It all started from some of the most fundamental questions ever raised in cosmology: How did the first luminous sources form and reionise the universe?
After five years of efforts and thanks to very thorough surveys of our sky, the COSMIC-DAWN (The Emergence of Black Holes and Galaxies in the Universe) project has taken an important step towards answering this question, and thereby towards understanding the transition from the cosmic ‘dark ages’ to the universe as we know it. The identification of more than 50 of the most distant quasars known to date, along with great progress in characterising the physical properties of early galaxies, make this project incredibly valuable to the scientific community.
Why is it important to better understand quasars and galaxies at the earliest epochs?
Dr Fabian Walter: By finding and studying quasars in the very early universe, we can obtain important information on the formation of the earliest supermassive black holes and their host galaxies. We now detect quasars at an age when the universe was only 750 million years old, that is, only about 1/20th of its current age. Still, we find supermassive black holes with masses exceeding 1 billion solar masses. These masses rival those of the more massive black holes found in the local universe. This puts challenging constraints on the rapid formation of supermassive black holes, as there is little time to form such structures. Likewise, accreting supermassive black holes reside in gas reservoirs that show significant chemical enrichment. This enrichment can only be explained by a previous generation of massive star formation. Again, this puts important constraints on star formation in the earliest massive galaxies in the universe.
How does the project differ from previous attempts at gathering this knowledge?
These quasars are extremely rare, and therefore large sky surveys are needed to select them. Our team had privileged access to the latest multi-wavelength sky survey, the so-called Pan-STARRS1 survey. This survey was executed at a dedicated observatory in Hawaii, and, among other things, was designed to find quasars at redshifts that were previously not accessible. This enabled new selection techniques (particularly due to a red filter that was accessible for the first time with this new facility). Thanks to our efforts, we were able to triple the known quasar population, and significantly push the redshift frontier to earlier cosmic ages.
What would you say were your most important findings?
Finding the quasars is time demanding and tedious, and establishing a large sample of high-z quasars has been of importance for the community. Once the quasars are found, they can be followed up with multi-lambda state-of-the-art observatories (both in space and on the ground). One of the most important findings was the fact that we could establish that some of the quasars live in major galactic overdensities that were already in place when the universe was less than 1 Gyr old. Our follow-up observations also established the presence of massive amounts of gas, sufficient to fuel future star formation, and provided evidence of ongoing star formation in the host galaxies.
How did you proceed to get to these results?
Some of the main results were obtained by targeting the newly-discovered quasars with the Atacama Large (Sub)millimetre Array (ALMA), a newly available radio interferometer, and the IRAM NOEMA observatory in the French Alps. The former facility is located in Chile, at an elevation of 5 000 m, and provides by far the best millimetre observations worldwide. These observations were essential to establishing the presence of galaxy overdensities around the quasars, as well as characterising the detailed gas/dust properties of the host galaxies.
What do you hope will be the impact of this project on the scientific community?
Our results put more stringent constraints on theories of early structure formation, as theoretical cosmological models need to explain and account for the presence of the billion solar mass black holes as well as the chemically enriched gas and dust. This observational project therefore is of clear relevance to both theoreticians and numerical simulations of the early universe.
What are your follow-up plans after the project ends, if any?
So far, we have only been able to focus on characterising the accreting supermassive black hole and the gas/dust in the quasar host galaxies. No stars have been able to be detected in the quasar hosts yet, even though the expectation is that a massive stellar component must exist in these quasars. This is likely due to the fact that the central bright quasar emission, powered by the accretion on the central supermassive black hole, outshines the stellar light. The next generation space telescope (JWST) has the potential to finally detect the early stars that constitute the quasar hosts. We plan to submit comprehensive observing programmes to observe a sample of quasars with JWST, with an anticipated launch date in early 2019.
COSMIC_DAWNCORDIS project page