Finding the highest redshift quasars
Quasars at z > 4.5 are our closest look at the first billion years of the universe. They tell us how supermassive black holes grew so large so early, what the galaxies around them were made of, and when the intergalactic gas was reionized. The problem is finding them: they are rare, and at these distances they look almost identical to the far more numerous cool stars in our own galaxy. Spectroscopy is the only certain confirmation, but it is far too expensive to run on every candidate, so the work begins with photometry and careful selection.
Three projects make up this line of work. I developed an SED fitting code to pick out quasar candidates at 4.5 < z < 7 from large photometric surveys, focused on the southernmost sky where almost nothing had been searched before. The result is a catalog of around 6,000 candidates, now being observed as part of the 4MOST Chilean AGN/Galaxy Evolution Survey. The method and the catalog are published in Astronomy & Astrophysics. Some of the candidates turned out to have radio detections, and spectroscopic follow-up at Palomar and Gemini North confirmed three radio-loud quasars at z > 5. These are the jetted, enormously powerful sources that are hardest to explain this early in cosmic time. I co-supervise Edgar Harutyunyan on their radio characterization, fitting the radio, infrared and optical data together to rank which remaining candidates are most worth observing. The Rubin Observatory, Euclid and Roman will image the sky deeper and wider than anything before them, but only if the photometric redshift methods can keep up. With the LSST AGN Science Collaboration I am building a mock photometric catalog of AGN at LSST depth, to test how well those methods will actually perform.Selecting high redshift quasars with SED fitting
The most extreme ones: radio-loud quasars at z > 5
Preparing for the next generation of surveys
