K. Mattila 1, K. Lehtinen 2, & D. Lemke 2
1 Observatory, P.O. Box 14, FIN-00014 University of Helsinki, Finland
2 Max-Planck-Institut für Astronomie, D-69117 Heidelberg, Germany
The spectrum of the unidentified infrared emission bands (UIRBs) between 5.8 and 11.6 um has been observed for the first time in the disk of an external galaxy where the interstellar radiation field is quite low as compared to nuclear and starburst regions of galaxies. In this talk we present the first results of an ISOPHOT-S observing project aimed at studying the UIRBs in the Sb spiral galaxy NGC891 seen edge-on. NGC891 is closely similar with our own Galaxy thus motivating a comparison with our ISOPHOT-S spectrophotometry of the diffuse galactic UIR emission (Mattila et al. 1996, AA 315, 353).
Our main results are the following:
(1) The main UIRBs at 6.2, 7.7, 8.6, and 11.3 um are clearly present in the disk of NGC891 at least to a distance of 10 kpc, both North and South of the centre. The spectra look very similar to those observed for the diffuse emission of the disk of our Galaxy as well as for bright galactic objects, such as reflection nebulae and HII regions.
(2) In order to compare the integrated emission of the UIRBs with the IRAS 12 um emission we have summed up the PHT-S spectra between 7.5 and 11.6 um (the IRAS band pass covers ca. 7.5 to 15 um). Taking into account an absolute calibration error of ca.20% for both IRAS and PHT-S we conclude that, at the very least ca. 50 - 60%, and possibly all of the IRAS 12 um emission is due to the UIR bands plus the weak underlying continuum seen in our spectra. The energy emitted (and absorbed) by the UIRB carriers is ca. 14% relative to the large grains. This is closely the same fraction (10%) as found for the diffuse emission of our Galaxy.
(3) The line intensities for the four UIR bands along the major axis show the following characteristic features: * a central peak;
* a ring of minimum intensity between 1 - 2.5 kpc;
* a ring of maximum intensity between 3.5 - 6 kpc;
* The profiles are not symmetric: the minimum appears deeper and the maximum broader and stronger on the northern side.
(4) Comparison of the UIRB distribution with other ISM components shows:
* the UIRB carriers have a distribution very similar to CO and large dust grains;
* a large-scale association of the UIRB emission with neutral hydrogen is not present;
* the UIRB emission does not follow the ionized gas distribution
(
,
[CII] 158 um); thus an essential part of the UIRB
emission
must come from the non-ionized ISM component.
(5) There are significant trends and differences of the line ratios between the different parts of NGC891:
* On the southern axis, outside of the molecular ring the 11.3/7.7 um ratio increases with increasing offset. The 8.6/7.7 um ratio shows the same tendency,but the 8.6/11.3 um ratio decreases slightly over this range.
* On the northern axis, outside the molecular ring, the 8.6/7.7 um ratio increases. Over the same range there is a modest decrease in the 11.3/7.7 um ratio. As a result the 8.6/11.3 um ratio shows even a stronger increase than the 8.6/7.7 um ratio.
* The line ratios for the central position do not differ from the values of the inner galactic disk.
We make an attempt to interpret the observed band ratios and their variations in NGC891 in terms of the PAH model. There are several possible mechanisms which influence the PAH band ratios and can explain the different types of band ratio variations as observed in NGC891:
* Hydrogenation/dehydrogenation of PAHs;
* The fraction of PAH cations, [PAH+]/[PAH];
* The fraction of compact and non-compact species in the PAH ion mixture;
* Size distribution of the PAH mixture.