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Astron. Astrophys. 315, L353-L356 (1996)

Spectrophotometry of UIR bands in the diffuse emission of the galactic disk*

K. Mattila1, D. Lemke2, L.K. Haikala1, R.J. Laureijs3, A. Léger4, K. Lehtinen1, Ch. Leinert2, and P.G. Mezger5

1 Observatory, P.O. Box 14, FIN-00014 University of Helsinki, Finland (mattila at cc.helsinki.fi)
2 Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
3 ISO Science Operations Team, ESA - Villafranca, Villafranca del Castillo, Aptdo 50727, E-28080 Madrid, Spain
4 Institut d'Astrophysique Spatiale, C.N.R.S., Université Paris XI, France
5 Max-Planck-Institut für Radioastronomie, Postfach 2024, D-53010 Bonn, Germany

Received 16 July 1996 / Accepted 2 August 1996

Abstract. The spectrum of the unidentified infrared (UIR) emission bands between 5.8 and 11.6 img1.gifm has been observed for the first time for the diffuse emission of the galactic disk where the interstellar radiation field (ISRF) is quite low. The UIR bands at 6.2, 7.7, 8.6, and 11.3 img1.gifm have absolute intensities which are img2.gif 1/10th of the values observed in planetary and reflection nebulae. However, the intensity ratios and band widths are similar, pointing to a common carrier for the UIR bands over a large range of different environmental parameters in the ISM. This mid-IR emission from a low-ISRF medium and the absence of any detectable continuum emission at 10 img1.gifm in the diffuse galactic spectra go against any significant contribution from grains at equilibrium temperature or emission during temperature spikes of very small silicate grains; it supports the identification of the UIR bands as being due to the PAH molecules.

Key words: interstellar medium: dust - infrared: ISM: bands - Galaxy: general

* Based on observations made with ISO, an ESA project with instruments funded by ESA member states (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA

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