active galaxies-a comprehensive study

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A PROJECT REPORT ON

ACTIVE GALAXIES: A COMPREHENSIVE STUDY

Prepared in partial fulfilment of the requirements of the

COURSE:- Astrophysics

COURSE NO.:- PHY C471

BIRLA INSTITUTE OF TECHNOLOGY AND SCIENCE,

PILANI K.K. BIRLA GOA CAMPUS

SEM II 2011-12

Submitted by

Ankit Asati (2008B3A8600G)

Tarun Barange (2008A3PS174G)

Vakul Saxena (2008B5A3319G)


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Preface

Project Title:

ACTIVE GALAXIES: A COMPREHENSIVE STUDY

Project Objectives:

To study Active Galaxies To theorize AGN and its components To make observations about Active Galaxies To identify current problems in this area To predict the future course of this research field


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Acknowledgement

We would like to extend our heartfelt gratitude to our instructor and mentor Dr.

P.K Das & Mr. T. K Jha for having constant faith in us throughout this project and

directing and supporting us in every possible way at each step.

Above all, we thank each and every one of those who have been instrumental in t

successful compilation and presentation of this report.


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TABLE OF CONTENTS

Preface 2

Acknowledgement 3

Abstract 5

1. Introduction 62. Theory of Active Galactic Nuclei 83. Types of Active Galaxies 174.

Observations and Measurement 26

5. Problems, Current and Future Trends 296. Conclusion 32

References 33


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Abstract

Active Galaxies were discovered in 1950s. Since then they have become a very

valuable tool for both cosmologist and physicts. By studying the Active Galaxies,

the scientists are uncovering extraordinary information about the early universe.

This project attempts to summarize some of the results of observations of active

galaxies and discuss its implications. The report starts with a brief introduction

and history of Active Galaxies. This is followed by the theory of AGN (Active

Galactic Nuclei). Thereafter, various types of active galaxies are explained. The

next segment discusses observational results made in past few years. Lastly, the

report discusses some of the current problems and unresolved issues about AGN

with further scope for research.


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1. INTRODUCTION

Active galaxies are galaxies which have a small core of emission embedded in an

otherwise typical galaxy. This core may be highly variable and very bright

compared to the rest of the galaxy. Models of active galaxies concentrate on thepossibility of a supermassive black hole which lies at the centre of the galaxy. The

dense central galaxy provides material which accretes onto the black hole

releasing a large amount of gravitational energy. Part of the energy in this hot

plasma is emitted as x-rays and gamma rays.

Fig.1:- A relativistic jet produced by M87, a giant Active Galactic Nucleus (AGN)

in the Virgo constellation

For "normal" galaxies, the total energy emitted by them can be thought of as the

sum of the emission from each of the stars found in the galaxy. For the "active"

galaxies, this is not true. The amount of energy emitted by such a galaxy is much

more than it should be, and this excess energy is found in the infrared, radio, UV,

and X-ray regions of the electromagnetic spectrum.


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1.1 Discovery

The issue of the high activity of galactic nuclei was first raised by the Soviet-

Armenian physicist Prof. Victor Ambartsumian in the early 1950s. Although the

idea concerning the activity of galactic nuclei was initially met with scepticism, itdid gain recognition as a result of a substantial amount of observational evidence

corroborating the same. The details of the discovery of various types of active

galaxies will be discussed in a later section.

Fig.2:- Instructional Figure showing various types of Active Galaxies depending

upon their viewing angle


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2. THEORY OF ACTIVE GALACTIC NUCLEI

It is generally believed that AGN must be powered by accretion onto

supermassive black holes situated at the center of the galaxies (whose masses

range from 106

to 109

times that of the Sun).

Gas accretingonto the black holereleases a large amount of gravitational potential energy:

Where is the efficiency and is the accretion rate.

Both the efficiency and the accretion rate vary across orders of magnitude

between different galaxies.

