ultraluminous

Ultraluminous is a descriptive term used in astronomy and related fields to denote an object or source whose luminosity is exceptionally high compared with typical objects of the same kind. The exact threshold that qualifies as ultraluminous varies by wavelength band and object class, and is not a single universal value.

In infrared astronomy, the term is most closely associated with ultraluminous infrared galaxies (ULIRGs). These are galaxies with infrared luminosities exceeding about 10^12 solar luminosities (Lsun). ULIRGs are often dust-shrouded systems where intense star formation, sometimes accompanied by active galactic nuclei, powers their extreme infrared output. Many ULIRGs are found to be gas-rich and frequently arise from galaxy mergers, and they contribute significantly to the cosmic star formation rate at certain epochs.

In X-ray astronomy, ultraluminous X-ray sources (ULXs) describe off-nuclear sources with X-ray luminosities greater than roughly 10^39 erg per second. ULXs are important for studying accretion physics, as their brightness exceeds the Eddington limit for typical stellar-mass black holes and neutron stars. They may represent super-Eddington accreting stellar remnants, neutron stars with strong magnetic fields, or, in some cases, intermediate-mass black holes; the exact nature of many ULXs remains an active area of research.

Beyond these categories, ultraluminous can be applied to other observational bands or transient phenomena to emphasize unusually high brightness. Overall, the term signals exceptional luminosity rather than a single, uniform class of objects.