Hickson 40 is a galaxy cluster in the constellation Hydra. It consists of 6 separate members and is in the stage of merging. Member galaxies of Hickson 40 interacted in the early stages of their formation to create a crowded galaxy sampler. Hickson 40 is so dense^[1] that it could exist within a region close to the diameter of the Milky Way's galactic disk.^[2]

Most galaxy clusters similar to Hickson 40 are normally part of larger galaxy clusters, but Hickson 40 is notably self-sustained. It is located in the constellation Hydra.

Researchers have observed Hickson 40 in many wavelengths, including visible light, radio, infrared, and X-ray.^[3] Majority of the galaxies within the cluster have a dense radio source in the cores, which could be proof for the existence of supermassive black holes. X-ray surveys have revealed that the galaxies are pushing closer together because of the amount of hot gas present. Infrared observations reveal fast rates of star formation.^[4] Hickson 40 is one of the most compact galaxy clusters ever observed, making it a valuable research point NASA. Research shows that galaxy cluster were more prevalant in the early universe, and helped the formation of quasars, whose luminosity from heated infalling material spread across the universe. Studying the formation of galaxies in other close groups like Hickson 40 have given astronomers a chance to sort out when and where galaxies assemble themselves, and what material they are composed of.^[5]

• Hickson Compact Group
• Supercluster
• Stephan's Quintet, a more famous HCG.