The Slug Nebula is one of the largest and most luminous Lyman-alpha nebula discovered so far extending over 450 kpc. The nebula sits at a distance of z=2.283 surrounds a bright quasar named UM287. It has a high surface brightness and is filimentary. This high surface brightness implies two things: (1) very large densities of cold and ionized gas, (2) very large colum densities of neutral hydrogen.
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