An international team of astronomers has identified the farthest example of a rare class of gamma-ray emitting galaxies. The so-called BL Lacertae object was discovered at cosmic dawn, within the first two billion years of the age of the universe. Today, the cosmos is 13.8 billion years old. The researchers from DESY, Universidad Complutense de Madrid, University of California Riverside and Clemson University report their discovery in The Astrophysical Journal Letters. They used one of the largest optical telescope in the world, Gran Telescopio Canarias (GTC), located at the Observatorio del Roque de los Muchachos on the island of La Palma.
Only a small fraction of the galaxies emits gamma rays, which is the most extreme form of light. Astronomers believe that these highly energetic photons originate from the vicinity of a supermassive black hole residing at the centers of these galaxies. When this happens, they are known as active galaxies. The black hole swallows matter from its surroundings and emits jets or, in other words, collimated streams of matter and radiation. Few of these active galaxies (less than one per cent) have their jets pointing by chance toward Earth. Scientists call them blazars, and these are one of the most powerful sources of radiation in the universe.
Blazars come in two flavors: BL Lacertae (BL Lac) objects and flat-spectrum radio-quasars (FSRQs). The current understanding about these mysterious astronomical objects is that FSRQs are relatively young active galaxies, rich in dust and gas that surround the central black hole. As time passes, the amount of matter available to feed the black hole is consumed and the FSRQ evolves to become a BL Lac object. “In other words, BL Lacs may represent the elderly and evolved phase of a blazar’s life, while FSRQs resemble an adult,” explains Vaidehi Paliya from DESY, first author of the paper.
“Since the speed of light is limited, the farther we look, the earlier in the age of the Universe we investigate,” says Alberto Domínguez from the Institute of Physics of Particles and the Cosmos (IPARCOS) at Universidad Complutense de Madrid and co-author of the study. The most distant FSRQ was identified at a distance when the age of the universe was merely a billion years. For comparison, the previously farthest BL Lac was found at an age when the Universe was around 2.5 billion years old, corroborating the hypothesis of the evolution from FSRQ to BL Lacs.
However, the newly discovered BL Lac object with the catalog number 4FGL J1219.0+3653, is substantially farther away than the previous record holder. “We have discovered a BL Lac existing even 800 million years earlier, this is when the Universe was less than 2 billion years old,” reports co-author Cristina Cabello, a graduate student at IPARCOS. “This finding challenges the current scenario that BL Lacs are actually an evolved phase of FSRQ,” adds IPARCOS professor Nicolás Cardiel, also a co-author of the paper. And IPARCOS professor and co-author Jesús Gallego concludes: “This discovery has challenged our knowledge of the cosmic evolution of blazars and active galaxies in general.”
Reference: “The First Gamma-Ray Emitting BL Lacertae Object at the Cosmic Dawn” by Vaidehi S. Paliya, A. Domínguez, C. Cabello, N. Cardiel, J. Gallego, Brian Siana, M. Ajello, D. Hartmann, A. Gil de Paz and C. S. Stalin, 27 October 2020, Astrophysical Journal Letters.