Forty years ago, astronomers using sensitive new imaging techniques discovered a class of large, faint galaxies they named low-surface-brightness galaxies. Giant low-surface-brightness galaxies (gLSBGs) are a subset whose masses are comparable to the Milky Way’s but whose radii are ten times bigger, as much as four hundred thousand light-years. These gLSBGs raise a problem for astronomers: despite being massive, the galaxy disks are (kinematically speaking) relatively inactive. The usual formation paradigm for high mass galaxies imagines them evolving out of galaxy mergers, a process that stirs up the disk and should make it kinematically active. Moreover, most gLSBGs are found with no other galaxies in their vicinities suggesting that collisions were probably not important in their formation.
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Source: Phys.org