**1. Introduction**

#### **1.1 The active galactic nucleus (AGN)**

The active galactic nucleus (AGN) is the existence of energetic phenomena in the nuclei, or core regions, of galaxies that cannot be clearly and directly explained to the interactions between the stars and interstellar medium. The source of radiation of AGN is definitely believed to emanate from the gravitational potential of gas from a supermassive black hole accreting mass at the core of the host galaxy. This emitted radiation by AGN is throughout the electromagnetic spectrum. The energy radiation in AGN is non-thermal unlike spectra of stars rather it is primarily as a result of the process of synchrotron radiation. In this scenario, power-law spectra, S<sup>ν</sup> � ν‐<sup>α</sup> and

high degree of linear polarization assigned to AGN object stand as evidence of synchrotron theory. AGNs have typical luminosity in the range of 10<sup>33</sup> to 1040WHz<sup>1</sup> [1].

Generally, an AGN possesses peculiar properties like, intense bright and point-like nucleus, radio cores with compact flat spectrum, highly ionized gas relativistically beaming out, variable fluxes observed on a wide timescale range from minutes upwards, extremely high luminosity from the range of 106 <sup>10</sup><sup>14</sup> solar luminosity, narrow, broad, and sometimes without lines of emission, extended radio jets and lobes.

#### **1.2 Classification of AGN**

The naming of AGN into classes and subclasses is mostly based on the exhibition of their properties or morphologies. Some AGN classes may be similar as a result of their evolution; some may be due to observed variability of luminosities [2]. Some AGN are classified based on their viewing angle as seen by the observer, which depends mostly on the obscuration of torus [3]. This brings about an unification scheme in AGNs due to the relativistic beaming and orientation effects.

Optically, AGN may be classified as type I or type II based on the optical spectral line emission. The classification of AGN using their response to radio-loudness is preferable because of the clearer observation obtained from the radio band compared to any bands' counterpart in the electromagnetic window. In this scenario, AGN can be classified as radio-loud and radio-quiet depending on their radio brightness. When they have ratios of radio (5GHz) to optical (B-band) flux F5*=*FB ≥10, they are called radio-loud, otherwise known as radio-quiet if F5*=*FB ≤10 [2, 4]. Since this work is entirely based on the observed radio properties of various AGN samples plus the clarification and consideration of radio window observation, the classification of AGNs based on radio-loudness is represented in **Figure 1**.

#### **1.3 Radio-loud AGNs**

Radio-loud sources emit more energies in the radio band than in the optical waveband, and hence possess F5*=*FB fluxes ≥ 10 [2]. There are about 15–20% of

**Figure 1.** *Classification of AGNs based on radio-loudness.*

radio-loud AGN. They are mostly elliptical galaxies in accordance with Hubble turning-fork proposition (population II)—meaning that they contain mostly of old stars with little interstellar gas. These objects are further classified into two: high luminosity objects and low luminosity objects.

#### **1.4 High luminosity objects**

They have total radio luminosity, P178 ≥1035WHz�<sup>1</sup> , with a highly ionized accretion disk [5]. These comprised of radio-loud quasars, Compact Steep-Spectrum Sources (CSSs) and Fanaroff-Riley class II radio galaxies. The works are mostly on these classes of AGN.

#### *1.4.1 Fanaroff-Riley class II radio galaxies*

These are known for their high luminous intensity with the possession of extended double lobes/jets where one side is Doppler enhanced. They also have smooth jets as a result of highly supersonic flows. The jets are also edge-brightened and terminate in hotspots [5–7]. They polarize linearly with the electric field vector being perpendicularly to the jets. In a unification scheme, [8] suggested that FR II sources are misaligned counterparts of core-dominated quasars.

#### *1.4.2 Quasars*

Quasars are classified further into core-dominated and lobe-dominated quasars. These are core-dominated if the radio emissions emanate mostly from the core, otherwise it is a lobe-dominated one.

The core-dominated ones possess properties like flat radio spectra with spectra index, α≤0*:*5, s<sup>ν</sup> � ν‐<sup>α</sup> ð Þ as a result of synchrotron self-absorption mechanism, cores with extremely brightness, broad emission lines and one-sided jets/lobes. These types of quasars dominated the survey at high frequencies and high redshifts. These classes of quasars show more asymmetry than the lobe-dominated counterpart [9].

On the other hand, lobe-dominated quasars, unlike the core-dominated, have two extended lobes straddling a weak compact core. They are also high luminosity sources with total luminosity, P178 ≥1035WHz�<sup>1</sup> . They are characterized by steep radio spectra (α> 0*:*5). They show spectra with broad emission lines; hence, they are referred to as broad-line region sources (BLRS). They also have higher redshifts when compared with radio galaxies [9].

### *1.4.3 Compact Steep-Spectrum Sources (CSSs)*

The CSS sources are characterized by sharp peaks exhibited in their radio spectra. As their names imply, they are compact bright radio sources with a population up to 30% [10]. They are also called "youth" scenario [10] being believed to be the younger phases of powerful large-scale extragalactic radio sources. They have a small radio size, D ≤15kpc, with a steep radio spectrum, α≥ 0*:*5, a very high radio luminosity, logP >1026WHz�<sup>1</sup> at frequency, ν = 2.7 GHz [10]. CSS radio sources exhibit brightness temperature up to 1010K [10]. Their radio structure is symmetric with low radio polarity and large Faraday rotation measures.

