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Astron. Astrophys. 359, 900-906 (2000)

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ISO observations of a sample of 60 µm peaker galaxies

R.J. Laureijs 1, D. Watson 2, L. Metcalfe 1, B. McBreen 2, B. O'Halloran 2, J. Clavel 1, K. Leech 1, P. Gallais 1, P. Barr 1, M. Delaney 3, L. Hanlon 2 and F. Quilligan 2

1 ISO Data Centre, Astrophysics Division, Space Science Department of ESA, Villafranca del Castillo, P.O. Box 50727, 28080 Madrid, Spain
2 Department of Physics, University College, Dublin 4, Ireland
3 Stockholm Observatory, 133 36 Saltsjöbaden, Sweden

Received 2 December 1999 / Accepted 22 April 2000


The sample of IRAS galaxies with spectral energy distributions that peak near 60 µm 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 [FORMULA]2-11 µm 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 µm, 7.7 µm and 8.6 µm is ubiquitous in the spectra. The PHOT-S spectrum of the H2 galaxy IRAS 23446+1519 exhibits a bright 11.04 µm line and an 8.6 µm feature of comparable size to its 7.7 µm feature. [S IV] emission at 10.5 µm was detected in three of four H2 galaxies and in one Seyfert 2 galaxy. The ratio of the 7.7 µm PAH feature to the continuum at 7.7 µm (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 µm to the flux at 60 µm is found, similar to that found in ultraluminous infrared galaxies. Silicate absorption at approximately 9.7 µm 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.

Key words: ISM: dust, extinction – ISM: molecules – galaxies: ISM – galaxies: Seyfert – galaxies: starburst – infrared: galaxies

Send offprint requests to: D. Watson (dwatson@bermuda.ucd.ie)

© European Southern Observatory (ESO) 2000

Online publication: July 13, 2000