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          Institute: MPI für extraterrestrische Physik     Collection: Infrared and Submillimeter Astronomy     Display Documents

ID: 10747.0, MPI für extraterrestrische Physik / Infrared and Submillimeter Astronomy
Ice features in the mid-IR spectra of galactic nuclei
Authors:Spoon, H. W. W.; Keane, J. V.; Tielens, A. G. G. M.; Lutz, D.; Moorwood, A. F. M.; Laurent, O.
Date of Publication (YYYY-MM-DD):2002-04
Title of Journal:Astronomy & Astrophysics
Journal Abbrev.:Astron. Astrophys.
Issue / Number:3
Start Page:1022
End Page:1041
Review Status:Peer-review
Audience:Experts Only
Abstract / Description:Mid infrared spectra provide a powerful probe of the conditions in dusty galactic nuclei. They variously contain emission features associated with star forming regions and absorptions by circumnuclear silicate dust plus ices in cold molecular cloud material. Here we report the detection of 6-8 mum water ice absorption in 18 galaxies observed by ISO. While the mid-IR spectra of some of these galaxies show a strong resemblance to the heavily absorbed spectrum of NGC 4418, other galaxies in this sample also show weak to strong PAH emission. The 18 ice galaxies are part of a sample of 103 galaxies with good S/N mid-IR ISO spectra. Based on our sample we find that ice is present in most of the ULIRGs, whereas it is weak or absent in the large majority of Seyferts and starburst galaxies. This result is consistent with the presence of larger quantities of molecular material in ULIRGs as opposed to other galaxy types. Like NGC 4418, several of our ice galaxy spectra show a maximum near 8 mum that is not or only partly due to PAH emission. While this affects only a small part of the galaxy population studied by ISO, it stresses the need for high S/N data and refined diagnostic methods, to properly discriminate spectra dominated by PAH emission and spectra dominated by heavy obscuration. The spectral variation from PAH emission to absorbed continuum emission near 8 mum shows strong similarities with Galactic star forming clouds. This leads us to believe that our classification of ice galaxy spectra might reflect an evolutionary sequence from strongly obscured beginnings of star formation (and AGN activity) to a less enshrouded stage of advanced star formation (and AGN activity), as the PAHs get stronger and the broad 8 mum feature weakens.
Free Keywords:galaxies : ISM; galaxies : nuclei; galaxies : Seyfert; galaxies : starburst; infrared : galaxies; infrared : ISM
External Publication Status:published
Document Type:Article
Communicated by:N. N.
Affiliations:MPI für extraterrestrische Physik
External Affiliations:European So Observ, Karl Schwartzschild Str 2, D-85748; Garching, Germany; European So Observ, D-85748 Garching, Germany; Kapteyn Astron Inst, NL-9700 AV Groningen, Netherlands; SRON, NL-9700 AV Groningen, Netherlands; Max Planck Inst Extraterr Phys, D-85741 Garching, Germany
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