The First Gamma-ray Emitting BL Lacertae Object at the Cosmic Dawn
Abstract
One of the major challenges in studying the cosmic evolution of relativistic jets is the identification of the high-redshift (z>3) BL Lacertae objects, a class of jetted active galactic nuclei characterized by their quasi-featureless optical spectra. Here we report the identification of the first γ-ray emitting BL Lac object, 4FGL~J1219.0+3653 (J1219), beyond z=3, i.e., within the first two billion years of the age of the Universe. The optical and near-infrared spectra of J1219 taken from 10.4 m Gran Telescopio Canarias exhibit no emission lines down to an equivalent width of 3.5 A supporting its BL Lac nature. The detection of a strong Lyman-α break at 5570 A, on the other hand, confirms that J2119 is indeed a high-redshift (z3.59) quasar. Based on the prediction of a recent BL Lac evolution model, J1219 is one of the only two such objects expected to be present within the comoving volume at z=3.5. Future identifications of more z>3 γ-ray emitting BL Lac sources, therefore, will be crucial to verify the theories of their cosmic evolution.