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The Physics of the Most Luminous Galaxies

Final Report Summary - EUROCAL (The Physics of the Most Luminous Galaxies)

The brightest galaxies in the Universe have been the underlying theme of the EuroCal project. Despite the prominence of these objects in the night sky, the details of the physics that drives and powers them remains in many cases elusive. The EuroCal project was designed to attack exactly these important open research questions.

The most luminous galaxies are powered in one of two ways: either by the infall of matter into a supermassive black hole at the center of the galaxy; or by the transformation of interstellar matter (gas and dust) into stars.

Despite the tremendous progress in their study over the past decades, black holes remain among the most mysterious objects in the Universe, representing the end-state of matter. Their gravity is so intense that not even light can escape their grasp. Unlike black holes that are the end products of stellar evolution, which have masses between a few and a few tens of solar masses, black holes at the centers of galaxies have masses which reach billions of times the mass of the Sun. When such a supermassive black hole accretes gas and stars, energy is released and radiated away. In such a case, the galaxy is said to host an active nucleus, and galaxies of this kind are collectively known as active galactic nuclei. The radiation from the nucleus can be absorbed by dust and re-radiated in infrared wavelengths.

Another observable feature of supermassive black holes are jets: highly collimated outflows that are sometimes launched by the black hole. In such jets, both matter as a whole and individual particles within the jet move at extremely high speeds approaching the speed of light. Emission from the jet extends over the entire electromagnetic spectrum. If the jet is closely aligned with our line of sight towards the host galaxy, relativistic effects boost the emission towards higher birghtness and higher frequencies, while at the same time enhancing the observed variability. A galaxy hosting such a closely aligned jet is known as a blazar.

The second way for energy to be produced in exceptionally luminous galaxies is str formation. The physics of star formation is οne of the most active research areas of both theoretical astrophysics and observational astronomy. Radiation from massive stars is emitted primarily in optical and ultraviolet wavelengths. This radiation can be also absorbed by dust, and re-emitted at infrared wavelengths.

The aim of the EuroCal project was to bring together scientists from three institutions: the Foundation for Research and Technology - Hellas in Heraklion, Greece, the Max-Planck Institute for Radio Astronomy in Bonn, Germany, and the California Institute of Technology, in Pasadena, California in the US, to exchange expertise on these aspects of luminous galaxies, explore areas of collaboration, and forge strong, lasting research partnerships to resolve the mysteries enshrouding the brightest galaxies in the Universe.

The project involved training workshops, long secondments of young students from the EU and their supervisors to Caltech, one of the top research institutions in the US, receiprocal secondments of Caltech scientists to the two EU partners, and the commissioning and operations of an innovative polarimeter - RoboPol - at the Skinakas Observatory operated by FORTH in Crete. These research activities have resulted in 53 publications that have already appeared in peer-reviewed journals, and are all freely available through the arXiv preprint server and the project website; additional research that is still ongoing; and new avenues for research and collaboration between institutions.

The RoboPol optopolarimetric monitoring program for blazars (jets that are powered by supermassive black holes) has been a flagship collaborative project between the partner institutions. RoboPol has monitored 100 blazars for three years, and has for the very first time demonstrated conclusively that coherent optopolarimetric variability – smooth changes in the polarization of light from blazars – is directly related to bright flaring in gamma rays, the highest-energy emission we observe from blazars. In this way, RoboPol has shown that the mechanism producing gamma rays in blazars and the mechanism that drives the variations in polarization of optical light have a common origin.

Active galaxies, the targets of RoboPol, were also observed in radio wavelengths and high-energy X-rays, with projects hosted at MPIfR and Caltech: the FGAMMA monitoring program, the OVRO monitoring program, and NuSTAR. Partner scientists had the opportunity to be trained in obtaining and analyzing data from all these different wavelength regimes, and use them for their science analysis and interpretation.

On the other side of ultra-luminous phenomena, star-formation efforts focused both on theory and observations. Partner scientists were trained in the analysis of data from the Herschel satellite that help map star formation both in its infancy as well as its bright main phase. They developed new ways to distinguish bright emission from star formation in a galaxy from emission by a black hole at its center using radio observations. They studied the interplay between black hole activity and star formation in galaxies. They studied the role of magnetic fields in star-forming regions, observationally through optical polarimetric mapping using background stars and through molecular line emission, as well as theoretically through magnetohydrodynamic simulations.

Finally, in a new project tying together optopolarimetry with interstellar medium science and cosmology, a new tomographic technique was developed to better map the interstellar dust that emits polarized light, and which constitutes the major hurdle in current attempts to detect an inflationary (early-universe) signature in the polarization of the cosmic microwave background. EuroCal members plan to apply the new technique over large areas of the sky in the next years, in a partnership that will endure beyond the end of EuroCal.

More information on the EuroCal project, including public-access links to all our papers (through the ArXiv archive), can be found on the project website:

Project EUROCAL is supported by the European Union’s Seventh Framework Programme, through an International Research Staff Exchange Scheme (IRSES) Marie Curie Action, under grant agreement PIRSES-GA-2012-316788.