A luminous infrared galaxy (LIRG) is a galaxy emitting more than 1011 times solar luminosity in the far infrared.
They are often a starburst galaxy and/or have an active galactic nucleus or a quasar. The relatively high infrared stems from dust heated by the electromagnetic radiation (EMR) of the AGN or star formation (SF), re-emitting the EMR in a cooler (infrared-intensive) black body spectrum, that can exceed the directly-viewed emission from the star formation or AGN. Generally, the higher the IR, the higher the star formation rate (SFR). LIRGs are among the brightest objects in the universe. Variations:
(Sources also cite these thresholds as applying to the galaxy's full bolometric luminosity, sometimes stipulating that most of its emission is within the infrared or far infrared; I presume this is the typical case for such bright galaxies.)
A LIRG is essentially the same as a submillimeter galaxy (SMG).