Hydroxyl megamasers are found in the nuclear region of a class of galaxies called luminous infrared galaxies (LIRGs), with far infrared luminosities in excess of one hundred billion solar luminosities, or LFIR > , and ultra-luminous infrared galaxies (ULIRGs), with LFIR > are favored. These infrared luminosities are very large, but in many cases LIRGs are not particularly luminous in visible light. For instance, the ratio of infrared luminosity to luminosity in blue light is roughly 80 for Arp 220, the first source in which a megamaser was observed.