ulir

Ulir is a rarely used lowercase shorthand in astronomy for ultra-luminous infrared sources, though the more common term is ULIRG, which stands for Ultra-Luminous Infrared Galaxy. In standard usage, ULIRG denotes a galaxy with an infrared luminosity exceeding about 10^12 solar luminosities, making it one of the most luminous classes of galaxies when observed in the infrared. The lowercase form ulir may appear as a typographical variant in some databases or informal texts, but it is not the preferred nomenclature in professional literature.

Most ULIRGs are dust-enshrouded systems undergoing interactions or mergers. The intense activity in these galaxies produces enormous rates of star formation and, in many cases, contributes significantly to the infrared output. The dust absorbs optical and ultraviolet light from young stars and active galactic nuclei, re-radiating it at infrared wavelengths. While ULIRGs are exceptionally bright in the infrared, their optical emission is often heavily obscured, complicating direct study of their central engines.

Energy sources within ULIRGs are typically a mix of starburst activity and active galactic nuclei (AGN). Mid- and far-infrared spectroscopy reveals features associated with star formation, such as polycyclic aromatic hydrocarbon (PAH) emissions, alongside indicators of AGN activity in others. The relative contribution of starburst and AGN processes varies among objects, and many ULIRGs are found to be mergers of gas-rich galaxies.

ULIRGs are important for understanding dusty star formation and galaxy evolution. They are linked to major merger-driven growth and are studied as a transitional phase that may lead to more quiescent, massive galaxies or luminous quasar activity. See also LIRG and ULIRG for related infrared-luminous galaxy classes.