SPECTRAL INDEX ASYMMETRY IN COMPACT STEEP SPECTRUM SOURCES

Abstract

The study investigated the spectral index of components of Compact Steep Spectrum sources (CSSs). The purpose of the study was to determine the symmetric nature of the CSSs. A sample of the CSS sources which is made up of 8 galaxies and 4 quasars was taken from the Extra Galactic Zoo for observation. These sources were observed at frequency ranges from 1.4-8.4 GHz. The spectral index of the Jet side and the Counter-Jet side lobes were estimated and their means were obtained. The result revealed asymmetry between the two sources which was more pronounced in galaxy (for Jet side) and in quasar (for Counter-Jet side). Further analysis revealed no statistically significant asymmetry in the mean values of CSSs and quasar sub-sample, while a statistically significant asymmetry exists for galaxy sub-sample.

 

Keywords:  Compact steep spectrum sources, quasar, galaxy, spectral index asymmetry.

TABLE OF CONTENT

Title page                                                                                                                                i

Certification                                                                                                                           ii

Dedication                                                                                                                              iii

Acknowledgments                                                                                                                 iv

Table of Contents                                                                                                                  v

Abstract                                                                                                                                  viii

List of tables                                                                                                   ix

List of figure                                                                                                  x

CHAPTER ONE:   INTRODUCTION

1.1     Extragalactic Radio Sources                                                                 1

1.2     Morphology/ Structures of Extragalactic Radio Sources                      2

1.2.1   The Core                                                                                               3

1.2.2 The Jets                                                                                                  6

1.2.3 The Lobes and Hotspots                                                                         8

1.3     Galaxy                                                                                                 9

1.4     Quasar                                                                                                  11

1.5     Active Galactic Nuclei (AGN)                                                             12

1.5.1 Types of Active Galactic Nuclei                                                            13

1.5.2 Unification of AGNs                                                                             17

1.6     Purpose of the Study                                                                            19

CHAPTER TWO: COMPACT STEEP-SPECTRUM SOURCES (CSSs)

2.1     Introduction                                                                                         20

2.2     Asymmetry Parameters                                                                        24

2.2.1 Jet Sidedness Asymmetry                                                                      24

2.2.2 Structural Asymmetry                                                                           25

2.2.3 Arm-Length Asymmetry                                                                       25

2.2.4 Lobe-Lobe Luminosity Ratio (Apparent Flux Ratio)                            27

2.2.5 Depolarization Asymmetry                                                                    28

2.2.6 Models of the Depolarization Asymmetry                                             29

2.2.7 Spectral-Index Asymmetry                                                                    30

CHAPTER THREE: DATA AND DATA ANALYSIS

3.1 Data                                                                                                          37

3.2 Data Analysis          34

HAPTER FOUR: RESULT, DISCUSSION AND CONCLUSION

4.1     Result                                                                                                  39

4.2     Discussions                                                                                          39

4.3     Conclusion                                                                                           42

4.4     Recommendation                                                                                 42

References                                                                                           44

 

CHAPTER ONE

INTRODUCTION

       1.1      EXTRAGALACTIC RADIO SOURCES

It is conveniently established that extragalactic radio sources (EGRS) radiate their energy by the commonly accepted incoherent synchrotron radiation process, that is, radiation from relativistic electrons spiraling in a magnetic field. The evidence lies in the shapes of the spectra of these sources. The spectra are of power law form in agreement with the synchrotron theory where the relativistic particles both gain and loose energy. This is also evident in the high degree of linear polarization.

The source of energy of these sources, the central engine, is thought to be associated with active galactic nuclei. Energy from these central engines appears to be transported to the outer lobes through a highly collimated beam of relativistic particles called jets. Although many extended extragalactic radio sources have two sided large scale lobes, the radio jet in these sources are overwhelmingly one-sided. There are at least three physical interpretations of the apparent one-sidedness of the jets in these sources:

1. The jets are intrinsically one-sided at a given time.
2. They are two-sided, but the counter jet is intrinsically faint even though it may carry a similar energy as the main jet, that is, one which is less dissipative.

