Long ago and far away: Viewing galaxy formation and evolution through the best cosmic magnifying glasses.

The finite speed of light enables us to look back in time as we peer out in the universe. This enables us to study how galaxies were formed and how they evolved throughout the history of the universe. However, it is difficult to study distant galaxies in detail: Each galaxy only covers a few pixels, even in the best images from the Hubble Space Telescope, and they are so faint that they are only barely detected even with our largest telescopes. Nature has provided us with a tool to overcome these obstacles: Light rays from distant galaxies are deflected in the gravitational field of a massive object, such as a cluster of galaxies, which acts as a lens. This phenomenon is known as gravitational lensing. If the distant galaxy and the lens is almost perfectly aligned along our line of sight, the light will be focused towards us and the distant source will be magnified. This enables us to see objects and study phenomena which would otherwise be unobservable. We have found that the brightest gravitationally lensed galaxy, called the "Sunburst Arc", is lensed into 12 multiple images of the same object. When we observe this galaxy, we are looking 11 billion years back in time, to a period less than 3 billion years after the Big Bang. The young stellar age and low metallicity observed in this galaxy makes it analogous to even earlier galaxies. The high-energy ultraviolet radiation of these earliest stars and galaxies ionized the hydrogen gas between the galaxies, however, the ionizing radiation from these first galaxies cannot be observed directly today, and it is not clear how the ultraviolet light could escape through the thick gas and dust inside the galaxies. In this project we found the first example of ionizing radiation escaping through a narrow open channel in an otherwise thick medium inside a galaxy. Such a mode of escape had been theoretically studied in the past, but no actual examples had been found among the small known sample of galaxies leaking ionizing radiation studied prior to our work. Our results lend credence to the idea that this was how ionizing UV radiation escaped from very early galaxies, shaping structures in the universe when it was less than 1 billion years old. By measuring absorption in the 12 lensed images originating from the same source we could also determine significant variations in the amount of intervening neutral hydrogen gas on spatial scales less than 1 kiloparsec, a hundred times smaller than the smallest scales that have been previously probed in absorption. We led the publication of these novel results in the journal Science. Using the bright Sunburst Arc as a backlight, we could measure absorption from a spiral galaxy along the line of sight, measuring the movement and chemical composition of gas surrounding the galaxy. We found asymmetries indicating that we measure cycling of gas in and out of the galaxy, as predicted by numerical simulations of galaxy evolution. Very massive binary stars are a likely source of X-rays and powerful stellar winds strong enough to carve out the channels through which ionizing radiation can escape from galaxies, as seen in the Sunburst Arc. This hypothesis is supported by a different study, published in Nature Astronomy, where we made the first measurement of gravitationally lensed X-ray emission from stellar populations in another distant galaxy, looking 9.5 billion years back in time. This galaxy also has properties similar to the first generation of galaxies, but we measured that it emits ten times more X-rays than similar galaxies do today. This pioneering work opens up a new wavelength window to study distant star-forming galaxies, complementing studies of electromagnetic radiation at longer wavelengths. We have also analysed the spectra of 19 distant, gravitationally lensed galaxies, measuring the ages and chemical composition of their dominant population of stars in more detail than previously achieved. We have used the Nordic Optical Telescope to monitor light variations in the hot luminous disk surrounding a distant supermassive black hole, seen 11 billion years back in time. The energy emitted from the immediate surroundings of such supermassive black holes play a crucial role in regulating star formation in galaxies. A preliminary analysis of the data we have collected indicate that we will be able to determine the mass of the black hole. The black hole mass determination will be used to discriminate between different models of the co-evolution of supermassive black holes and their surrounding galaxies. We have also used the Nordic Optical Telescope to discover new examples of gravitationally lensed galaxies that are bright enough to be good targets for detailed follow-up studies with the upcoming James Webb Space Telescope.

Prosjektet har gjort omfattende bruk av Hubble Space Telescope og i vesentlig grad styrket den norske vitenskapelige utnyttelsen av dette ESA/NASA-prosjektet. Det vil også lede frem til flere norske bidrag i en tidlig fase av den vitenskapelige utnyttelsen av det fremtidige James Webb Space Telescope. Resultatene fra dette prosjektet har resultert i flere internasjonale pressemeldinger (fra ESA, NASA og AAAS), basert på en norskledet publikasjon i Science som har i vesentlig grad bidratt til å øke synbarheten for det norske forskningsmiljøet innen et internasjonalt høyprofilert grunnforskningsområde (galakser i det tidlige univers).   Det allerede etablerte faglige samarbeidet mellom prosjektleder og forskere ved ledende amerikanske institusjoner har blitt sterkere og bredere som resultat av dette prosjektet, og samarbeidet forventes å vare gjennom tiden JWST er operativ.

Gravitational lenses act as cosmic "magnifying glasses", bringing into view details of the distant universe which would otherwise be unobservable through even our best telescopes and instruments. The project seeks to exploit a set of the most powerful such lenses for studying distant galaxies: (A) In the course of an ongoing programme to follow up galaxy cluster candidates detected by the ESA Planck mission, a serendipitous discovery has been made of what is most likely the brightest gravitationally lensed galaxy in the Universe (Dahle et al. 2016). This discovery presents a unique opportunity to conduct a high-resolution physical study of an actively star forming galaxy, while looking back to a period when the universe was only 25% of its present age. A postdoctoral position is sought to analyse the wide range of ultraviolet, optical, infrared, and millimetre-wavelength data we are proposing for this remarkable object. (B) Another remarkable lens system, where a distant quasar is multiply lensed into six images by a galaxy cluster acting as a foreground lens, presents an unprecedented (and time-critical !) possibility to accurately determine the mass of the supermassive black hole powering the quasar. The time delays between the different quasar images, measured from an ongoing monitoring programme at the Nordic Optical Telescope (NOT), allows us to confidently predict a dramatic surge of brightness of the two brightest quasar images during 2016 - 2018. By mounting a spectroscopic and multi-band photometric campaign covering this period, we will be able to test theories for the co-evolution of supermassive black holes and their surrounding galaxies. (C) Studies of the earliest, most distant galaxies is a key objective of the upcoming James Webb Space Telescope (JWST). A strong gravitational survey using NOT is proposed to identify and characterise the high-mass, high-redshift galaxy cluster lenses which will be optimal for such future studies with JWST.