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COLD GASS is an Institut de radioastronomie millimétrique (IRAM) survey of carbon monoxide (CO) in a selection of over 350 massive galaxies identified both in Sloan Digital Sky Survey (SDSS) (in the optical) and the Galaxy Evolution Explorer (GALEX) (in ultraviolet). The goal is estimates of their molecular hydrogen (through the use of alpha CO with detected CO spectral lines) and its ratio with atomic hydrogen (based on the 21-cm line). The galaxies were drawn from the GALEX Arecibo SDSS Survey (GASS).
xCOLD GASS is a follow-on effort extending the survey to include galaxies of a smaller mass range, bringing the total above 500.