Astron. Astrophys. 342, 643-654 (1999)

5. Conclusions

(1) We have observed for the first time the UIR band spectra at 6.2, 7.7, 8.6, and 11.3 µm in the diffuse emission of an external galaxy disk, the nearby edge-on Sb spiral NGC 891.

(2) The UIR band emission between 5.9 and 11.7 µm is 9% of the total infrared emission of the galaxy.

(3) The UIR band emission is visible along the major axis of the galaxy up to a distance of 10 kpc.

(4) The distribution of the UIR band intensities is closely similar to that of the 1.3 mm continuum emission by cold dust and to that of the ¹²CO(1-0) line intensity, whereas the 21-cm HI emission has a substantially broader distribution. The distributions of the H and [CII] 158 µm line emissions are also different from the ones for the UIR bands. We conclude that the UIR band carriers are mainly associated with areas of neutral molecular gas.

(5) The UIR band ratios show substantial variations along the major axis of NGC 891. Adopting the PAH model the band ratios suggest a low dehydrogenation degree of the PAHs at the southern end of the major axis, and an increase of the fraction of PAH cations, especially the non-compact ones, along the northern major axis of NGC 891.

© European Southern Observatory (ESO) 1999

Online publication: February 23, 1999
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