Evidence for a dwarf galaxy without dark matter challenges traditional galaxy formation models

Astronomers using the WM Keck Observatory in Mauna Kea, Hawaii’s Island have discovered compelling evidence for the dark matter-deficient d-star galaxy FCC 224.

This discovery challenges the traditional cosmological paradigm, which assumes dark matter is a fundamental component of the galactic layer. Previously, dark matter-deficient galaxies without ongoing star formation were claimed only in another location, the NGC 1052 group, but new observations suggest that such objects may be more widespread than previously thought.

The rare galaxy was featured in two complementary studies. The first study, led by doctoral candidate Yimeng Tang at the University of California, Santa Cruz, will be featured in the Astrophysical Journal to analyze FCC 224’s unique Global Star Cluster System.

The second, led by doctoral candidate Maria Lusa Bazzo at the University of Swinburne and the European Southern Observatory, will be published in Astronomy and Astrophysics, explore the contents of galaxy dark matter and possible scenarios for its formation.

“Spherical clusters are often used as proxy to estimate the amount of dark matter contained in galaxies,” Buzzo said. “For some reason, this galaxy (FCC 224) has an extraordinarily bright cluster, at least not a dark problem within its internal regions. The existing galaxy formation model within the standard cosmic paradigm can now explain how this galaxy has turned out.”

Buzzo’s study used high-resolution spectroscopy from Keck Cosmic Web Imager (KCWI) at Keck Observatory to investigate the dark matter content of FCC 224. She investigated the similarities claiming to previously claimed dark matter-free dwarf galaxies DF2 and DF4 within the NGC 1052 Galaxy Group to determine whether FCC 224 belongs to the same class of galaxies.

Buzzo’s work is based on Tang’s first finding, using imaging from NASA’s Hubble Space Telescope (HST), to discover that FCC 224 has similar star clusters to other dark matter-deficient galaxies. Their research suggests that these types of star clusters may be associated with a lack of dark matter, challenging traditional models of the galactic layer. Keck Observatory/KCWI data was obtained through the proposal of Swinburne University under the lead investigator (PI) Duncan Forbes, whilst HST data was obtained through a programme led by Pi Aaron Romanowsky.

“KCWI’s new, high-resolution red arm was able to measure movement within the galaxy very accurately, which can be used to track its dark material content,” explained Buzzo. “The high spectral resolution of KCWI allowed us to accurately measure the movement (velocity dispersion) within the FCC 224 stars and spherical clusters,” Tang said. “These measurements confirmed that galaxies exhibit very low velocity dispersion, a characteristic of the dark matter deficiency.”

Both teams emphasize the need for further observation to understand the mechanisms behind the FCC 224 formation. One of the main hypotheses is that external interactions, such as high-speed collisions, may have isolated dark, visible problems in the galaxy. This is a formation scenario known as the “bullet dwarf model.” However, additional data is required to test this theory.

“FCC 224 serves as an important data point for identifying and studying other dark matter-deficient galaxies,” added Buzzo. “Increasing the sample size will allow us to improve our understanding of the role of dark matter in these rare galaxies and in the dwarf galaxies layer.”

The discovery of FCC 224 outside the NGC 1052 group suggests that dark matter-deficient galaxies may be more common than previously thought, raising fundamental questions about the need for dark matter in galaxy formation. As observational capabilities improve, astronomers aim to conduct larger investigations to identify similar galaxies and investigate their origins.