Fig.3:- A schematic of showing the accretion of stellar material around the black

hole at the galactic center

The above schematic shows 3 most important components of an Active Galaxy,

namely the Central Black Hole, Jetand theAccretion Disk. These are discussed

on the next page:


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2.1 Black Hole

Black holesare completely specified by their mass M, angular momentum J, and

charge Q (likely ~zero).

The mass of the black hole can be measured by Gas disk kinematics:

At the same time the radius of black hole can be measured by Stellar kinematics.

If the velocity dispersion in the galaxy is , black hole will dominate motion of

stars with a radius rBH, the black holes sphere of influence, given by:

On the basis of above parameters, the black holes are classified into the following

categories:

a)Q=0, J=0: Schwarzschild black holeThese are spherically symmetric and the solution has two important radii for us:

[i] Schwarzschild radius:- The black holes event horizon is located at this

distance from the singularity at its centre. It is calculated using the expression:

No matter, radiation or information can propagate outwards through this radius.

[ii] Least Stable Circular Orbit:- It is calculated using the expression:

Outside Rms, test particles can orbit indefinitely in stable circular orbits, whereas

inside Rms orbits are unstable, particles spiral rapidly past the event horizon and

into the black hole. Thus, Rms defines the inner edge of the gas disk in AGN andsets a minimum orbital period. Roughly,


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b)Q=0, J and M arbitrary: Kerr black holeThis kind of Black hole has a preferred rotation axis, implied by its axisymmetric

solution.

Since this is a spinning black hole, we can define the amount of angular

momentum via a dimensionless spin parameter:

Here the maximum value of spin parameter a can be 1 which corresponds to themaximum angular momentum. The Kerr hole cannot spin beyond this limit.

2.1.1 Eddington Limit:- For an AGN with an observed (bolometric) luminosity L,

we can estimate the minimum mass of the black hole involved and this mass is

called the Eddington Limit.

Suppose the gas around the black hole is spherically symmetric and fully ionized

hydrogen, then at distance r, the flux is:

This is flux of energy. Since momentum of a photon of energy E is E/c, momentum

flux due to radiation is:

This is the pressure that would be exerted on a totally absorbing surface at

distance r from the source.

But force exerted on the gas depends upon the opacity (i.e. the fraction of the

radiation absorbed per unit mass of gas).

Minimumforce is given by the absorption due to free electrons. This is given by

the Thomson cross-section:


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( )

Outwardradiation force on a single electron is:

Inwardforce due to gravity of a central point mass M is:

Setting Frad = Fgrav, and solving for L, we get:

This luminosity is known as the Eddington limit. It is the maximum luminosity of

a source of mass M, which is powered by spherical accretion of gas.

Conversely, if a source with observed luminosity L is radiating at the Eddington

limit, the mass would be:

This is a minimum mass - source could actually be radiating at much less than the

Eddington limit.

2.1.2 Fuelling Active Galactic Nuclei

AGN requires huge amount of power (1044 - 1046 erg s-1). But the question is from

where do these AGN gets the fuel to generate such an enormous power and how

fast must gas be supplied to the black hole to produce typical AGN luminosities?

To answer these questions we define the efficiency of the accretion process :

A mass m of gas at infinity has zero potential energy. Energy available if the gas

spirals in to radius r is:


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This is actually an upper limit - not all the potential energy will be radiated as the

gas falls in.

Actual efficiency of disk accretion onto a black hole is estimated to be:

Schwarzschild black hole: = 0.06

Kerr black hole: = 0.42

Standard estimate is= 0.1. By this, mass flow needed to sustain a black hole is:

2.2 Accretion Disk

Luminosity of AGN derives from gravitational potential energy of gas spiralling

inward through an accretion disk.Thus we derive the structure of the disk, and

characteristic temperatures of the gas.

a)Deriving Structure:

Gravitational acceleration in vertical direction is:

If the gas is supported against gravity by a pressure gradient, force balance in the

vertical direction gives:


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Assuming the disk is isothermal in the vertical direction with sound speed cs.,the

pressure is then:

Solve for the vertical structure, we get:

where h is the vertical scale height of the disk.

