The First Gamma-ray Emitting BL Lacertae Object at the Cosmic Dawn

TL;DR

This paper reports the discovery of the first gamma-ray emitting BL Lac object at a redshift greater than 3, providing new insights into the cosmic evolution of relativistic jets in active galactic nuclei.

Contribution

It presents the identification of a high-redshift gamma-ray BL Lac object, supporting models of their evolution and rarity in the early universe.

Findings

First gamma-ray BL Lac at z>3 identified

Optical spectra show no emission lines, confirming BL Lac nature

Supports predictions of BL Lac evolution models at high redshift

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 $\gamma$-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.