The First Gamma-ray Emitting BL Lacertae Object at the Cosmic Dawn
  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 $\gamma$-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 $\sim$3.5 A supporting
its BL Lac nature. The detection of a strong Lyman-$\alpha$ break at $\sim$5570
A, on the other hand, confirms that J2119 is indeed a high-redshift
($z\sim3.59$) 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 $z>3$
$\gamma$-ray emitting BL Lac sources, therefore, will be crucial to verify the
theories of their cosmic evolution.