Here, h2 can be estimated as:

The thickness of the disk as a fraction of the radius is given by the ratio of the

sound speed to the orbital velocity. A disk for which (h/R)


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Redistribution of angular momentum within a thin disk is a diffusive process - a

narrow ring of gas spreads out under the action of the disk viscosity. With

increasing time:

Massall flows inward to small R and is accreted and

Angular momentumis carried out to very large R by a vanishingly small fraction

of the mass.

b) Deriving temperature

Consider gas flowing inward through a thin disk.

Potential energy per unit mass at radius R in the disk is:

Suppose mass dM flows inward distance dR. Change in potential energy is:

Half of this energy goes into increased kinetic energy of the gas. If the other half is

radiated, luminosity is:


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We divide by the radiating area, 22RdR to get luminosity per unit area.

Equate this to the rate of energy loss via blackbody radiation: This gives the radial temperature distribution as:

(

)

This is a correct dependence on mass, accretion rate, and radius, but has a wrong

prefactor. Thus we need to account for:

Radial energy flux through the disk (transport of angular momentum also

means transport of energy) and

Boundary conditions at the inner edge of the disk

Correcting for this, radial distribution of temperature is:

[ { }]

Where Rin is the radius of the disk inner edge.

Accretion produces a variety of physical phenomena. Some prominent examples

are:

Very high luminosity from a point source in the nucleus. The small physicalsize of the emission region allows rapid variability.

Broad spectral lines due to Doppler shift of gas orbiting the black hole. X-ray emission from high temperature plasma close to the black hole. Mechanical power in the form of outflows and jets from the central regions.


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In the most powerful AGNs, these phenomena dominate over starlight. What we

see in a particular system reflects both the accretion rate and the viewing angle

to the central angle. This is the basis for the classification of active galaxies into

various categories, which are described in the next section.


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3. TYPES OF ACTIVE GALAXIES

There are several types of active galaxies: Seyfert, Quasars and Blazars. Most

scientists believe that, even though these types may look very different to us, they

are actually the same thing viewed from different directions.

Quasars are active galaxies which are situated at a great distance from our own

Milky Way galaxy. Some of the observed quasars have been reported to be about

12 billion light-years away.

Blazars emit high intensity of radiation in the radio frequency range, which led

scientists to conclude that they are AGNs whose jets point in the general direction

of the earth. This accounts for the rapid variability and compact features of

blazars.

Contrary to blazars, if the jet does not point towards the earth at all, and the

dusty disk of material lying in the plane of the galaxy is in the way, we would

observe the characteristics of the Seyfert galaxies.

By redshift measurements, it has been concluded that Seyfert galaxies are much

closer to the earth than Quasars or Blazars.

The discovery and characteristics of each type of active galaxy are now describedas follows:

3.1 Seyfert Galaxies: Discovery and Characteristics

3.1.1 Discovery

The class of Seyfert galaxies was first recognized by Carl Seyfert in a 1943 paper,

which discussed the set of (mostly spiral) galaxies whose spectra showed

unusually broadened emission lines from bright, star-like nuclei. In retrospect,

these were hints that large masses might be involved, to produce such high gas

velocities without spraying the material right out of the galaxy, and that the

phenomenon was concentrated in a small volume, thus giving the star-like

appearance to the nuclei.

3.1.2 Characteristics

While Seyfert nuclei had been occasionally observed earlier - in fact, NGC 1068was among the first few galaxies whose redshift was measured - this was the first

definition of a class of similar objects. Seyfert nuclei were divided into two classes


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by Ed Khachikian and Daniel Weedman, based on whether all their emission lines

had similar amounts of broadening.

In Type 1 nuclei, certain emission lines were much broader - the ones thatcould originate at the highest gas density.

In Type 2 nuclei, all lines have broadly similar widths. Both kinds havevery similar sets of emission lines seen, implying the simultaneous

existence of atoms in states normally associated with a huge range of

density, temperature, and incident radiation.

