  At luminosities above ~10^{11} L_sun, infrared galaxies become the dominant
population of extragalactic objects in the local Universe (z < 0.5), being more
numerous than optically selected starburst and Seyfert galaxies, and QSOs at
comparable bolometric luminosity. At the highest luminosities, ultraluminous
infrared galaxies (ULIGs: L_ir > 10^{12} L_sun), outnumber optically selected
QSOs by a factor of ~1.5-2. All of the nearest ULIGs (z < 0.1) appear to be
advanced mergers that are powered by both a circumnuclear starburst and AGN,
both of which are fueled by an enormous concentration of molecular gas
(~10^{10} M_sun) that has been funneled into the merger nucleus. ULIGs may
represent a primary stage in the formation of massive black holes and
elliptical galaxy cores. The intense circumnuclear starburst that accompanies
the ULIG phase may also represent a primary stage in the formation of globular
clusters, and the metal enrichment of the intergalactic medium by gas and dust
expelled from the nucleus due to the combined forces of supernova explosions
and powerful stellar winds.
