ultraviolet dropout

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The disappearance of an extremely distant galaxy from a telescope image when viewed through an ultraviolet filter, caused by the absorption of the galaxy's emitted ultraviolet radiation by intergalactic hydrogen gas.
• noun A technique that identifies galaxies as extremely distant by comparing images taken through visible light (in which distant objects are often detectable) and ultraviolet filters (in which they are not).