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Issue A&A
Volume 405, Number 3, July III 2003
Page(s) 999 - 1012
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20030440



A&A 405, 999-1012 (2003)
DOI: 10.1051/0004-6361:20030440

Unidentified infrared bands in the interstellar medium across the Galaxy

J. Kahanpää1, K. Mattila1, K. Lehtinen1, C. Leinert2 and D. Lemke2

1  Observatory, University of Helsinki, PO Box 14, 00014 Helsingin yliopisto, Finland
2  Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

(Received 26 September 2002 / Accepted 18 March 2003 )

Abstract
We present a set of 6-12  $\rm {\mu m}$ ISOPHOT-S spectra of the general interstellar medium of the Milky Way. This part of the spectrum is dominated by a series of strong, wide emission features commonly called the Unidentified Infrared Bands. The sampled area covers the inner Milky Way from l =  -60°to +60° with a ten-degree step in longitude and nominal latitudes b = 0°, $\pm$1°. For each grid position the actual observed direction was selected from IRAS 100  $\rm {\mu m}$ maps to minimize contamination by point sources and molecular clouds. All spectra were found to display the same spectral features. Band ratios are independent of band strength and Galactic coordinates. A comparison of total observed flux in band features and IRAS 100  $\rm {\mu m}$ emission, a tracer for large interstellar dust grains, shows high correlation at large as well as small (1´) scales. This implies a strong connection between large dust grains and the elusive band carriers; the evolutionary history and heating energy source of these populations must be strongly linked. The average mid-infrared spectrum of the Milky Way is found to be GROUP the average spectrum of spiral galaxy NGC 891 GROUP The common spectrum can therefore be used as a template for the 6-12  $\rm {\mu m}$ emission of late-type spiral galaxies. Finally, we show that interstellar extinction only weakly influences the observed features even at $\lambda = 10\rm {\mu m}$, where the silicate absorption feature is strongest.


Key words: ISM: lines and bands -- ISM: dust, extinction -- infrared: ISM -- Galaxy: disk -- galaxies: ISM

Offprint request: J. Kahanpää, jere.kahanpaa@helsinki.fi

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