Abstract:
One of the major challenges in studying the cosmic evolution of relativistic jets is the identification of the highredshift (z > 3) BL Lacertae objects (BL Lacs), a class of jetted active galactic nuclei characterized by their quasifeatureless optical spectra. Here we report the identification of the first γ-ray emitting BL Lac, 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 Å supporting its BL Lac nature. The detection of a strong Lyα break at ∼5570 Å, on the
other hand, confirms that J2119 is indeed a high-redshift (z ∼ 3.59) 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.
