IRAS Galaxies

Abstract

Approximately thirty of the luminous infrared galaxies highlighted by the IRAS survey have now been mapped at high resolution using the millimeter arrays. In virtually all cases, extremely high gas concentrations are found in their nuclei (≤2 kpc) and in several cases, the gas mass fraction is extremely high (25-100%). Optical and near infrared imaging of the most luminous infrared galaxies shows significant distortions, double nuclei, and/or tidal tails indicating that these galaxies have probably undergone a recent galactic interaction. The dense molecular gas probably plays a pivotal role in the evolution of such dynamically disturbed systems: being dissipative the gas readily sinks to the central regions where it may fuel a nuclear starburst and possibly build up and fuel a central active galactic nucleus. In this picture, the IRAS galaxies may be crudely characterized by three parameters: the initial mass of ISM in the progenitor galaxies, the time since the interaction, and the fraction of the total luminosity attributed to young stars versus an active nucleus. In this scenario, systems such as the antennae (NGC 4038/39) may represent the early stages of a merger before the gas has been deposited in the nucleus; Arp 220 the phase in which a large nuclear gas concentration is presently fueling a starburst/AGN, and NGC 1068 the later phase in which much of the central gas concentration has been consumed and the residual gas exists in a circumnuclear ring or arms.