Searching for the Largest Radio Galaxies

Abstract: The origins of radio astronomy and the discovery of the first radio galaxies are described which showed that the radio emission of active galaxies is very diverse in shape and can reach a size of many times their optical extent. In 1974 the first "giant" radio galaxy (GRG) was discovered, several times larger than any previously known one. Since 2012, when about 100 such GRGs larger than 1 Megaparsec (3.3 million light years) had been reported in literature, the author is performing his own search for GRGs and maintains a list of currently nearly 7000 GRGs, with more than half of these found on his own or his students at the Departamento de Astronom\'ia of Universidad de Guanajuato. An analysis of the very largest GRGs does not reveal any single property of these that would explain why they could grow to such large sizes. Recent advances in radio telescopes have led to vast amounts of images rich in GRGs, but due to the complexity of identifying their host galaxies only a fraction of these images can be searched with visual inspection by humans. Currently available machine algorithms and citizen science projects are prone to erroneous identifications and also leave unnoticed a substantial fraction of GRGs, such that supervision of the results by experts is essential to produce reliable results.