• The jets have intrinsically similar emissivity, but relativistic Doppler beaming enhances the surface brightness of the jet that is closer to the line of sight.

 

1.2           MORPHOLOGY/ STRUCTURES OF EXTRAGALACTIC RADIO SOURCES

The radio sources associated with radio galaxies and quasars come in a variety of structures and sizes. At one extreme of the range are unresolved flat spectrum compact sources coincident with the optical nucleus. At the other end are complex structures, many hundreds of kiloparsec. In extent, with a halo, lobes, a compact nucleus and jet-like features connecting the nucleus to the extended region. The development of large radio telescope arrays, which makes use of the earth’s rotational aperture synthesis and sophisticated image restoration techniques made it possible to obtain and see these features. The most sensitive of such instrument is the Very Large Array (VLA), located in

New Mexico. This consists of an array of twenty-seven dish antennae, each

25m in diameter. The array is Y-shaped and the distances between the dishes can be varied. Using the VLA, it is possible to make high sensitive radio maps with an angular resolution of ≤ 1 arcsec. Resolution better than 50milliarcsec at

5GHz can be obtained with the Multi-Element Radio linked Interferometry Network (MERLIN) which is an array of radio telescopes spread over Britain; with separations up to 217 km. Resolutions of 1milliarcsec can be obtained using the Very Long Baseline Interferometry (VLBI). The VLBI uses antennae spread over different continents. The baseline is therefore thousands of kilometers long. The signals from the antennae are independently recorded and correlated later. The Very Long Baseline Array (VLBA) consists of ten dish antennae, each of 25m diameters, spread from Hawaii to Virgin Islands.

The primary features in an Extragalactic Radio Source are the core, lobes, jets and hotspot. Not all of these features are observable in all sources and quite often the morphology is too complex to provide separation into these parts. It is however convenient to view a radio source being built from them and consider any complex or ill-defined features to be produced from disturbances in the source itself or an interaction with the inter-galactic environment.

       1.2.1            THE CORE

Radio cores are flat spectrum components associated with the power source in the nucleus of the optical identification. The flat spectrum is the result of synchrotron self-absorption. Nevertheless they are usually resolved by VLBI into a core component with a milliarcsecond jet. Cores are found in almost all radio quasars and in 80% of all radio galaxies. It is safe to assume that they will be found in all sources when observations are made with sufficient sensitivity and dynamic range.

Cores have been observed to be very compact, about 0.1-1 kpc (Dallacase et al., 2002). They however produce vast amount of radio power (over 1000 times that of a normal galaxy). No known nuclear process can produce this whole conversion of energy. The big question is what then is responsible for the high radio luminosity and compact structure? Radio astronomers are arriving at a consensus that the high radio luminosity and compact nature indicated by rapid flux variability can be attributed to a super-massive black hole which is the “central engine” residing within the galaxy nucleus into which matter is accreting (Akujor et al, 1991). In this picture, a spinning black hole is fuelled by interstellar gas which is drawn in by the hole’s intense gravitational field.

The matter falling inwards crashes into a flat swirling “accretion disc” that envelopes the hole, except for two funnel-shaped openings above the poles through which matter is accelerated out of the funnel to create two opposite jets.

Many reasons are given to support the idea of a spinning black hole as the

‘central engine’ residing within the nucleus of galaxies. For example, the jets on parsec and kiloparsec scales usually are fairly well aligned. It is thought that only a massive black hole with its spin axis corresponding to the jet axis is capable of producing jets that can keep memory of their direction over several years ~ 10⁶ years.

A commonly agreed physical process that can facilitate sufficient energies is the release of potential energy by matter falling into a deep potential well. Matter falling from infinity into a potential well will have a velocity, matter accreting to such a deep well can release ~30% of its rest mass as energy by synchrotron radiation, or by some other non-thermal processes. In the case of the black hole, the effective radius, where non-coherent emission is produced, is the radius of the accretion disc, r < 10¹⁶cm. An important parameter for accretion models is the “Eddington Limit” (Begelman, et al., 1984), defined as.

