Active galaxies and quasars harbor a black hole accretion driven power house in their centres (see f.i. the Wikipedia description). The accretion process generates unusually strong optical, infrared, X-ray and occasionally also radio emission, which makes active galaxies among the most luminous and exotic objects in the Universe. Owing to their high luminosity, active galaxies can be traced to the outer edges of the Universe, which makes them even more interesting. It has also been shown that active galaxies are prolific star-formers: as such they can be considered as the locations of excessive fireworks in the Universe.
Together with his students and collaborators throughout the world, Barthel has studied nearby and distant active galaxies and quasars with cutting edge instruments on earth and in space. Combining their various properties, he was among the first to propose that seemingly different classes of active galaxies can be unified, i.e., belong to one and the same parent population whereby the viewing angle differentiates the classes. His involvement with infrared astronomy in space has allowed him in recent years to study the nuclear accretion power in relation to the global host galaxy star-formation.