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Astron. Astrophys. 351, 447-458 (1999)

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Mid-infrared and far-ultraviolet observations of the star-forming ring of M 31 *

L. Pagani 1, J. Lequeux 1, D. Cesarsky 2, J. Donas 3, B. Milliard 3, L. Loinard 4 and M. Sauvage 5

1 DEMIRM, Observatoire de Paris, 61 Avenue de l'Observatoire, 75014 Paris, France
2 Institut d'Astrophysique Spatiale, Bat. 121, Université Paris XI, 91450 Orsay CEDEX, France
3 Laboratoire d'Astronomie Spatiale, B.P. 8, Traverse du Siphon, 13376 Marseille 12 CEDEX, France
4 IRAM, 300 Rue de la Piscine, Domaine Universitaire, 38406 St Martin d'Hères CEDEX, France
5 SAp/DAPNIA/DSM, CEA-Saclay, 91191 Gif sur Yvette CEDEX, France

Received 27 July 1998 / Accepted 23 August 1999

Abstract

We present mid-IR images of a 15´[FORMULA]15´ field in the south-west part of the Andromeda galaxy M 31 obtained with the ISOCAM camera (6" pixels) on board ISO. These broad-band images complement spectro-imaging observations of smaller fields (Cesarsky et al. 1998). We also present a 20" resolution far-UV image of a larger field at 200 nm obtained with the balloon-borne telescope FOCA 1000. These images are inter-compared and also compared with H I , CO(1-0) and H[FORMULA] maps. The mid-IR emission as seen through wide-band filters centered at 7 and 15 µm is extremely well correlated with the distribution of neutral gas as shown by the H I and CO(1-0) maps, while the correlation is poorer with the distribution of the ionized gas seen through its H[FORMULA] emission. There is some correlation with the UV radiation, but it appears that the contribution of UV photons to the excitation of the carriers of the mid-IR emission is not dominant in most of M 31. The spectro-imaging observations of Cesarsky et al. (1998) show that the mid-IR spectra of several regions of M 31, two of which are in the presently studied area, are dominated by a strong emission band at 11.3 µm while emission in the other classical Aromatic Infrared Bands (AIBs) at 6.2, 7.7 and 8.6 µm is faint or absent. This result is precised, and we find that the mid-IR spectral variations are not clearly related to the UV radiation field. The present observations have important consequences on our understanding of excitation of the interstellar mid-IR emission in general. In particular, we conclude that like for M 31, excitation in the Galactic cirruses may not be dominated by UV photons but rather by another mechanism which remains to be identified (visible photons?). The UV excitation appears to become important when the UV radiation density is of the order of 2 times that near the Sun.

Key words: galaxies: individual: M 31 – galaxies: ISM – ISM: dust, extinction – infrared: ISM: lines and bands

* 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 the participation of ISAS and NASA.

Send offprint requests to: james.lequeux@obspm.fr

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© European Southern Observatory (ESO) 1999

Online publication: November 3, 1999
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