ULIRGs

Ultra-Luminous Infrared Galaxies (ULIRGs) are galaxies with infrared luminosities L_IR greater than 10^12 times the Sun’s luminosity (integrated over 8–1000 microns). They are among the most radiant galaxies known, with the bulk of their energy emitted in the infrared after dust absorbs shorter-wavelength light from young stars and active nuclei.

The primary energy sources in ULIRGs are intense star formation (starbursts) and/or accretion onto a central supermassive black hole (AGN). The surrounding dust absorbs much of the optical and ultraviolet radiation and re-radiates it in the far-infrared, making ULIRGs extraordinarily bright at infrared wavelengths.

Most ULIRGs are interacting or merging systems. Galaxy mergers drive large reservoirs of gas toward the center, triggering prodigious star formation and often fueling the central black hole. The result is a compact, dusty nucleus with strong infrared emission and disturbed optical morphologies.

ULIRGs are typically identified in infrared surveys, with IRAS being pivotal in their discovery. Optical counterparts are often highly obscured and disturbed; mid-infrared spectra show features associated with star formation, such as polycyclic aromatic hydrocarbon (PAH) bands, and various silicate absorption features. The relative contribution of star formation and AGN activity tends to increase with luminosity and redshift.

Evolutionarily, ULIRGs are proposed to evolve into optically bright quasars and eventually into quiescent elliptical galaxies as gas is exhausted or expelled. Notable examples include Arp 220 and Markarian 231.