ULIRG

Luminous infrared galaxies or LIRGs are galaxies with luminosities, the measurement of brightness, above 10¹¹ L_☉. LIRGs are more abundant than starburst galaxies, Seyfert galaxies and quasi-stellar objects at comparable luminosity. Infrared galaxies emit more energy in the infrared than at all other wavelengths combined. A LIRG's luminosity is 100 billion times that of our sun.

Galaxies with luminosities above 10¹² L_☉ are ultraluminous infrared galaxies (ULIRGs). Many of the LIRGs and ULIRGs are showing interactions and disruptions. Many of these types of galaxies spawn about 100 new stars a year as compared to ours which spawn one a year; this creates the high level of luminosity.

More luminous than ULIRGs are the hyper-luminous infrared galaxies, HLIRG.

The most luminous class are the extremely luminous infrared galaxies, ELIRG.

Infrared galaxies appear to be single, gas-rich spirals whose infrared luminosity is created largely by the formation of stars within them. However some galaxies' luminosity comes from an active galactic nucleus or AGN. These AGNs reside in compact regions at the centres of galaxies and have higher than normal luminosity. The emission of the AGN has been observed in the radio, infrared, visible, ultra-violet, X-ray and gamma ray wavebands. These types of galaxies were discovered in 1983 with IRAS. In some cases a LIRG's luminosity may come from starburst, the creation of stars, and also an AGN.

These galaxies emit more energy in the infrared portion of the spectrum, not visible to the naked eye. The energy given off by LIRGs is comparable to that of a quasar, which formerly was known as the most energetic object in the universe.

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