Detection of widely distributed UIR band emission in the disk of NGC 891

The spectrum of the unidentified infrared (UIR) emission bands between 5.9 and 11.7 mu m has been observed for the first time in the disk of an external galaxy. We have used the low-resolution spectrometer of the ISOPHOT instrument aboard ISO. The UIR bands at 6.2, 7.7, 8.6, and 11.3 mu m have absolute intensities which are similar to the values observed for the diffuse emission of our own Galaxy. The UIR bands between 5.9 and 11.7 mu m contribute ~ 9% of the total IR radiation of NGC 891. The intensity ratios and band widths in the NGC 891 disk emission are similar to the diffuse emission of the Milky Way, pointing to a common carrier for the UIR bands in the two galaxies. However, there are some notable variations of the band ratios along the major axis of the galaxy. Based on observations made 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