THE SPATIAL EXTENT OF (U)LIRGs IN THE MID-INFRARED. I. THE CONTINUUM EMISSION

We present an analysis of the extended mid-infrared (MIR) emission of the Great Observatories All-Sky LIRG Survey sample based on 5-15 mu m low-resolution spectra obtained with the Infrared Spectrograph on Spitzer. 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 9.7 mu m. More than 30% of the galaxies have a median FEE lambda larger than 0.5, implying that at least half of their MIR emission is extended. Luminous Infrared Galaxies (LIRGs) display a wide range of FEE in their warm dust continuum (0 less than or similar to FEE13.2 mu m less than or similar to 0.85). The large values of FEE13.2 mu m that we find in many LIRGs suggest that the extended component of their MIR continuum emission originates in scales up to 10 kpc and may contribute as much as the nuclear region to their total MIR luminosity. The mean size of the LIRG cores at 13.2 mu m is 2.6 kpc. However, once the IR luminosity of the systems reaches the threshold of L-IR similar to 10(11.8) L-circle dot, slightly below the regime of Ultra-luminous Infrared Galaxies (ULIRGs), all sources become clearly more compact, with FEE13.2 (mu m) less than or similar to 0.2, and their cores are unresolved. Our estimated upper limit for the core size of ULIRGs is less than 1.5 kpc. Furthermore, our analysis indicates that the compactness of systems with L-IR greater than or similar to 10(11.25) L-circle dot strongly increases in those classified as mergers in their final stage of interaction. The FEE13.2 mu m is also related to the contribution of an active galactic nucleus (AGN) to the MIR emission. Galaxies which are more AGN dominated are less extended, independently of their L-IR. We finally find that the extent of the MIR continuum emission is correlated with the far-IR IRAS log(f(60 mu m)/f(100 mu m)) color. This enables us to place a lower limit to the area in a galaxy from where the cold dust emission may originate, a prediction which can be tested soon with the Herschel Space Telescope.