Fig.4:- Example spectra of Type I and Type II Seyfert Galaxies

3.2 Quasars: Discovery and Characteristics

3.2.1 Discovery

The discovery of quasars was the result of an investigation into their anomalous

characteristics. A few strong radio sources stubbornly resisted identification with

any obvious visible-light counterpart until positional accuracies from radio

observations reached only a few seconds of arc. Some radio sources appeared to

be nothing more than galactic stars, but their spectra were very peculiar, with

strong, broad emission features at wavelengths that didn't match any plausible

features expected from stars - young, old, or exploding. It took some time for

Maarten Schmidt at Palomar Observatory to show that these were indeedfamiliar spectral features, but redshifted to an unprecedented degree. The name


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quasi-stellar radio source (soon shortened to quasar) was coined for these

enigmatic objects. As it turned out, many similar objects are not strong radio

sources, and these are distinguished as quasi-stellar objects (QSOs), though both

are often lumped together as quasars. Quasars enable us to gain new insights into

the phenomenon of active nuclei. To be so bright at the large distances implied bytheir redshifts, they had to be much more luminous than any ordinary galaxies -

hundreds of times brighter. Yet they must be tiny, with most of the light coming

from a region no larger than our own solar system. This was found from the fact

that quasars vary (in both visible light and radio core output) in timescales so

short that the object responsible (which cannot be any larger than the distance

light travels in this timescale) must by only a light-day or so in size.

Fig.5:- The X-ray image of a quasar PKS 1127-145, located 10 billion light-years

away from earth


Quasars are the most luminous class of AGN. They constitute a small fraction

(about 5-10%) of the total AGNs, characterized by strong radio emission. The

emission from the galactic nucleus dominates the light coming from the host

galaxy.


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(a) (b)

Fig.6:- Images of two distant Quasars in (a) a spiral host, and (b) an elliptical host

using HST imaging

Their spectra are very similar to those of the Seyfert galaxies, except that:

Stellar absorption lines are very weak, if detectable at all. Quasars can all be classified as Type 1 in Seyfert jargon, i.e. their spectra

consist of broad lines too.

Fig.7:- A Typical Quasar Spectrum


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3.2.2.1 Spectral Energy Distribution in Quasars

Quasars have a very broad spectral energy distribution (SED) which cannot be

described as blackbodies.

Their flux can be characterized roughly by the following power-law:

Where:

is the power-law index

C is a constant

is the specific flux (i.e. per frequency interval, units of erg s-1 cm-2 Hz-1)We will integrate to get the power between frequency 1 and 2:


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3.3 Blazars: Discovery and Characteristics

3.3.1 Discovery

A new member in the AGN family was added in the late 1970s, with theidentification of a few mysterious objects from variable-star catalogues as highly

variable nuclei of distant galaxies. Named after their prototype, such BL Lacertae

objects have almost perfectly featureless spectra - the nucleus produces a smooth

rainbow of radiation, which can be bright enough to swamp the surroundinggalaxies and has no tell-tale emission or absorption lines to measure its redshift.

Redshifts have been measured, either from the surrounding galaxy or by waiting

for the object to appear unusually dim so it doesn't drown out the emission lines

from surrounding gas.

Fig.8:- A schematic figure showing the structure of a Blazar


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BL Lac objects are most notable for being strongly and rapidly variable at all

wavelengths, in both intensity and polarization. Their properties are usually

thought to reflect our viewing the jet of a radio galaxy almost along its own axis,so our view is dominated by Doppler-boosted radiation from the jet rather than

the more usual view of the nucleus and its surroundings. Some quasars with

unusually weak emission lines share some of these variability properties as well,

so they and BL Lac objects may be lumped together as blazars.

Fig.9:- Time varying spectrum of a BL Lac object

Blazars produce light at all wavelengths, ranging from being strong radiosources all the way to being sources for gamma-rays.

By observing them for long periods, it was noted that they tend to havevariable brightness at different wavelengths (got brighter and fainter overtime).

