Search and analysis of giant radio galaxies with associated nuclei (SAGAN)

Abstract

We present the first results of a project called SAGAN, which is dedicated solely to the studies of relatively rare megaparsec-scale radio galaxies in the Universe, called giant radio galaxies (GRGs). We have identified 162 new GRGs primarily from the NRAO VLA Sky Survey with sizes ranging from ∼0.71 Mpc to ∼2.82 Mpc in the redshift range of ∼0.03−0.95, of which 23 are hosted by quasars (giant radio quasars). As part of the project SAGAN, we have created a database of all known GRGs, the GRG catalogue, from the literature (including our new sample); it includes 820 sources. For the first time, we present the multi-wavelength properties of the largest sample of GRGs. This provides new insights into their nature. Our results establish that the distributions of the radio spectral index and the black hole mass of GRGs do not differ from the corresponding distributions of normal-sized radio galaxies (RGs). However, GRGs have a lower Eddington ratio than RGs. Using the mid-infrared data, we classified GRGs in terms of their accretion mode: either a high-power radiatively efficient high-excitation state, or a radiatively inefficient low-excitation state. This enabled us to compare key physical properties of their active galactic nuclei, such as the black hole mass, spin, Eddington ratio, jet kinetic power, total radio power, magnetic field, and size. We find that GRGs in high-excitation state statistically have larger sizes, stronger radio power, jet kinetic power, and higher Eddington ratio than those in low-excitation state. Our analysis reveals a strong correlation between the black hole Eddington ratio and the scaled jet kinetic power, which suggests a disc-jet coupling. Our environmental study reveals that ∼10% of all GRGs may reside at the centres of galaxy clusters, in a denser galactic environment, while the majority appears to reside in a sparse environment. The probability of finding the brightest cluster galaxy as a GRG is quite low and even lower for high-mass clusters. We present new results for GRGs that range from black hole mass to large-scale environment properties. We discuss their formation and growth scenarios, highlighting the key physical factors that cause them to reach their gigantic size.

Radio observations of the merging galaxy cluster system Abell 3391-Abell 3395

“…However, about 10 percent of GRGs have now been found to re-side in cluster environments. According to Dabhade et al (2020a), about 820 GRGs larger than 0.7 Mpc are known to date. However, owing to their large extent and low surface brightness, they are notoriously difficult to detect.…”

MIGHTEE: are giant radio galaxies more common than we thought?

“…Mack et al 1998;Malarecki et al 2015). However, ∼10 per cent of GRGs have now been found to reside in cluster environments (Komberg & Pashchenko 2009;Dabhade et al 2020b;Tang et al 2020) and Lan & Prochaska (2020) recently found no difference between the environments of GRGs and that of galaxy control samples. Another…”

“…size 2 of 4.89 Mpc (Machalski et al 2008), though the majority of known GRGs are below 2 Mpc in extent (Dabhade et al 2020b).…”

“…Fewer than 1000 GRGs have been found to date. Dabhade et al (2020b) recently compiled a comprehensive catalogue of all 820 GRGs known. At the time of writing, a further six have been reported by Ishwara-Chandra et al (2020) and five by Tang et al (2020).…”

“…For example, 162 GRGs have only very recently been discovered in NVSS data, despite this survey having already existed for two decades. These were found via the Search and Analysis of Giant radio galaxies with Associated Nuclei (SAGAN) project, which rigorously combined newly-developed automated pattern recognition techniques (Proctor 2016) with careful manual inspection (Dabhade et al 2017(Dabhade et al , 2020b. Despite such efforts, and the existence of many thousands of 'normal' sized (< 0.7 Mpc) radio galaxies (RGs), GRGs remain scarce (e.g.…”