Search

The opacity of spiral galaxy disks, from counts of distant galaxies, is compared to HI column densities. The opacity measurements are calibrated using the ``Synthetic Field Method'' from Gonzalez et al (1998) and Holwerda et al. (2005a). When compared for individual disks, the HI column density and dust opacity do not seem to be correlated as HI and opacity follow different radial profiles. To improve statistics, an average radial opacity profile is compared to an average HI profile. Compared to dust-to-HI estimates from the literature, more extinction is found in this profile. This difference may be accounted for by an underestimate of the dust in earlier measurements due to their dependence on dust temperature. Since the SFM is insensitive to the dust temperature, the ratio between the SFM opacity and HI could very well be indicative of the true ratio. Earlier claims for a radially extended cold dust disk were based on sub-mm observations. A comparison between sub-mm observations and counts of distant galaxies is therefore desirable. We present the best current example of such a comparison, M51, for which the measurements seem to agree. However, this remains an area where improved counts of distant galaxies, sub-mm observations and our understanding of dust emissivity are needed., 8 pages, 5 figures, 1 table, accepted by A&A

In order to study the nature of the far-infrared emission observed in radio-loud active galaxies, we have obtained 1.2 mill observations with the IRAM 30 m telescope for a sample of eight radio-loud active galaxies. In all objects we find that the 1.2 mm emission is dominated by non-thermal emission. An extrapolation of the non-thermal radio spectrum indicates that the contribution of synchrotron emission to the far-infrared is less than 10% in quasars. and negligible ill the radio galaxies. The quasars in the sample show signs of relativistic beaming at millimeter wavelengths, and the quasar 3C 334 shows evidence for strong variability.

We have obtained 2.5-45 mu m spectra of a sample of compact H II regions, YSOs and evolved stars in order to study the origin and evolution of interstellar Polycyclic Aromatic Hydrocarbon molecules (PAHs). Besides the well-known, strong PAH bands at 3.3, 6.2, 7.7, 8.6, and 11.2 mu m, these spectra reveal for the first time, a ubiquitous emission plateau from 15 to 20 mu m While the overall shape of this plateau is very similar in all sources, the detailed profiles vary from source to source. In particular, some sources show a distinct emission feature at 16.4 mu m. Moreover, the integrated intensity of this plateau varies relative to the PAH emission features by a factor 10 in our sample.We attribute this 15-20 mu m plateau to a blend of many emission features due to the interstellar or circumstellar PAH family present in these sources. Laboratory studies and quantum chemical calculations show that PAH molecules invariably possess emission features in this wavelength region, arising from C-CC bending modes which cause in- and out-of-plane distortion of the carbon skeleton. These modes are very sensitive to the molecular structure of the specific PAHs present and hence different molecules emit at different wavelengths. Analysis of the available data on the IR characteristics of PAHs show that a collection of PAHs will give rise to a broad plateau in this region.We have analyzed the size distribution of PAHs giving rise to the IR emission spectra of the sources in our samples. While much of the 15-20 mu m plateau is thought to arise in relatively large PAHs and PAH clusters, we attribute the 16.4 mu m feature to the small end of the interstellar PAH size distribution. We conclude that the observed increased strength of the 15-20 mu m plateau relative to the shorter wavelength IR emission features in regions of massive star formation is caused by a preponderance of larger PAHs and PAH clusters in those sources. Possibly this reflects the importance of coagulation in the dense molecular cloud environment from which these stars are formed.