Röttgering’s main interest is the origin and evolution of active and star-forming galaxies, clusters of galaxies and large-scale structures in the Universe.
He carried out numerous projects based on radio, mm-submm, optical, IR and X-ray observations taken with ground- and space-based observing facilities. These interests led to many important results, including:
The discovery of a large number of distant radio galaxies including the most distant radio source known to date at a redshift of z = 5.1. Mapping with the Hubble Space telescope of their stellar content, kinematical studies of associated enormous Lyα halos and HI absorption, and dust and CO measurements established that distant radio galaxies are among the most massive galaxies in the early Universe that form stars at vigorous rates.
Studies of faint and extremely red galaxies showed them to be highly clustered, providing compelling evidence that the majority of them are the progenitors of nearby elliptical galaxies. Detection of dust emission showed that at least some are associated with extremely dusty starbursts.
The discovery that powerful radio galaxies reside in proto-clusters, large ensembles of galaxies that can be considered the progenitors of nearby massive clusters of galaxies. Subsequent multi-wavelength studies revealed important information about the galaxy content and the overall kinematics of these early structures.
Interferometry observations using ESO’s 8.4-meter VLT telescopes of the nucleus of the nearby active galaxy, NGC 1068, proved the existence of a torus-like dust structure only a few parsecs across, with important implications for AGN unification theories.
The discovery and study of an enormous 2 Mpc linear radio-emitting shock associated with a massive merging cluster provided definitive evidence for shock-induced particle acceleration in merging clusters.
LOFAR observations that yielded the first detection of carbon recombination lines outside our Milky Way. This opened up detailed studies of the cold-neutral medium, an important gas phase within galaxies.
The development of new radio reduction techniques to deal with ionospheric-induced phase corruptions. Their application to data from the LOFAR radio telescope resulted in the deepest low-frequency radio maps ever produced.
Spectroscopy of a very bright galaxy in the early Universe revealing a highly unusual spectrum which could be due to either the long-sought-after primordial Pop III stars or to a directly collapsed black hole.
The discovery that clusters of galaxies can contain a population of mildly relativistic electrons that only low-frequency telescopes like LOFAR can observe.
LOFAR observations show that the cosmic web glows in the radio, providing evidence that accretion shocks associated with forming filaments can accelerate particles to such a speed that they emit synchrotron radiation.
LOFAR and Keck observations of a radio source with a size of 7Mpc, the largest so far discovered, with important implications for understanding how magnetic fields, relativistic particles and metals are transported far away from their prime source.
His second interest is the development of astronomical observing facilities.
Röttgering has played a leading role in developing the €150 million pan-European radio telescope, LOFAR. As the first chair of the astronomical research committee, he guided the project's initial phases. As Principal Investigator (PI) for the project 'LOFAR for astronomy', he secured €7 million in funding, enabling the four Dutch-led LOFAR Key projects—focused on reionisation, transient radio sources, cosmic rays, and surveys of the low-frequency sky—to (i) conduct essential technical and software work for their Key projects and (ii) participate in the commissioning of LOFAR capabilities specific to their Key projects. As PI of the LOFAR Survey Key project, he is directing a large science team of 350 European scientists to conduct extensive and in-depth surveys of the low-frequency radio sky. Building on the results from these surveys, Röttgering and his team are currently engaged in numerous scientific projects related to the formation and evolution of Active Galactic Nuclei (AGN), galaxies, and clusters of galaxies. He has also been heavily involved with ground- and space-based optical and infrared interferometers, including MIDI on the VLT interferometer, ESA's Darwin, and NASA's Terrestrial Planet Finder (TPF) space interferometers. He initiated the Dutch contribution to Euclid, ESA’s dark energy mission, and has chaired the Euclid consortium board for five years. With a launch in 2023, Euclid is providing high-resolution optical and infrared images, along with photometric redshifts for virtually all of the 5 × 10^7 radio sources from the LOFAR surveys.
