The thesis describes infrared radiation and dust properties of the Magellanic Clouds, together with comparisons with emission at other wavelength regimes. Observations of the SMC and LMC were made... Show moreThe thesis describes infrared radiation and dust properties of the Magellanic Clouds, together with comparisons with emission at other wavelength regimes. Observations of the SMC and LMC were made with the IRAS satellite. Maps are presented at wavelengths of 12, 25, 60 and 100 _m. From these maps a list of discrete infrared sources is extracted. This source list is compared to known object lists (foreground stars, SMC stars, H_ nebulosities, clusters, supernova remnants, planetary nebula and dark clouds). There is a good correlation between HII regions and infrared emission from the SMC. For the LMC a similar presentation and discussion is given as for the SMC. A list of discrete sources is extracted, which is compared to similar source lists. As for the SMC, there is also a very good correlation of the infrared emission peaks with HII regions in the LMC. The Galactic infrared foreground emission is discussed. Foreground filaments disturb the image of this Cloud, and the IRAS data show significant variations in the foreground. The relation between the infrared foreground emission and the atomic hydrogen content is found to be non-linear. An estimate for the foreground infrared emission is derived, based on the average gas-to-dust ratio. The foreground towards the LMC varies from 0.07 to 0.17 magnitude, with an average of 0.10. A more or less constant foreground of 0.08 magnitude towards the SMC is found. Global properties of the Magellanic Clouds are discussed. Integrated infrared flux densities, infrared sizes, infrared luminosities and dust temperatures are derived. The infrared morphology is discussed, together with the temperature distribution. It turns out that the Greater 30 Doradus Region emits about 40 % of all infrared radiation. The density of the interstellar radiation field in the Clouds is derived. On average the fields are 7 times stronger than the Solar Neighbourhood field. A relation is found between infrared emission and the H_ supergiant shells in the LMC. Comparisons of the infrared maps are presented with ultraviolet data, H_ maps and with radio continuum observations. The SMC is 10 to 20 times weaker than the LMC in the infrared. The thesis describes dust properties in the Magellanic Clouds. The mid-infrared (12 and 25 _m) emission in the Clouds is less than that in the Local Group spiral galaxies. It is most likely that very small grains are responsible for this emission. Dust masses are derived for the combined warm (~50 K) and cool (25 K) dust components. The mass present in the cold (15 K) dust component is estimated to be between 1/3 and 1 times the mass of cool dust. Total dust masses of 550000 solar masses for the LMC and 24000 solar masses for the SMC are derived. Dust column density maps are presented for both Clouds. From a comparison of the infrared maps with atomic hydrogen maps, similar non-linear relations are found as for the Galactic foreground. Global properties of the Clouds are discussed in relation with irregular galaxies and in relation with Local Group spiral galaxies. The infrared colours of the Clouds are similar to the colours of other irregular galaxies. There are some exceptions, NGC 6822 and NGC 1569 have much lower f12 _m/f25 _m ratios than the Clouds, but their f60 _m/f100 _m ratios are similar. The luminosity-to-mass ratio of the SMC is similar to that of other irregular galaxies. That ratio of the LMC is an order of magnitude higher, and is comparable to that of spiral galaxies such as IC 342, M101and M33. Relative to the irregular galaxies, the gas-to-dust ratio in the LMC is low. The LMC gas-to-dust ratio is probably not very different from that in M31 and M33. For the LMC a ratio 3 times higher than for the Milky Way is found; for the SMC the ratio is 50 times higher than for the Milky Way. The LMC value agrees well with that of other authors, based on photometric B and V measurements and Ly_ absorption. The SMC value is, however, 3 to 6 times higher than those ratios. This difference is explained by Galactic foreground difficulties in photometric observations: high foreground extinction values from Chapter IV suggest large variations on small scales for individual stars. A new fit is made to the EB-V and NHI data, showing agreement with the dust mass based on infrared data. These findings also agree with the integrated ratio L100 _m/MHI, which is about 30 times lower for the SMC than for the Galaxy. Show less
