The infrared continuum radiation of NGC 1808

Free access article

Issue		A&A Volume 377, Number 3, October III 2001


Page(s)		735 - 744
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20010969

A&A 377, 735-744 (2001)
DOI: 10.1051/0004-6361:20010969

A PAH and polarisation study

R. Siebenmorgen¹, E. Krügel² and R. J. Laureijs³

¹ European Southern Observatory, Karl-Schwarzschildstr. 2, 85748 Garching b. München, Germany
² Max-Planck-Institut for Radioastronomy, Auf dem Hügel 69, Postfach 2024, 53010 Bonn, Germany
³ ISO Data Centre, Villafranca del Castillo, PO Box 50727, 28080 Madrid, Spain

(Received 1 June 2001 / Accepted 29 June 2001 )

Abstract
The paper is devoted to the understanding of the infrared emission of nuclear regions in galaxies. a) ISO data of NGC 1808 are presented: spectro-photometry from 5.1 to 16.4 $\mu$ m; a $25''\times 25''$ map at 6 $\mu$ m and 170 $\mu$ m photometry. b) The data are complemented by a polarisation measurement at 170 $\mu$ m ( $2.5\pm0.4\%$ at position angle $94\degr\pm5\degr$ ) and a map at 6 $\mu$ m. In the map, the degree of polarisation goes up to 20% in the outer regions. We argue that the polarisation is produced by emission of big grains and exclude very small grains and PAHs or scattering and extinction. c) The mid infrared spectrum shows, beside the main emission bands, a so far unknown plateau of PAH features in the ${\geq} 13 \mu$ m region. d) The total spectrum can be fit under the assumption of optically thin emission. However, such a model fails to reproduce the 25 $\mu$ m point and implies that the mid infrared is due to very small grains and PAHs. These particles would then also have to be responsible for the 6 $\mu$ m polarisation, which is unlikely. e) To avoid these difficulties, we successfully turn to a radiative transfer model whose major feature is the existence of hot spots produced by the dust clouds around OB stars. We demonstrate the decisive influence on the mid infrared spectrum of both the PAHs and the hot spots.

Key words: polarisation -- infrared: galaxies -- galaxies: ISM -- galaxies: magnetic fields -- galaxies: individual: NGC 1808

Offprint request: R. Siebenmorgen, rsiebenm@eso.org

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.