Abstract: Water is one of the most abundant molecules in interstellar molecular gas, but very difficult to detect through our wet atmosphere. The Herschel Space Observatory has for the first time observed the full far-infrared and submillimetre spectra of nearby (ultra)luminous infrared galaxies or (U)LIRGs, revealing luminous H2O emission lines in many such objects. Interestingly, the H2O emission appears to be mostly excited by the absorption of far-IR photons. As such, H2O lines form a unique probe of local conditions, measuring the intensity of the local far-infrared radiation field. The inferred radiation intensity reveal emission at the blackbody limit, implying the presence of dust disks that are optically thick even in the far-infrared. Under those conditions the infrared radiation pressure can become significant and may play a role in driving molecular outflows. While the Herschel observations were mostly spatially unresolved, with the advent of ALMA it is now possible to obtain spatially resolved images of H2O emission in high-redshift galaxies. I will show the first results of these observations. Morphological differences between the observed lines have direct implications for the interpretation of the results. I will conclude by showing that molecular outflows in high-redshift galaxies can be traced in detail with lines of the OH+ molecule, opening a new window on the role of such outflows in luminous high-redshift galaxies.  

   Paul van der Werf studied Astronomy at the University of Groningen, The Netherlands. At the same time he studied music at the Conservatory in the same town. After obtaining his PhD in 1989, he moved to Garching (Germany) as postdoc at the Max-Planck-Institut fuer Extraterrestrische Physik, before obtaining an ESO Fellowship. He moved to Leiden in 1994, as Fellow of the Royal Netherlands Academy of Arts and Sciences. He is a faculty member at Leiden University since 1998, Director of Education of the Astronomy Department since 2011, and Professor of Extragalactic Astrophysics since 2012. In 2010 he was Distinguished Visitor at the Institute of Astronomy of the University of Edinburgh (UK). His principal research interests are (ultra)luminous infrared galaxies at low and high redshift, the evolution of star forming galaxies, and the interstellar medium of galaxies. He has over 300 refereed publications which have been cited over 13000 times. In his spare time he maintains a low-level musical career as a member of the Bach Ensemble Amsterdam.