Extended mid-infrared emission from VV 114: Probing the birth of a ULIRG

We present our 5-16 mum spectro-imaging observations of VV114, an infrared luminous early-stage merger of two galaxies VV114E and VV114W, taken with the ISOCAM camera on-board the Infrared Space Observatory. We find that only 40% of the mid-infrared (MIR) flux is associated with a compact nuclear region of VV114E, while the rest of the emission originates from a rather diffuse component extended over several kpc in the regions between VV114E and VV114W. This is in stark contrast with the very compact MIR starbursts usually seen in luminous and ultraluminous infrared galaxies. A secondary peak of MIR emission is associated with an extra-nuclear star forming region of VV114W which displays the largest Halpha equivalent width in the whole system. Comparing our data with the distribution of the molecular gas and cold dust, as well as with radio observations, it becomes evident that the conversion of molecular gas into stars can be triggered over large areas at the very first stages of an interaction. This extended star formation along with the extreme nuclear starburst observed in VV114E can easily lead to the heating of dust grains found in the tidally disturbed disks of the progenitor galaxies and subsequently traced via their MIR emission. The presence of a very strong continuum at the 5-6.5 mum range in the spectrum of VV114E indicates that an enshrouded active galactic nucleus (AGN) may contribute to similar to 40% of its MIR flux. We finally note that the relative variations in the UV to radio spectral properties between VV114E and VV114W provide evidence that the extinction-corrected star formation rate of similar objects at high z, such as those detected in optical deep surveys, cannot be accurately derived from their rest-frame UV properties.