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 chose n to represent different phases of a merger sequence of galaxy-galaxy inter action with special emphasis on early/intermediate stages of merging. The m id-infrared emission of these LIGs shows extended structures for the early and intermediate mergers, indicating that most of the mid-infrared luminosi ties are not from a central active galactic nucleus. Both the infrared hard ness (indicated by the IRAS 12, 25, and 60 mu m flux density ratios) and th e peak-to-total Aux density ratios of these LIGs increase as projected sepa ration of these interacting galaxies become smaller, consistent with increa sing star formation activities that are concentrated to a smaller area as t he merging process advances. These observations provide among the first obs ervational constraint of largely theoretically based scenarios.