2000A&A...359..900L

The sample of IRAS galaxies with spectral energy distributions that peak near 60µm are called Sixty Micron Peakers (SMPs or 60PKs). Their generally peculiar and amorphous morphologies, hot dust and lack of a cirrus component have been interpreted as being indicative of a recent interaction/merger event. Mid-infrared spectra of eight SMPs, obtained with ISOPHOT-S in the ∼2-11µm band are presented. Four of the observed sources are HII region-like (H2) galaxies, three are Seyfert 2 and one is unclassified. Emission attributed to Polycyclic Aromatic Hydrocarbons (PAHs) at 6.2µm, 7.7µm and 8.6µm is ubiquitous in the spectra. The PHOT-S spectrum of the H2 galaxy IRAS 23446+1519 exhibits a bright 11.04µm line and an 8.6µm feature of comparable size to its 7.7µm feature. [S IV] emission at 10.5µm was detected in three of four H2 galaxies and in one Seyfert 2 galaxy. The ratio of the 7.7µm PAH feature to the continuum at 7.7µm (PAH L/C) divides the eight SMPs at a ratio greater than 0.8 for H2 and less than 0.8 for Seyfert galaxies. An anti-correlation between PAH L/C and the ratio of the continuum flux at 5.9µm to the flux at 60µm is found, similar to that found in ultraluminous infrared galaxies. Silicate absorption at approximately 9.7µm was observed in the Seyfert 2 galaxy, IRAS 04385-0828 and in IRAS 03344-2103. The previously unclassified SMP galaxy IRAS 03344-2103 is probably a Seyfert 2.