luminous infrared galaxy

A luminous infrared galaxy (LIRG) is a galaxy emitting more than 10¹¹ times solar luminosity (L_Sun) in the far infrared. They are often a starburst galaxy and/or have an active galactic nucleus (AGN) or a quasar. The relatively high infrared stems from dust heated by the electromagnetic radiation (EMR) of the AGN and/or of 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 more IR, the higher the star formation rate (SFR). LIRGs are among the brightest objects in the universe. Terms for extreme instances:

• ultraluminous infrared galaxy (ULIRG): 10¹² times solar luminosity in far infrared.
• hyperluminous infrared galaxy (HLIRG): 10¹³ times solar luminosity in far infrared.
• extremely luminous infrared galaxy (ELIRG): 10¹⁴ times solar luminosity in far infrared.

(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 submillimeter galaxy (SMG) is essentially the same as a LIRG.