Ice features in the mid-IR spectra of galactic nuclei

Free access article

Issue		A&A Volume 385, Number 3, April III 2002


Page(s)		1022 - 1041
Section		Diffuse matter in space
DOI		http://dx.doi.org/10.1051/0004-6361:20020147

A&A 385, 1022-1041 (2002)
DOI: 10.1051/0004-6361:20020147

Ice features in the mid-IR spectra of galactic nuclei

H. W. W. Spoon^{1, 2}, J. V. Keane², A. G. G. M. Tielens^{2, 3}, D. Lutz⁴, A. F. M. Moorwood¹ and O. Laurent⁴

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
    e-mail: spoon@astro.rug.nl; amoor@eso.org
² Kapteyn Institute, PO Box 800, 9700 AV Groningen, The Netherlands
    e-mail: j.keane@astro.rug.nl; tielens@astro.rug.nl
³ SRON, PO Box 800, 9700 AV Groningen, The Netherlands
⁴ Max-Planck-Institut für Extraterrestrische Physik (MPE), Postfach 1312, 85741 Garching, Germany
    e-mail: lutz@mpe.mpg.de

(Received 28 November 2001 / Accepted 24 January 2002)

Abstract
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 $\mu$ m 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 $\mu$ m 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 $\mu$ m 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 $\mu$ m feature weakens.

Key words: galaxies: ISM -- galaxies: nuclei -- galaxies: Seyfert -- galaxies: starburst -- infrared: galaxies -- infrared: ISM

Offprint request: H.W.W. Spoon, spoon@astro.rug.nl

SIMBAD Objects

What is OpenURL?

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.

If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.