ELFs are remarkable objects, discovered independently by a number of groups soon after the IRAS catalogue was released (see Norris, Allen & Roche 1988). These optically undistinguished galaxies produce the bulk of their enormous luminosities at far-infrared (FIR) wavelengths (30 to 300 µm). They are probably AGNs or bursts of star formation cloaked in a large mass of gas and dust that reprocesses most of the optical photons to the FIR. This "laundering" can make it difficult to distinguish between possible alternative power sources. This has lead to considerable debate recently over the roles played by starbursts (introduced below) and Seyfert nuclei in powering ELFs and the question is not yet resolved (e.g., Harwit et al. 1987; Sanders et al. 1988b; Condon et al. 1991).
Starburst galaxies convert large fractions of their interstellar medium (ISM) into stars at a furious and unsustainable rate. In contrast, star formation in most galaxies is a well regulated, orderly, slow process. Starburst galaxies produce large optical luminosities from the combined output of many high-mass stars. The bursts are sometimes triggered by galaxy-galaxy interactions.
Despite the obscuring dust, parts of the life cycle of ELFs have been established. The progenitors of ELFs are thought to be normal spirals, since ELFs are rich in gas and dust. Clearly, the presently observed luminous phase must be only transitory. If ELFs are powered by starburst activity then the star formation would exhaust any reasonable supply of ISM in about 108 yr (Chapter 6).
ELFs are often involved in interacting systems, appearing disturbed and displaying tidal tails. Interactions are efficient at driving large amounts of ISM inwards and concentrating it into the nuclear region. There, it forms a reservoir to feed star formation or an AGN. Interactions do this by tidally inducing a bar in the stellar distribution of one or both galaxies; Barnes & Hernquist 1991. Bars can funnel ISM efficiently inward since parcels of gas ahead of the bar feel a component of gravitational attraction back towards the bar. The gas decelerates and falls down the potential well. Supporting these numerical results, large concentrations of molecular material have been seen within the central kpc in many ELFs by millimetre-wave interferometers (Sanders et al. 1988a).
The future in store for ELFs is not so clear. Sanders et al. (1988b) suggest that ELFs are the progenitors of quasars and that they currently harbour AGN cores that are hidden by powerful dusty circumnuclear starbursts. According to this view, they will evolve to blow away the surrounding gas and dust. Then, as the starburst fades, the full glory of the embedded quasar core will be revealed to our view.