Observations of their spectra show there are virtually no emission orabsorption lines - this is a fairly unusual characteristic for a galaxy, especially

an Active Galaxy. The radiation coming from these objects is known to be non-thermal (not

related to heat). The spectra do look rather uniform and flat, close to being

continuous spectra. The optical light from these objects is also highly

polarized, indicating the influence of a strong magnetic field.


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Fig.10:- A typical spectrum of a BL Lac object

3.4 Radio Galaxies

3.4.1 Discovery

After World War II, radio telescope technology enabled us to gain further insight

into the nature of active galaxies. What looked like normal galaxies with visiblelight telescopes looked really strange with radio telescopes. In some cases,

galaxies were producing more light at radio wavelengths than at the visible light

wavelengths. Galaxies that exhibit such anomalous features at radio wavelengths

are called Radio Galaxies.

Fig.11:- Radio image showing two jets shooting out of the center of the active

galaxy Cygnus A


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Radio galaxies are the type of active galaxies that are very luminous at radio

wavelengths, with luminosities up to 1039 W between 10 MHz and 100 GHz.

It is convenient to divide Radio Galaxies into two types, conventionally called

radio-quiet and radio-loud. In the radio-loud objects a contribution from the

jet(s) and the lobes 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.

A brief comparison between the two is summarized below:

Radio Loud Radio QuietContains a high spin supermassive

black hole at the centre.

Contains a low spin supermassive black

hole at the centre.

Produce relativistic jets, which are

origins of radio emission.

Do not produce any relativistic jets.

Jets are powered by spin energy

extracted from black hole.

Jets are produced by accretion disk

(blackbody plus non-thermal coronal

emission)

Example: Cen A, M87 Example: Circinus

Fig.12:- Cen A, as seen in a radio(orange) and visible composite image


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4. OBSERVATIONS AND MEASUREMENT

AGN are multiwavelength emitters. To understand the physical processes at work

in the AGN environment it is crucial to characterize AGN emission/absorption

throughout the entire wavelength spectrum, from radio wavelengths to X-rays,i.e. the AGN spectral energy distributions (SEDs). This requires observing them

with many telescopes. One of the main tools for analyzing the SEDs of AGN is the

comparison of SED models (templates) with observations.

Most observational work on the AGN has been carried out on the type 2 sources

Seyfert 2 rather than Seyfert 1 and radio galaxies rather than quasars for the

simple reason that a direct view of the AGN in type 1 objects results in an

excessive glare that makes it difficultto see faint extra nuclear structures. The

presence of thick obscuration close to the nucleus and in our line-of-sight in type

2 sources results in a very effective natural coronagraph.

Fig.13:-A typical picture of Active Galaxy taken by HST (Hubble Space Telescope)


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4.1 Observational CharacteristicsThere is no single observational signature of an AGN. Some of the historicallyimportant features that have allowed systems to be identified as AGN are: Nuclear optical continuum emission. This is visible whenever we have a direct

view of the accretion disc. Jets can also contribute to this component of the

AGN emission. The optical emission has a roughly power-law dependence on

wavelength.

Nuclear infra-red emission. This is visible whenever the accretion disc and itsenvironment are obscured by gas and dust close to the nucleus and then re-

emitted ('reprocessing'). As it is thermal emission, it can be distinguished from

any jet or disc-related component.

Broad optical emission lines. These come from cold material close to the centralblack hole. The lines are broad because the emitting material is revolving

around the black hole with high speeds, emitting photons at varying Doppler

shifts. Narrow optical emission lines. These come from more distant cold material,

and so are narrower than the broad lines.

Radio continuum emission. This is always due to a jet. It shows a spectrumcharacteristic of synchrotron radiation.

X-ray continuum emission. This can arise both from a jet and from the hotcorona of the accretion disc via scattering processes: in both cases it shows apower-law spectrum. In some radio-quiet AGN there is a `soft excess' in the X-

ray emission in addition to the power-law component. The origin of the soft

excess is not clear at present.