L_E=  4 GMm_pT = 1.3 x 10⁴⁷ M_o

Where M_o is measured in solar masses (2 x10³⁰ kg). Masses of 10⁸ – 10⁹ M_o are needed to produce the observed luminosities. T is the temperature in Kelvin, m_p is the mass of the particles, G is the universal gravitational constant and M is the mass of the black hole.

The required released energy is equal to the total conversion of L_E/C² into radiation energy, so nuclear (fusion) processes with efficiency of < 1% would be an inappropriate source of the energy of extragalactic radio sources as that would require an enormous rate of consumption of mass. However, accretion into a massive black hole which permits the conversion into radiation of up to

32% of the rest mass of accreting matter seems to provide a viable mechanism. Nevertheless, there are still many questions which have not been fully answered. For instance, why are the cores of quasars relatively stronger than those of radio galaxies? Could it be due to the orientation effects? Or is the effect intrinsic?

 

       1.2.2            THE JETS

Jets are the linear features linking the core to the extended lobe structures and may be visible over all or only part of their inferred length. A working definition of a jet is given by Bridle et al (1984) and it may be one or two-sided, smooth or knotty, with spectral index, α ≥ 0.6.

The jets of Compact Steep Spectrum Sources (CSSs) have been observed to show striking features like misalignments and bending of the jet, which are not typical of radio galaxies. Possible explanations to these features are accumulating.  Take the quasar 3C43 for example: It is a Compact Steep

Spectrum quasar having a misaligned and asymmetric structure with a remarkably bent jet. Akujor et al., (1991), suggested that a strong interaction between the jet and dense interstellar medium is a more plausible explanation for this phenomenon and may be common in a sub class of CSS quasars. The large misalignment between this kpc- scale jet and the outer extended radio sources seen in 3C216 and 3C446 are rather typical for quasars in the CSS class of radio sources. In many cases for example, 3C 309.1, 3C 147 and 3C 380, the jets show sudden bends at the end of a linear or slightly curved structure. Many reasons have been given to account for bends and distortion of jets in Compact Steep Spectrum Sources. These include orbital motion or possible precession of the active nucleus and the interaction of the radio beam with ambient gas, resulting in either disruption or bending of the jet, (Breugel et al.,1984). The asymmetries in structure and the observed bend in the jets may be related and could be due to interaction of the jet with line emitting gas that is asymmetrically distributed within the gas vicinity.

A theoretical basis for this hypothesis can be found in numerical simulations which have shown that gas clouds can be very effective in deflecting and/or slowing out-flowing plasma (Liang, 1988). The model for CSSs predict that dense knots of emission line gas, will be found adjacent to bends in the radio jets and at the position of the brighter radio feature which is the hotspots, both in distorted and linear sources.

Another mechanism responsible for this distortion in CSSs is the relativistic beaming effect. If a radio source is viewed by an observer at an inclination angle θ and the jets are flowing at speeds close to the speed of light, relativistic beaming will concentrate their emission in the forward direction. The Doppler equations for the approaching and receding jets respectively are

=  1 –                                      (1)

and

                                    =  1 +                                   (2)

 

So a jet coming towards an observer will appear much brighter than an identical jet going away. This means that for speeds ν = c and moderate orientation angles of say 45⁰, d could be quite large ≥ 20 Hz.

       1.2.3             THE LOBES AND HOTSPOTS

The lobes are the extended regions of radio emitting plasma and consist of material which has passed through the shocks and may be flowing back towards the nucleus to form bridges. Bridges are the inner lobe regions of high luminosity radio sources which join with the outer halves of the source structure and contain the oldest and most radioactively aged plasma in the source

(Sanghera et al, 1992). These lobes are generally characterized by hotspots which are radio bright components located near the outer extremes of powerful extended sources, with a linear size of 1 kpc.

