Spatial extent of (U)LIRGs in the MIR. I.

We calculate the fraction of extended emission (FEE) as a function of wavelength for the galaxies in the sample, FEE_{lambda}_, defined as the fraction of the emission which originates outside of the unresolved component of a source at a given distance.

We find that the FEE_{lambda}_ varies from one galaxy to another, but we can identify three general types of FEE_{lambda}_: one where FEE_{lambda}_ is constant, one where features due to emission lines and polycyclic aromatic hydrocarbons appear more extended than the continuum, and a third which is characteristic of sources with deep silicate absorption at 9.7um. The mean size of the LIRG cores at 13.2um is 2.6kpc. However, once the IR luminosity of the systems reaches the threshold of L_IR_~10^11.8^L_{sun}_, slightly below the regime of Ultra-luminous Infrared Galaxies (ULIRGs), all sources become clearly more compact, with FEE_13.2um_<~0.2, and their cores are unresolved.