How Many Ulirgs are Mergers?
Ultraluminous IRAS Galaxies (ULIRGs) are unusual galaxies with luminosities > 1012 L ⊙ (H0 = 75 kms-1Mpc-1), comparable to optically selected quasars, with up to 90% of this emitted in the 8–1000 µm region. Initial work on nearby ULIRGs suggested that all of these objects were in interacting or merging systems. Larger samples (e.g. Leech et al 1994) suggested that this might only be true for 2/3 of the objects. The triggering of ULIRG-type activity in the non-merging galaxies would then be difficult to understand.