Hotspots having special indices in the range (0.5-1.0) and are generally somewhat flatter than the lobes in which they are imbedded. High resolution observation have shown that in many sources the lobes contain single, bright, very compact primary hot spot, generally to one side of the lobe and set back from the leading edge along with a secondary hotspot (Laing, 1988). Thus a hotspot is where a jet from the nucleus hits a medium, producing a shock in which bulk-kinetic energy of the beam is converted to random motion. The energetic particles diffuse from the hotspot to the lobes, producing a continuous supply of energy. They are interpreted as the “working surface” where the jets terminate at a strong shock, which converts a significant fraction of the jet kinetic energy into relativistic particles.

 

       1.3               GALAXY

This can be seen as any number of large scale aggregates of stars, gas and dust that constitute the universe, containing an average of one hundred billion solar masses and ranging in diameter from 1500 – 300,000 light-years. The recognition that galaxies are independent star systems came from a study of the Andromeda galaxy (1926 – 29) by Edwin P. Hubble that indicated the great distances at which the Milky Way and other galaxies are located. Previously, the galaxies had been classified with luminous gas cloud or bright nebula within the Milky Way. The sun and its solar system, as well as the visible stars are all in the Milky Way galaxy. Billions of galaxies are within the optical range of the largest telescopes; in 1996 analysis of photographs taken from the Hubble space telescope, the estimated number of galaxies increased from 10 billion to 50

billion.

A galaxy is held together by the gravitational attraction between its constituent parts while its rotational motion prevents it from collapsing on itself. Just as gravitation binds individual stars into galaxies, it also acts to hold clusters of galaxies together. Many large galaxies have smaller galaxies called satellite galaxies. The galaxies nearest the Milky Way form a cluster called the local group. The local group includes the Andromeda galaxy, which is similar to the Milky Way and the magellanic clouds which are satellite galaxies of the Milky Way. The vast majority of observed galaxies are classified as either spiral or elliptical, with a small minority, example, the Magellanic Clouds, classified as irregular according to Hubble. Although estimates of the age of the Universe are controversial, if it takes as much as 15 billion years, then it is estimated that the first galaxies were formed 12.8 – 13.5 billion years ago.

Many galaxies radiate a large fraction of their energy in forms other than visible light. With the development of radio astronomy, many radio-galaxies were discovered. Other galaxies radiate strongly in the infra-red, ultraviolet, or x-ray parts of the spectrum.

       1.4               QUASAR

It is one of the class of blue celestial objects having the appearance of stars when viewed through a telescope and currently believed to be the most distant and most luminous object in the universe; the name is shortened from quasistellar radio sources (QSR). Quasars were discovered as the visible counterparts of certain discrete celestial sources of radio waves. Similar starlike objects that do not emit radio waves were subsequently discovered and named quasi-stellar objects (QSO). Although their visible light is faint, the quasars are optically brighter than the galaxies with which radio sources had been identified before 1963. Before their spectra were studied carefully, it was believed that quasars were stars in our galaxy. However, the lines in their spectra have enormous red shifts that seem to imply that they were receding from the Milky Way with speed as great as 95% of the speed of light. Only shift toward the red end of the spectrum have been observed for quasars; blue-shifted ones that would indicate a quasar approaching our galaxy have not yet been found. If quasars were simply objects been ejected from near-by galaxies at high speeds and not the distant objects they appear to be, then some would have blue-shifts. If Hubble’s law for the expansion of the Universe is directed to include quasars, they would be many billion light-years away and consequently as luminous intrinsically as 1000 galaxies combined. To account for such brilliant light, astronomers believed that quasars are supper-massive black holes in galactic nuclei, releasing energy by the accretion of matter through a rotating viscous disk.

       1.5                ACTIVE GALACTIC NUCLEI (AGN)

An Active Galactic Nuclei (AGN) is an object in the center of a galaxy whose spectrum cannot be explained just by starlight. It is a compact region at the Centre of a galaxy that has a much higher than normal luminosity over at least some portion, and possibly all of the electromagnetic spectrum. A galaxy hosting an AGN is called an active Galaxy. The radiation from AGN is a result of accretion of mass by a supper massive black hole at the Centre of its host galaxy. AGNs are the most luminous persistent sources of electromagnetic radiation in the Universe, and as such can be used as a means of discovering distant objects.  The light emitted by the nucleus equals (Seyfert galaxy) or even exceeds Quasi-Stellar Object (QSO), the total emission of the rest of the galaxy. This led to the idea that QSOs were “stars with peculiar properties” (Ajit et al., 1999) as they looked like stars (they were point sources) but their spectra showed unusual properties for starlight.

