Mid-Infrared Images of Luminous Infrared Galaxies in a Merging Sequence

We report mid-infrared observations of several luminous infrared galaxies(LIGs) carried out with the Infrared Space Observatory. Our sample was chosento represent different phases of a merger sequence of galaxy-galaxy interactionwith special emphasis on early/intermediate stages of merging. The mid-infraredemission of these LIGs shows extended structures for the early and intermediatemergers, indicating that most of the mid-infrared luminosities are not from acentral active galactic nucleus (AGN). Both the infrared hardness (indicated bythe IRAS 12, 25, and 60 $\micron$ flux density ratios) and the peak-to-totalflux density ratios of these LIGs increase as projected separation of theseinteracting galaxies become smaller, consistent with increasing star formationactivities that are concentrated to a smaller area as the merging processadvances. These observations provide among the first observational constraintof largely theoretically based scenarios.Comment: 10 pages, 3 figures, please refer to ApJ Letters for the final versio