ULIRG

An ultra-luminous infrared galaxy (ULIRG) is a galaxy with an infrared luminosity L_IR exceeding 10^12 solar luminosities (L_sun), measured over 8–1000 microns. This extreme infrared output is due to large quantities of dust absorbing ultraviolet and optical photons from intense star formation and/or accretion onto a supermassive black hole, re-emitting the energy in the infrared. ULIRGs are typically enshrouded in dust, making them faint in optical wavelengths but bright in the infrared.

Most ULIRGs in the local universe are gas-rich galaxies undergoing mergers or strong interactions. Their morphology often shows multiple nuclei, disturbed isophotes, and tidal tails. They are more common at higher redshifts (z ~ 0.5–2) and contribute significantly to the cosmic star-formation rate density during that epoch. The ULIRG class is defined in contrast with LIRGs (10^11–10^12 L_sun) and hyper-luminous infrared galaxies (HLIRGs; >10^13 L_sun).

Energy sources in ULIRGs can include both intense starburst activity and accretion onto a central active galactic nucleus, with the relative contribution varying case by case. Disentangling the two requires multi-wavelength data, including mid-infrared spectroscopy (PAH features, silicate absorption), X-rays, and submillimeter observations. Notable examples include Arp 220, often cited as the prototypical ULIRG; Mrk 231, a ULIRG with a luminous AGN; and the dual-nucleus system NGC 6240.

ULIRGs are important for understanding extreme star formation, dust obscuration, and the co-evolution of galaxies and their central black holes, particularly in the context of gas-rich mergers triggering rapid growth.