       1.5.1             TYPES OF ACTIVE GALACTIC NUCLEI

It is convenient to divide AGN into two classes, conventionally called radioquiet and radio-loud. In the radio-loud objects, the emission contribution from the jet(s) and the lobes that they inflate dominates the luminosity of the AGN, at least at radio wavelengths but possibly at some or all others. Radio-quiet objects are simpler since jet and jet-related emission can be neglected. AGN terminology is often confusing, since the distinctions between the different types of AGN sometimes reflect historical differences in how the objects were discovered or initially classified, rather than real physical differences.

Extended

Radio Sources

Compact Radio

Sources

Serfert I qalaxies or

Radio quiet quarsars

Serfert II Galaxies

 

Figure 1.0: A schematic classification of AGN

 

       A                 Radio-quiet AGN

1. Low Ionization Nuclear Emission-line Regions (LINERs); As the name suggests, these systems show only weak nuclear emission-line regions, and no other signatures of AGN emission. It is debatable whether all such systems are true AGN (powered by accretion on to a supermassive black hole). If they are, they constitute the lowest-luminosity class of radio-quiet AGN. Some may be radio-quiet analogues of the low-excitation radio galaxies.
2. Seyfert galaxies;Seyferts were the earliest distinct class of AGN to be identified. They show optical range nuclear continuum emission, narrow and occasionally broad emission lines, occasionally strong nuclear X-ray emission and sometimes aweak small-scale radio jet. Originally they were divided into two types known as Seyfert 1 and 2: Seyfert 1s show strong broad emission lines while Seyfert 2s do not, and Seyfert 1s are more likely to show strong low-energy X-ray emission. Various forms of elaboration on this scheme exist: for example, Seyfert 1s with relatively narrow broad lines are sometimes referred to as narrow-line Seyfert 1s. The host galaxies of Seyferts are usually spiral or irregular galaxies.

• Radio-quiet quasars/QSOs; these are essentially more luminous versions of

Seyfert 1s: the distinction is arbitrary and is usually expressed in terms of a limiting optical magnitude. Quasars were originally ‘quasi-stellar’ in optical images as they had optical luminosities that were greater than that of their host galaxy. They always show strong optical continuum emission, X-ray continuum emission, and broad and narrow optical emission lines. Some astronomers use the term QSO (Quasi-Stellar Object) for this class of AGN, reserving ‘quasar’ for radio-loud objects, while others talk about radio-quiet and radio-loud quasars. The host galaxies of quasars can be spirals, irregulars or elliptical. There is a correlation between the quasar’s luminosity and the mass of its host galaxy, in that the most luminous quasars inhabit the most massive galaxies

(elliptical).

       B                 Radio-loud AGN

1. Radio-loud quasars behave exactly like radio-quiet quasars with the addition of emission from a jet. Thus, they show strong optical continuum emission, broad and narrow emission lines, and strong X-ray emission, together with nuclear and often extended radio emission.
2. “Blazars” (BL Lac objects and OVV quasars) classes are distinguished rapidly variable, polarized optical, radio and X-ray emission. BL Lac objects show no optical emission lines, broad or narrow, so that their redshifts can only be determined from features in the spectra of their host galaxies. The emissionline features may be intrinsically absent or simply swamped by the additional variable component. In the latter OVV quasars behave more like standard radioloud quasars with the addition of a rapidly variable component. In both classes of source the variable emission is believed to originate in a relativistic jet oriented close to the line of sight. Relativistic effects amplify both the luminosity of the jet and the amplitude of variability.

