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Giant radio sources (GRSs) defined to be > 0.7 Mpc are the largest single objects in the Universe and can be associated with both galaxies (GRGs) and quasars (GRQs). They are important for understanding the evolution of radio galaxies and quasars whose sizes range from pc to Mpc scales and are also valuable probes of their environment. These radio-loud active galactic nuclei (RLAGN) interact with the interstellar medium of the host galaxy on small scales and the large-scale intracluster or intergalactic medium for the GRSs. With several new and sensitive surveys over the last few years, the number of known GRSs has increased many fold which has led a resurgence of interest in the field. This review article summarises our current understanding of these sources based on nearly five decades of research and discusses the importance of the Square Kilometer Array (SKA) in addressing some of the outstanding questions., Comment: Accepted for publication in the Journal of Astrophysics and Astronomy for Special Issue on Indian Participation in the SKA. Guest Editors: Abhirup Datta, Nirupam Roy, Preeti Kharb and Tirthankar Roy Choudhury). 31 pages, 8 figures, and 4 tables. Comments are welcome

Giant radio quasars (GRQs) are radio-loud active galactic nuclei (AGNs), propelling megaparsec-scale jets. In order to understand GRQs and their properties, we have compiled all known GRQs ("the GRQ catalogue"), and a subset of small (size <700 kpc) radio quasars (SRQs) from the literature. In this process, we have found 10 new FR-II GRQs, in the redshift range of 0.66 < z < 1.72, which we include in the GRQ catalogue. Using the above samples, we have carried out a systematic comparative study of GRQs and SRQs, using optical and radio data. Our results show that the GRQs and SRQs statistically have similar spectral index and black hole mass distributions. However, SRQs have higher radio core power, core dominance factor, total radio power, jet kinetic power and Eddington ratio compared to GRQs. On the other hand, when compared to giant radio galaxies (GRGs), GRQs have higher black hole mass and Eddington ratio. The high core dominance factor of SRQs is an indicator of them lying closer to the line of sight than GRQs. We also find a correlation of the accretion disc luminosity with the radio core and jet power of GRQs, which provides evidence for disc-jet coupling. Lastly, we find the distributions of Eddington ratios of GRGs and GRQs to be bi-modal, similar to that found in small radio galaxies (SRGs) and SRQs, which indicate that size is not strongly dependent on the accretion state. Using all of these, we provide a basic model for the growth of SRQs to GRQs., Comment: Accepted for publication in Astronomy & Astrophysics. 9 figures, and 2 tables. This is the third paper in the series. Comments are welcome. "SAGAN Project website this http https://sites.google.com/site/anantasakyatta/sagan"

We present original GMRT radio observations of the galaxy cluster Abell~725, at a redshift of 0.09, along with other archival observations. Our GMRT maps reveal two steep-spectrum diffuse filaments in the cluster, along with a previously reported arc-like structure, and a wide-angle tail (WAT) radio source associated with the Brightest Cluster Galaxy (BCG) at the periphery of the cluster. The bent morphology of the WAT indicates that its jets have been swept back by the dynamic pressure resulting from the motion of the BCG through the surrounding intracluster medium. The BCG associated with the WAT hosts a black hole whose mass we estimate to be 1.4$\pm0.4 \times10^{9} \Msun$. We observe a 2\arcmin (195\,kpc in projection) offset between the BCG and the X-ray centroid of the galaxy cluster, which, along with other dynamic features, indicates the cluster's early stage of evolution. The WAT radio galaxy, the arc and the filaments have spectral indices $\alpha_{612}^{240}= -0.46\pm 0.15$, $-0.8\pm0.3$, and ($-1.13\pm 0.48$, $-1.40\pm 0.50$), respectively. The WAT and the arc are connected structures, while the filaments are detached from them, but are found to be along the trail of the WAT. Based on the morphology of the components, and the progressive steepening of the components from the core of the WAT to the filaments, we propose that this system is a radio galaxy with trailing antique filaments., Comment: 12 Pages, 7 Figures, 1 Appendix, 5 Tables, Accepted for publication in MNRAS

Radio galaxies with jets of relativistic particles are usually hosted by massive elliptical galaxies with active nuclei powered by accretion of interstellar matter onto a supermassive black hole. In some rare cases (<5%), their jets drive the overall structure to sizes larger than 700 kpc, and they are called giant radio galaxies (GRGs). A very small fraction of the population of such radio galaxies contains molecular and atomic gas in the form of rings or discs that can fuel star formation. The origin of this gas is not well known; it has sometimes been associated with a minor merger with a gas-rich disc galaxy (e.g. Centaurus A) or cooling of material from a hot X-ray atmosphere (e.g. cooling flows). The giant radio jets might be the extreme evolution of these objects, and they can teach us about the radio galaxy evolution. We selected 12 targets from a catalogue of 820 GRGs that are likely to be in a gas-accretion and star formation phase. The targets were selected from the mid-infrared to contain heated dust. We report here the results of IRAM-30m observations, the molecular gas content, and the star formation efficiency, and we discuss the origin of the gas and disc morphology. Three out of our 12 targets are detected, and for the others, we report significant upper limits. We combine our three detections and upper limits with four additional detected GRGs from the literature to discuss the results. Most of the GRG targets belong to the main sequence, and a large fraction are in the passive domain. Their star formation efficiency is comparable to normal galaxies, except for two galaxies that are deficient in molecular gas with a short (~200Myr) depletion time, and a quiescent gas-rich giant spiral galaxy. In general, the depletion time is much longer than the lifetime of the giant radio jet., Comment: Accepted for publication in A&A journal on 16 September 2020. 13 pages, 9 figures, and 5 tables. This is the second paper under project SAGAN https://sites.google.com/site/anantasakyatta/sagan . Comments are welcome

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 NVSS 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, GRQs). 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. 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 (ER) 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. We find that GRGs in high-excitation state statistically have larger sizes, stronger radio power, jet kinetic power, and higher ER than those in low-excitation state. Our analysis reveals a strong correlation between the ER 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. We find that the probability of BCG as a GRG is quite low. 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. Abridged., Comment: Accepted for publication in Astronomy & Astrophysics. 14 figures, 7 tables and 7 montages. Comments are welcome. "SAGAN Project website https://sites.google.com/site/anantasakyatta/sagan"