Mid-infrared observations of the ultraluminous galaxies IRAS 14348-1447, IRAS 19254-7245, and IRAS 23128-5919

We present a study of the three ultraluminous infrared galaxies IRAS 14348-1447, IRAS 19254-7245, and IRAS 23128-5919, based on mid-infrared (MIR) spectro-imaging (5-18 mum) observations performed with ISOCAM. We find that the MIR emission from each system, which consists of a pair of interacting late type galaxies, is principally confined to the nuclear regions with diameters of 1-2 kpc and can account for more than 95% of their IRAS 12 mum flux. In each interacting system, the galaxy hosting an active galactic nucleus (AGN) dominates the total spectrum and shows stronger dust continuum (12-16 mum) relative to the Unidentified Infrared Band (UIB) emission (6-9 mum), suggestive of its enhanced radiation field. The MIR dominant galaxy also exhibits elevated 15 mum/Halpha and 15 mum/K ratios which trace the high extinction due to the large quantities of molecular gas and dust present in its central regions. Using only diagnostics based on our mid-infrared spectra, we can establish that the Seyfert galaxy IRAS 19254-7245 exhibits MIR spectral features of an AGN while the MIR spectrum of the Seyfert (or LINER) member of IRAS 23128-5919 is characteristic of dust emission principally heated by star forming regions.