• Radio galaxies: These objects show nuclear and extended radio emission. Their other AGN properties are heterogeneous. They can broadly be divided into low-excitation and high excitation classes. Low-excitation objects show no strong narrow or broad emission lines and the emission lines they do have may be excited by a different mechanism. Their optical and x-ray nuclear emission is consistent with originating purely in a jet. By contrast, high excitation objects

(narrow-line radio galaxies) have emission line spectra similar to those of Seyfert 2s. The small class of broad-line radio galaxies, which show relatively strong nuclear optical continuum emission probably, includes some objects that are simply low-luminosity radio-loud quasars. The host galaxies of radio galaxies, whatever their emission-line types are essentially always elliptical. In

Table 1.1, we present the classification of AGN.

This classification is orientation and objects show overlap. The line of sigth decreases to the right of the Table. Luminosity increases from top to bottom.

Based on Urry & Padovani (1995) and Krolik (1990).

Table 1.1 – Classification of AGN.

Type 2                    Type 1                             Type 0

Radio quiet LINER Seyfert 2      Seyfert 1

Radio quiet quarsars

Radio loud    NLRG FR I             BLRG                              BL Lacs/Blazars

NLRG FR 11            Lobe dominated quasars Core dominated Quasars

 

 

 

1.5.2 UNIFICATION OF AGNs

The Unification model is currently the most accepted model to explain the different AGNs. It says that Type 0, 1 and 2 AGN, with the same radio loudness, might be the same phenomenon, but seen with different orientations

(Barthel 1989; Urry & Padovani1995).

Table 1.1 shows schematically how the type of AGN depends on the orientation towards the observer. Astronomers are also trying to understand what produces the difference between radios loud and radio quiet AGN and attempting to unify them according to their radio loudness. A very interesting model explains the difference in loudness with the spin of the central black hole, where fast spinning (Wilson & Willis 1980) central black holes would produce radio AGN while slow spinning central black holes would produce a radio quiet AGN. However, x-ray observations (Ajit et al., 1999) suggest that the majority of super-massive black holes rotate rapidly. Furthermore, the evolution of massive black holes in galactic nuclei is still not well understood and the difference between radio loud and radio quiet AGN is more complicated.  Other phenomena must be taken Into account,  such as the size of the accretion disk, mass-to-energy conversion efficiency of the AGN, merging rates, size and angular momentum of the black hole (Peterson 2000). The physics are still not completely understood, but the current, most accepted model for the AGN consists of a super-massive black hole in the center, surrounded by a small accretion disk (<1kpc), which is responsible for the huge amount of radiation we observe (Perley, 1998). The accretion disk widens in the outer regions, forming a torus (at 1 to a few tens of parsecs) that surrounds the whole AGN  as the matter falls into the central hole, some of it will escape from the

disk forming jets (which can extend for mega parsecs in the largest radio sources). The inner cloud (1 parsecs) of the host galaxy form the broad line region (BLR) as the gas rotates faster closer to the black hole and the Doppler effect widens the emission line of these clouds, and the outer clouds form the narrow line region (NRL) which can extend for a few kpc. Although the basics of the model are probably correct, recent observations (e.g. Garrington et al, 1988) suggest that some modifications must be made. The accretion mechanism could be more complicated than just a simple thin disk (e.g. Gopal-Krisha & Witta, 2004), the torus scenario is slowly being substituted by matter dragged by out flowing winds and creating the obscuring region. This wind could also affect the emission mechanisms of the inner regions (Elvis 2006). The Unification model still has flaws beyond these corrections. While it explains most of the observed AGN, there are exceptions. Some of  these exceptions can be explained as special cases where the source lacks a BLR, a special distribution dust, etc. However, the unification model does not consider factors that should be important in the life of AGN and their host galaxy, such as age, evolution and interaction with other objects

       1.6                PURPOSE OF THE STUDY

The purpose of this study is to determine the spectral index of components of Compact Steep Spectrum Sources, and to know if they are symmetric or not.

SPECTRAL INDEX ASYMMETRY IN COMPACT STEEP SPECTRUM SOURCES

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