Executive Summary : | Most galaxies host supermassive black holes (SMBHs) at their centers, which grow by accreting matter and are observed as active galactic nuclei (AGN). AGN can be identified by their extreme radiation across the entire electromagnetic spectrum. Radio-loud AGN ejects two oppositely directed, relativistic, magnetised jets along the rotational axis of the blackhole, known as radio galaxies (RG) or giant radio galaxies (GRG). Interruption of jet production has been observed in a rare group of RGs/GRGs, classified as double-double radio galaxies (DDRGs). In DDRGs, new pairs of radio lobes are seen between the AGN and old diffuse lobes. Two epochs of jet activity are well aligned and maintain long-term steadiness in their outflow directions. However, in few DDRGs, the new jet may not follow the previous jet direction, referred to as misaligned DDRGs (mDDRG). Recent work indicates that repeated jet activity is not very common among radio sources and large linear sizes are characteristic of most known DDRGs. DDRGs can be sub-classified further on the basis of their length-scales. Small size DDRGs (sDDRG) have projected linear extents below 0.7 Mpc, while giant size DDRGs (gDDRG) have projected linear-size above 0.7 Mpc. Both sDDRG and gDDRG can preserve lifelong unidirectional jet outflow, but mDDRGs may represent the prior evolution stage of well-aligned DDRGs. The important question is the relationship between sDDRGs, gDDRGs, and mDDRGs, and whether there exist particular host galaxy characteristics leading to the formation of these distinct populations of episodic sources. |