requiring advanced telescopes for detection and further study.
  4. Fraternali’s findings suggest potential discrepancies in gravitational physics, prompting a reevaluation of established theories.
  5. Continued scrutiny of these enigmatic galaxies, including with cutting-edge technologies like the James Webb Space Telescope, promises further insights into the cosmos’ mysteries.


Astronomers mapped out the stars (shown here in blue) and gas (green) of the strange galaxy known as AGC 114905.
Javier Román and Pavel Mancera Piña

Filippo Fraternali and his team identified six mysteriously diffuse galaxies resembling sprawling cities of stars and gas. Unlike most galaxies, including the Milky Way, these did not appear to be surrounded by significant amounts of dark matter, which typically contributes to their gravitational cohesion. Focusing on one such galaxy situated roughly 250,000 light-years away, the scientists directed the 27 radio telescope antennas of the Very Large Array in New Mexico for a detailed examination.

After accumulating data over 40 hours, they charted the distribution of stars and gas, confirming their initial suspicions: “The dark matter content that we infer in this galaxy is much, much smaller than what you would expect,” remarks Fraternali, an astronomer at Kapteyn Astronomical Institute of the University of Groningen in the Netherlands.

The potential discovery of more such galaxies by Fraternali’s team or others could challenge the prevailing understanding of dark matter, which has dominated the field for over two decades. Their findings were published in the Monthly Notices of the Royal Astronomical Society.

Building upon decades of observations and simulations, scientists have regarded dark matter as the underlying framework of the universe, with invisible particle clumps acting as its “joints” and hosting galaxies of varying sizes. However, Fraternali’s observation is not the first deviation from this pattern.

Several years ago, Pieter van Dokkum and his team at Yale identified similar galaxies lacking in dark matter using the Hubble telescope. “These galaxies that we found in 2018, they created a lot of controversy and discussion and follow-up work because they were unexpected and difficult to explain,” van Dokkum explains.

While those galaxies were situated in crowded environments, where neighboring galaxies might strip away dark matter, Fraternali’s galaxy appears relatively isolated. This absence of dark matter poses a significant puzzle: “How do you get stars and gas in that location together without the help of dark matter?” van Dokkum wonders.

Dubbed “ultra-diffuse galaxies,” these anomalies defy convention. Despite their vast dimensions, their mass is minuscule, with some as expansive as the Milky Way but containing a fraction of its stars.

Their transparency makes them challenging to detect in the night sky. “They’re slightly fainter in the center, so they’re difficult to detect. Now, with better telescopes and deeper observations, they have become more well known,” explains Mireia Montes, an astronomer at the Space Telescope Science Institute in Baltimore.

Beginning in the 1960s, Vera Rubin and others inferred the existence of dark matter while studying the rotational velocities of stars within galaxies. This led to the realization that the mass required to maintain their orbital motions exceeded that of visible matter. Dark matter, which neither emits nor reflects light, was proposed to account for this discrepancy.

However, Fraternali’s findings suggest that for ultra-diffuse galaxies, dark matter may not be necessary. The observed rotation speeds align with the mass of stars and gas clouds, eliminating the need for additional unseen mass. Montes and her team plan to further investigate these galaxies, particularly their outskirts, to determine if there’s any undetected matter. Nonetheless, the enigma of this ghostly galaxy persists.

Fraternali emphasizes a crucial factor regarding their galaxy, known as AGC 114905: its inclination. Unlike galaxies shaped like flying saucers, whose edge-on view simplifies observation, AGC 114905 is tilted at over 30 degrees, necessitating adjustments in mass measurements. Incorrect estimations could leave room for dark matter.

However, assuming the correctness of the team’s calculations, the exact nature of this exceptional galaxy remains uncertain. Is it an incomprehensible cosmic anomaly or a precursor to fundamental challenges to dark matter theories?

Thus far, AGC 114905 defies existing explanations for the origin of ultra-diffuse galaxies. Speculations range from gravitational interactions between past galaxies to remnants of stellar explosions. Yet, none of these theories convincingly accommodate the peculiarities of AGC 114905.

In fact, AGC 114905 appears incompatible with models incorporating dark matter. Despite decades of simulations, no existing model replicates this galaxy’s characteristics.

Fraternali’s observations and others like it suggest a need for alternative explanations to dark matter. These deviations from predicted behaviors may indicate a fundamental misunderstanding of gravitational physics.

Among the proposed alternatives, Modified Newtonian Dynamics (MOND) offers a promising avenue. By adjusting Newton’s law of gravity, MOND attempts to resolve discrepancies in galactic rotational speeds without invoking dark matter. However, it fails to explain galaxies like AGC 114905.

The implications of AGC 114905 on dark matter theories remain uncertain. Fraternali and his peers will continue scrutinizing these enigmatic galaxies, aided by advanced tools like the James Webb Space Telescope, in pursuit of answers. “It’s not like we’re probing the edge of the universe, or trying to see a tiny planet next to a star. It’s actually doable with the tools that we have,” van Dokkum remarks, underscoring the excitement surrounding this ongoing exploration.

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