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Issue A&A
Volume 399, Number 3, March I 2003
Page(s) 833 - 855
Section Galactic structure and dynamics
DOI http://dx.doi.org/10.1051/0004-6361:20021719



A&A 399, 833-855 (2003)
DOI: 10.1051/0004-6361:20021719

ISOCAM view of the starburst galaxies M 82 , NGC 253 and NGC 1808

N. M. Förster Schreiber1, M. Sauvage1, V. Charmandaris2, 3, O. Laurent1, 4, P. Gallais1, I. F. Mirabel1, 5 and L. Vigroux1

1  CEA/DSM/DAPNIA/Service d'Astrophysique, CE Saclay, 91191 Gif-sur-Yvette Cedex, France
2  Astronomy Department, Cornell University, Ithaca, NY 14853, USA
3  Observatoire de Paris (Chercheur Associé.) , LERMA, 61 avenue de l'Observatoire, 75014 Paris, France
4  Max-Planck-Institute für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
5  Instituto de Astronomía y Física del Espacio, cc 67, suc 28, 1428 Buenos Aires, Argentina

(Received 22 August 2002 / Accepted 15 November 2002 )

Abstract
We present results of mid-infrared $\lambda = 5.0$- $16.5~\mathrm{\mu m}$ spectrophotometric imaging of the starburst galaxies M 82 , NGC 253 , and NGC 1808 from the ISOCAM instrument on board the Infrared Space Observatory . The mid-infrared spectra of the three galaxies are very similar in terms of features present. The $\lambda \ga 11~\mathrm{\mu m}$ continuum attributed to very small dust grains (VSGs) exhibits a large spread in intensity relative to the short-wavelength emission. We find that the 15 $\,\mu$m dust continuum flux density correlates well with the fine-structure [ $\ion{Ar}{ii}$] 6.99 $\,\mu$m line flux and thus provides a good quantitative indicator of the level of star formation activity. By contrast, the $\lambda = 5$- $11~\mathrm{\mu m}$ region dominated by emission from polycyclic aromatic hydrocarbons (PAHs) has a nearly invariant shape. Variations in the relative intensities of the PAH features are nevertheless observed, at the $20\%$- $100\%$ level. We illustrate extinction effects on the shape of the mid-infrared spectrum of obscured starbursts, emphasizing the differences depending on the applicable extinction law and the consequences for the interpretation of PAH ratios and extinction estimates. The relative spatial distributions of the PAH, VSG, and [ $\ion{Ar}{ii}$] 6.99 $\,\mu$m emission between the three galaxies exhibit remarkable differences. The $\la$ 1 kpc size of the mid-infrared source is much smaller than the optical extent of our sample galaxies and $70\%$- $100\%$ of the IRAS 12 $\,\mu$m flux is recovered within the ISOCAM $\leq 1.5~\mathrm{arcmin^{2}}$ field of view, indicating that the nuclear starburst dominates the total mid-infrared emission while diffuse light from quiescent disk star formation contributes little.


Key words: galaxies: individual: M 82 , NGC 253 , NGC 1808 -- galaxies: ISM -- galaxies: starburst -- infrared: galaxies -- infrared: ISM

Offprint request: N. M. Förster Schreiber, forster@strw.leidenuniv.nl




© ESO 2003

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