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Astron. Astrophys. 315, L89-L92 (1996)

ISOCAM mapping of the Whirlpool galaxy M 51*

M. Sauvage1, J. Blommaert2, F. Boulanger3, C.J. Cesarsky3, D.A. Cesarsky1, F.X. Désert3, D. Elbaz1, P. Gallais1,2, G. Joncas4, L. Metcalfe2, K. Okumura2, S. Ott2, R. Siebenmorgen2, J.L. Starck1, D. Tran1, and L. Vigroux1

1 CEA/DSM/DAPNIA/Service d'Astrophysique, C.E. Saclay, F-91191 Gif-sur-Yvette Cedex, France
2 ISO Science Operations Centre, Astrophysics Division of ESA, Villafranca del Castillo, P.O. Box 50727, E-28080 Madrid, Spain
3 Institut d'Astrophysique Spatiale, Université Paris-Sud, Bât. 121, F-91405, Orsay France
4 Département de Physique, Obs. Astronomique du Mont Mégantic, Université Laval Sainte-Foy, Québec G1K7PK, Canada

Received 16 July 1996 / Accepted 29 August 1996

Abstract. We have obtained the first images of the 6.75 and 15 img1.gifm continuum emission in the Whirlpool galaxy with ISOCAM, the infrared camera of the ISO (See Kessler et al. 1996). These data reveal the well-developed, spiral arms of the galaxy, in addition to diffuse emission between arms and in the outer regions of the disk. There is a striking correspondence with the 15 img1.gifm emission and the Himg2.gif emission over the galaxy. We tighten this link by looking at the properties of the individual HII regions and show that the 15 img1.gifm emission traces massive star formation and is not affected by extinction as is Himg2.gif . The data show systematic mid-infrared color variations perpendicular to the arms, which we interpret as two distinct dust components contributing to the mid-infrared emission. We examine the implication of these color variations in light of our current understanding of dust at these wavelength.

Key words: galaxies: individual: M 51 - galaxies: ISM - galaxies: spiral - infrared: interstellar: continuum - stars: formation

* Based on observations 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 participation of ISAS and NASA.

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