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No PAHs in M 31

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

¹ Institut d'Astrophysique Spatiale, Orsay, France

² DEMIRM, Observatoire de Paris

³ Service d'Astrophysique, CEA

⁴ IRAM, Grenoble

⁵ Laboratoire d'Astronomie Spatiale, Marseille

We have observed the mid-IR spectra of four fields in M31 with the ISOCAM CVF and imaged a large fraction of the star-forming ring in this galaxy with the ISOCAM filters LW2 (5-8 $\rm\mu m$ ) and LW3 (12-18 $\rm\mu m$ ). The spectra are dominated by an interstellar emission band at 11.3 $\rm\mu m$ while the other bands at 6.2, 7.7, and 8.6 $\rm\mu m$ are faint or absent. Our interpretation is that we see the emission of tiny hydroganted amorphous grains released by carbon stars. The ultraviolet field of M 31 is too weak to graphitize these grains and to enable them to emit the 6-9 $\rm\mu m$ bands, except very near star-formation regions. The UV field is considerably larger than in our Galaxy and in most spiral and irregular galaxies. This allows the processing of the grains and the emission of the ``normal'' and spectrum in these galaxies.

We will compare mid-IR images of M 31 with HI, CO and H $\rm\alpha$ images and with a new far-UV map. There is an excellent correlation of the mid-infrared emission with the distribution of interstellar gas. The correlation with the UV emission is very poor, mainly due to extinction effects.

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"The Universe as seen by ISO", 20 - 23 October 1998, Paris: Abstract Book