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

Abstract

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 mu m) 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 mu m flux. In each interacting system, the galaxy hosting an active galactic nucleus (AGN) dominates the total spectrum and shows stronger dust continuum (12-16 mu m) relative to the Unidentified Infrared Band (UIB) emission (6-9 mu m), suggestive of its enhanced radiation field. The MIR dominant galaxy also exhibits elevated 15 mu m/Hα and 15 mu m/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. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, Netherlands and the UK) and with participation of ISAS and NASA.