Optical versus mid-infrared spectroscopic classification of ultraluminous infrared galaxies

The origin of the huge infrared luminosities of ultraluminous infrared gala xies (ULIGs) is still in question. Recently, Genzel and colleagues performe d a mid-infrared (MIR) spectroscopic survey of a large number of ULIGs and found the major energy source in them to be massive stars formed during rec ent starburst activity; similar to 70%-80% of the sample are predominantly powered by starbursts. However, previous optical spectroscopic observations classified the majority of ULIGs either as Seyferts or as LINERs. In order to reconcile this difference, we compare types of emission-line activity f or a sample of ULIGs that have been observed in both optical and MIR. We co nfirm the results of previous studies that the majority of ULIGs classified as LINERs on the basis of optical emission-line diagnostics turn out to be starburst-dominated galaxies on the basis of MIR diagnostics. Since MIR sp ectroscopy can probe the more heavily reddened, inner parts of the ULIGs, a nd since it is quite unlikely that the inner parts are powered by starburst s while the outer parts are powered by nonstellar ionization sources, the m ost probable resolution of the dilemma is that the optical emission-line ne bulae with LINER properties are powered predominantly by shock heating driv en by superwind activity; i.e., blast waves driven by the collective effect of a large number of supernovae in the central region of galaxy mergers.