Part I: M51 is observed at a wavelength of 850 mum, where most radiation is emitted by dust. We find prominent spiral arms and an underlying exponential disk. The properties of the disk and the arms are studied. Part II and III: Gas in Galaxy Centers is exposed to intense radiation from a starburst region, an Active Galactic Nucleus (AGN), or both. The OB stars in starforming regions mostly radiate in the far-ultraviolet (FUV) and accreting black holes mostly in the X-ray regime. FUV and X-ray photons lead to a totally different chemical composition of gas clouds. Therefore, molecules such as H2, CO, HCN and HCO+ emit different line intensities. In the thesis, we model the line emission of the molecules in these gas clouds with Photon Dominated Regions models (FUV: PDR) and X-ray Dominated Region models (X-rays: XDR). These models are applied to nearby active galaxies such as NGC 253 and NGC 1068.Leiden Observatory LKBFUBL - phd migration 201
