Gamma-ray emission from young radio galaxies and quasars

According to radiative models, radio galaxies and quasars are predicted to produce gamma rays from the earliest stages of their evolution. Exploring their high-energy emission is crucial for providing information on the most energetic processes, the origin and the structure of the newly born radio jets. Taking advantage of more than 11 yr of Fermi-LAT data, we investigate the gamma-ray emission of 162 young radio sources (103 galaxies and 59 quasars), the largest sample of young radio sources used so far for such a gamma-ray study. We separately analyse each source and perform the first stacking analysis of this class of sources to investigate the gamma-ray emission of the undetected sources. We detect significant gamma-ray emission from 11 young radio sources, 4 galaxies, and 7 quasars, including the discovery of significant gamma-ray emission from the compact radio galaxy PKS 1007+142 (z = 0.213). The cumulative signal of below-threshold young radio sources is not significantly detected. However, it is about one order of magnitude lower than those derived from the individual sources, providing stringent upper limits on the gamma-ray emission from young radio galaxies (Fγ < 4.6 × 10-11 ph cm-2 s-1) and quasars (Fγ < 10.1 × 10-11 ph cm-2 s-1), and enabling a comparison with the models proposed. With this analysis of more than a decade of Fermi-LAT observations, we can conclude that while individual young radio sources can be bright gamma-ray emitters, the collective gamma-ray emission of this class of sources is not bright enough to be detected by Fermi-LAT.