Ultra-diffuse galaxies (UDGs) have very low luminosities, comparatively few stars, and little star-formation activity as compared with normal galaxies of similar sizes. Commonly found in galaxy clusters, UDGs come in a wide range of sizes and shapes, many of them being round and smooth like dwarf elliptical galaxies, others showing distorted shapes from having experienced tidal disruptions; some having total masses of as much as one hundred billion solar-masses. In addition to being interesting in their own right, these galaxies are important to astronomers because their diffuse structures are valuable in models trying to recover information about the dark matter halos that help keep them self-contained; indeed most of their mass is thought to be in the form of dark matter.
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Source: Phys.org