ISO observations of a sample of 60 mu m peaker galaxies

Abstract

The sample of IRAS galaxies with spectral energy distributions that peak near 60 mu m 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 ~ 2-11 mu m band are presented. Four of the observed sources are H II region-like (H2) galaxies, three are Seyfert 2 and one is unclassified. Emission attributed to Polycyclic Aromatic Hydrocarbons (PAHs) at 6.2 mu m, 7.7 mu m and 8.6 mu m is ubiquitous in the spectra. The PHOT-S spectrum of the H2 galaxy IRAS 23446+1519 exhibits a bright 11.04 mu m line and an 8.6 mu m feature of comparable size to its 7.7 mu m feature. [S IV] emission at 10.5 mu m was detected in three of four H2 galaxies and in one Seyfert 2 galaxy. The ratio of the 7.7 mu m PAH feature to the continuum at 7.7 mu m (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 5.9 mu m to the flux at 60 mu m is found, similar to that found in ultraluminous infrared galaxies. Silicate absorption at approximately 9.7 mu m 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.