They are CSS radio galaxies if they have double lobes with weak jets and cores emitting weakly, otherwise CSS quasars if they exhibited brighter cores and jets [11]. Majority of CSS radio jets are one-sided and superluminal [11]. From the theory of orientation-based unification scheme, the morphological difference between the CSS radio galaxies and CSS quasars is that objects seen close to the line of sight of the observer are referred to CSS quasars otherwise radio galaxies [2].

#### **1.5 Low luminosity radio sources**

Low luminosity radio sources have total radio luminosity, P178 <1026WHz<sup>1</sup> with less ionized accretion disks [12]. Examples of these sources include FR I radio galaxies and BL Lacertae objects.

#### *1.5.1 Fanaroff and Riley class I (FR I) radio galaxies*

According to [12], FR I radio galaxies are characterized by extended double-lobed with low frequency, ν 400 MHz. FR I has RFR <0*:*5 as the source faints away from the nucleus, while FR II with brightness further away from the nucleus has RFR ≥ 0*:*5. RFR ratio is the ratio of the distance between the regions of highest surface brightness to the lowest brightness contour of the central galaxy.

Moreover, FR I sources are symmetric with smooth and continuous jets which begin as one-sided nearer the core and two-sided at a few kilo parsecs away. FR I sources are located in rich clusters that highly emit x-ray gas. The x-ray gas sweeps back and distorts the FR I radio structure as it moves across the interstellar cluster, hence giving FR I object narrow-angle-tail or wide-angle-tail according to the strength of the ram pressure of the gas [13–14].

#### *1.5.2 BL Lacertae sources (BL Lacs)*

BL Lacs objects are the most violent AGN known. They have properties like very weak or sometimes no radio emission lines, compact radio core, rapid and high peak variable fluxes, superluminal flows. They are also known as blazars just like optically violently variable (OVV) quasars.

### **1.6 Radio quiet objects**

These are objects that emit more in the optical window than they do in the radio band. AGN sources are said to be radio quiet if they emit more of their energy in the optical waveband than in the radio waveband. They have properties like F5*=*FB fluxes ≤10, low luminosity at 6 cm less than 1026WHz<sup>1</sup> , short jets, few relativistic particles and total weakness of the radio sources [15]. Examples of these objects are radio quiet quasars and seyferts (seyfert I and seyfert II). In this work, it is centered more on high luminosity radio-loud objects like FR II radio galaxy and quasar.

### **1.7 Features observed in AGNs**

The observational morphological features of a radio source are radio core, jets, lobes and hotspots, though every source may not exhibit all these features.

*Evolution of Radio Source Components and the Quasar/Galaxy Unification Scheme DOI: http://dx.doi.org/10.5772/intechopen.106244*

#### *1.7.1 Radio core*

This is the core engine where the energetic radio emission mechanism in EGRSs is assumed to originate. This core is divided into steep spectrum cores and ultra-compact flat spectrum [16]. The steep spectrum cores are characterized in some radio galaxies by having more extended radio cores of few kilo parsecs in size and steeper spectra (α≥ 0*:*5), example as found in seyferts and some spiral galaxies. On the other hand, the ultra-compact cores' counterparts possess properties like 1kpc in sizes and flat radio spectra (α<0*:*5), signifying synchrotron self-absorption that arises as a result of the re-absorption of some radiate relativistic electrons within the radio source. Quasars objects' core emissions appear to be more powerful than that of the radio galaxies.

### *1.7.2 Jets*

These are the conduits through which the high-energy particles are transporting from the cores to the other extended radio structures. A typical radio jet is expected to be at least four times as long as its width, in line with the radio core and separable from other features at high resolution [17]. The radio jets which can be one or two-sided exhibit properties like emissions with steep spectra of spectral index, α ̴0.5–0.9.

#### *1.7.3 Radio lobes*

These are one of the extended structures of radio sources that cover a range of hundred kilo parsec to a few mega parsecs. These are characterized by possession of non-thermal steep spectra of spectral indices, α ≥0*:*5, with a high degree of polarization at high frequency. They exhibit morphological features like tail, plumes, bridges and haloes. Tails are structures formed as a result of deflected plasma interacting with external medium, while plumes are extended regions with low luminosity that faints away from the whole source. Bridges occur in the inner lobe regions of radio galaxies, while the haloes are low surface brightness structures containing old aged plasma [18].

#### *1.7.4 Hotspots*

Hotspots are the brightest region of the extended radio structure formed at the end of the lobe where kinetic power of the jets is converted into random motion within the relativistic plasma and strengthened magnetic fields [19]. They have a linear size of 1kpc and steep spectra, α ̴0.5–1 slightly flatter than that of the surrounding diffuse emission [20]. See **Figure 2**.

It has been established that the appearance of EGRSs is substantially modified by relativistic beaming and orientation of the radio axes with the line of sight, leading to asymmetries in the observed radio structures. Similarly, radio sources are known to undergo some form of cosmological as well as temporal evolution. However, the amount of relativistic beaming and the nature of the evolution present in different classes and subclasses of the EGRSs are still a subject of intensive research. In particular, different source samples show a wide range of the amount and nature of temporal evolution as reported in literature. Hence, the aim of this work is to analytically examine the observed radio properties of different samples of EGRSs for radio source structural asymmetry, use relativistic beaming and source orientation model to explain any

**Figure 2.** *Unified structure of an typical extragalactic radio source [21].*

observed structural asymmetry in the radio sources and finally develop a model that can unambiguously explain the temporal evolution in extragalactic radio sources.
