ultraluminous infrared galaxy

The term ultraluminous infrared galaxy (ULIRG) indicates a galaxy emitting more than 10¹² times solar luminosity (L_Sun) in the far infrared. Such a galaxy also falls under the term luminous infrared galaxy (LIRG, more than 10¹¹ times L_Sun in far infrared) and potentially under the term hyperluminous infrared galaxy (HLIRG, more then 10¹³ times L_Sun 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 LIRG is often a starburst galaxy and/or has an active galactic nucleus (AGN). The infrared would be from black-body radiation of heated clouds (at a much lower temperature than stars themselves), e.g., by the young stars the galaxy is creating, and with enough dusty clouds to hide the new stars, the infrared may be the only observable sign of star formation. A ULIRG is typically a low-redshift galaxy, and often the result of a merger. When distant, it is a useful sign of star formation.