“The Cosmic Misfits” –25% of All Galaxies Have an Indefinable, Irregular Shape (NASA/ESA Hubble)





Most galaxies possess a majestic spiral or elliptical structure. About a quarter of galaxies, though, defy such conventional, rounded aesthetics, instead sporting a messy, indefinable shape. Known as irregular galaxies, this group includes NGC 5408, the galaxy that has been snapped here by the NASA/ESA Hubble Space Telescope.

John Herschel recorded the existence of NGC 5408 in June 1834. Astronomers had long mistaken NGC 5408 for a planetary nebula, an expelled cloud of material from an aging star. Instead, bucking labels, NGC 5408 turned out to be an entire galaxy, located about 16 million light-years from Earth in the constellation of Centaurus (The Centaur).

In yet another sign of NGC 5408 breaking convention, the galaxy is associated with an object known as an ultraluminous X-ray source, dubbed NGC 5408 X-1, one of the best studied of its class. These rare objects beam out prodigious amounts of energetic X-rays. Astrophysicists believe these sources to be strong candidates for intermediate-mass black holes. This hypothetical type of black hole has significantly less mass than the supermassive black holes found in galactic centers, which can have billions of times the mass of the sun, but have a good deal more mass than the black holes formed when giant stars collapse.

The NASA/ESA Hubble Space Telescope image belwo shows a galaxy known as UGC 11411. It is a galaxy type known as an irregular blue compact dwarf (BCD) galaxy.




BCD galaxies are about a tenth of the size of a typical spiral galaxy such as the Milky Way and are made up of large clusters of hot, massive stars that ionize the surrounding gas with their intense radiation. Because these stars are so hot they glow brightly with a blue hue, giving galaxies like UGC 11411 their characteristic blue tint. With these massive stars being less than 10 million years old, they are very young compared to stellar standards. They were created during a starburst, a galaxy-wide episode of furious star formation. UGC 11411 in particular has an extremely high star formation rate, even for a BCD galaxy.

Unusually for galaxies with such intense star-forming regions, BCDs don’t contain either a lot of dust, or the heavy elements that are typically found as trace elements in recently formed stars, making their composition very similar to that of the material from which the first stars formed in the early universe. Because of this astronomers consider BCD galaxies to be good objects to study to improve our understanding of primordial star-forming processes. The bright stars in the image are foreground stars in our own Milky Way galaxy.

The Daily Galaxy via European Space Agency
Image credit: ESA/Hubble & NASA, Judy Schmidt


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