One of the “darkest” galaxies ever discovered has been identified

The hundreds of billions of stars contained in most galaxies in the Universe make them visible from enormous distances. There is, however, a rare class of galaxies that remains virtually invisible. These so-called low-surface-brightness galaxies are dominated by dark matter and contain only a few faint stars. 

Such an object, codenamed CDG-2, may be one of the galaxies with the highest concentration of dark matter ever discovered. Dark matter is a strange form of matter, which does not interact with electromagnetic radiation and, for that very reason, cannot be directly detected by our telescopes.    

Detecting galaxies with low surface brightness is very difficult. Nevertheless, David Li of the University of Toronto in Canada and his team confirmed the existence of 10 such galaxies and identified two additional candidate dark galaxies, using an advanced statistical method based on the search for relatively compact clusters of globular star clusters. 

Globular clusters are usually found in the galactic halo surrounding each galaxy and typically consist of anywhere from a few tens of thousands to a few million very old stars, clustered within a spherical region that usually does not exceed 150 light-years in diameter. It is precisely this high stellar density of globular clusters that ensures their gravitational cohesion for many billions of years, making them reliable detectors of such faint galaxies. 

To confirm the detection of one of the two candidate dark galaxies, astronomers used the Hubble and Euclid space telescopes, as well as the Subaru ground-based telescope in Hawaii. High-resolution images from Hubble initially revealed a group of four globular clusters in the Perseus galaxy cluster, one of the largest and closest galactic clusters to Earth, about 300 million light-years away. Subsequently, analysis of the data collected by the three telescopes revealed that the four globular clusters are surrounded by a faint, diffuse glow,  which is strong evidence for the existence of an underlying galaxy. 

According to LI, “this is the first galaxy to be detected exclusively through its population of globular clusters… and by conservative estimates, these four clusters represent the entire population of globular clusters in the galaxy CDG-2”.

Preliminary analysis suggests that CDG-2 has a luminosity of approximately 6 million suns, and that its four globular clusters account for 16% of its visible matter. In fact, according to the relevant data, 99% of its total mass appears to consist of dark matter, a fact which, if confirmed, would make CDG-2 the galaxy with the highest concentration of dark matter detected to date. It is speculated that a significant portion of its initial hydrogen gas content—from which many more stars could have formed— was likely dispersed due to the gravitational interactions of CDG-2 with other galaxies in the Perseus cluster. 

The study was published in the scientific journal The Astrophysical Journal Letters.

Source: NASA’s Hubble Identifies One of the Darkest Known Galaxies - NASA Science

Photo: The low-surface-brightness galaxy CDG-2 was discovered thanks to the identification of four globular clusters [Credit: NASA, ESA, Dayi Li (University of Toronto); Image Processing: Joseph DePasquale (STScI)].