Ghosts of the Cosmos

Ultra-Diffuse Galaxies (UDGs) are enormous, faint galaxies that challenge models of cosmic evolution. They can be as large as our own Milky Way—about 60,000 light-years across—yet contain 100 to 1,000 times fewer stars. This scarcity of stars makes them appear ghostly and transparent, with a surface brightness so low it is fainter than the background light of the night sky. Many are found in galaxy clusters, such as the Coma Cluster located about 330 million light-years from Earth. Their existence in these high-traffic, gravitationally violent environments is a puzzle. The intense tidal forces within a cluster should shred such flimsy collections of stars.

The modern study of these objects accelerated in 2015 with discoveries made by the Dragonfly Telephoto Array, an innovative telescope composed of multiple commercial camera lenses. This instrument is specifically designed to detect faint structures by minimizing internal scattered light. One of its most famous discoveries is Dragonfly 44, a UDG in the Coma Cluster. It has a mass comparable to the Milky Way, estimated at one trillion solar masses, yet emits only 1% of the light. This suggests that Dragonfly 44 might be composed of up to 99.99% dark matter, the substance that holds galaxies together. The motions of its stars move too quickly to be held by the gravity of the visible matter alone.

The Dark Matter Dilemma

The leading theory for UDG survival is that they are protected by immense halos of dark matter. For a galaxy like Dragonfly 44, this massive, invisible scaffolding would provide the gravitational glue needed to withstand the disruptive forces within its cluster. The large number of globular clusters—dense, spherical collections of stars—orbiting some UDGs also suggests a large total mass. Dragonfly 44, for instance, has around 90 such clusters.

This model was complicated by the discovery of other UDGs, such as NGC 1052-DF2 and NGC 1052-DF4, which appear to have almost no dark matter at all. These galaxies contradicted to the leading theory and challenged fundamental ideas of galaxy formation. The existence of a galaxy without dark matter implies that dark matter is a separable component of galaxies, not an intrinsic property. The debate over these objects is ongoing, with some studies suggesting the original distance measurements to these galaxies were incorrect, which would alter the calculations of their mass. Other theories propose that UDGs are "failed" galaxies that lost their star-forming gas early in their history, or that they are former satellite galaxies that had their dark matter stripped away by a much larger neighbor.

Galaxies With 1,000 Times Fewer Stars

Ghosts of the Cosmos

The Dark Matter Dilemma

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References

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Practical Information

Galaxies With 1,000 Times Fewer Stars

Ghosts of the Cosmos

The Dark Matter Dilemma

💡Fun Facts

References

There's more in the app

Practical Information

Nearby curiosities

Nearby curiosities