The detection of high-redshift (z>3) blazars by the Fermi Large Area Telescope (LAT) is of great astrophysical importance as they are extreme objects whose energetics remain a mystery.More importantly, high-z blazars tend to host massive black holes and can be used to constrain the space density of heavy black holes in the early Universe. The first detection with the LAT of five gamma-ray emitting blazars beyond z=3.3, has been announced and presented at the American Physical Society meeting in Washington in January 2017, and a paper describing the results has been submitted to The Astrophysical Journal Letters.
Light from the most distant object, NVSS J151002+570243 (z=4.31), began its journey to us when the universe was 1.4 billion years old. These objects have steeply falling gamma-ray spectral energy distributions and, those that have been observed in X-rays, a very hard X-ray spectrum, both typical of powerful blazars.Their Compton dominance (ratio of the inverse Compton to synchrotron peak luminosities) is also very large (> 20). All of these properties place these objects among the most extreme members of the blazar population. The radio-loudness may play a key role in rapid black hole growth in the early Universe. Fermi LAT could have detected just the tip of the iceberg, the first examples of a galaxy population that previously has not been detected in gamma rays
This research was led by researchers at the Clemson University, South Carolina, USA and at the ASI Space Science Data Center (SSDC), Rome, Italy, including D. Gasparrini (ASI SSDC and INFN). They began by searching for the most distant sources in a catalog of 1.4 million quasars, a galaxy class closely related to blazars. Because only the brightest sources can be detected at great cosmic distances, they then eliminated all but the brightest objects at radio wavelengths from the list. With a final sample of about 1,100 objects, the scientists then examined LAT data for all of them, resulting in the detection of five new gamma-ray blazars.
More details in the NASA's press release, ASI news, and in the forthcoming ApJ Letter.