X-ray line emission. This is a result of illumination of cold heavy elements bythe X-ray continuum. Fluorescence gives rise to various emission lines, the

best-known of which is the iron feature around 6.4 KeV. This line may be

narrow or broad: relativistically broadened iron lines can be used to study the

dynamics of the accretion disc very close to the nucleus and therefore the

nature of the centralblack hole.

4.2 Unified Model ConceptUnified models of AGN unite two or more classes of objects, based on the

traditional observational classifications, by proposing that they are really a single

type of physical object observed under different conditions. The currently

favoured unified models are 'orientation-based unified models' meaning that

they propose that the apparent differences between different types of objects

arise simply because of their different orientations to the observer. This

unification is broadly classified into 2 types:


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a) Radio-quiet unification: At low luminosities, the objects to be unified areSeyfert galaxies. The unified models propose that in Seyfert 1s the observer

has a direct view of the active nucleus. In Seyfert 2s it is observed through

an obscuring structure which prevents a direct view of the optical

continuum, broad-line region or (soft) X-ray emission. The key insight of

orientation-dependent accretion models is that the two types of object can

be the same if only certain angles to the line of sight are observed.

At higher luminosities, quasars take the place of Seyfert 1s, but, as already

mentioned, the corresponding 'quasar 2s' are elusive at present. If they do

not have the scattering component of Seyfert 2s they would be hard to

detect except through their luminous narrow-line and hard X-ray emission.

b)Radio-loud unification: These galaxies can be unified with narrow-lineradio galaxies in a manner directly analogous to the Seyfert 1/2 unification

(but without the complication of much in the way of a reflection

component: narrow-line radio galaxies show no nuclear optical continuum

or reflected X-ray component, although they do occasionally show

polarized broad-line emission). The large-scale radio structures of these

objects provide compelling evidence that the orientation-based unified

models really are trueHowever, the population of radio galaxies is completely dominated by low-

luminosity, low-excitation objects. These do not show strong nuclear

emission lines broad or narrow they have optical continua which

appear to be entirely jet-related and their X-ray emission is also consistent

with coming purely from a jet, with no heavily absorbed nuclear

component in general.

Fig.14:- Unification of Active Galaxies

by viewing angle.


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5 PROBLEMS, CURRENT AND FUTURE TRENDS5.1 Problems

Despite many years of effort, observational studies have not found a strongcorrelation between the presence of any proposed fueling mechanism and

low-luminosity AGN.

The unexplained FeII spectrum is one of the unsolved problems ofAGNstudy.

5.2 Current TechnologyThe most recent contribution to the study has come from high spatial resolution

X-ray maps of the nucleus, the jets and radio hot-spots and the surroundingcluster environment. Detailed studies of Cygnus A with Chandra ACIS show what

can be achieved .This is shown in the following figure:

Fig.15:- An X-ray image of Cygnus A with the CHANDRA ACIS instrument.

The principal components in above figure are:

thermal X-ray emission from the intracluster gas a limb-brightened cavity containing the relativistic gas fed by the jets


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synchrotron self-Compton emission from the radio hot-spots X-ray emission from the jets themselves, soft emission from an extended nuclear source and Hard emission from an unresolved nuclear source.

The extended nuclear source, on a kilo parsec scale, appears to result from the

Thompson scattering of nuclear X-ray photons in a very highly (photo) ionized

gas extending along the radio axis. The Thompson optical depth, while it can

explain the X-rays, is too small to produce the extended optical polarization

which must, therefore result from dust scattering. The spatial distribution of this

soft X-ray component appears to match very closely the emission from the

extended coronal lines of [Fe X], [Fe XI], [Ar XI] and [S XII]. This is strong

evidence in favor of the photoionization model for the production of the coronal

line spectra in AGN in gas with a very high ionization parameter.

5.3 Future ScopeDue to rapid advances in infrared and other detector technology, the

development of adaptive optics for ground based work and the commitment to

infrared missions from space organizations such as NASA, ESA and ISAS, thefuture of AGN study is extremely bright. Some of the proposed astronomical

projects in future are:

a)The James Webb Space Telescope

Fig.16:- A Schematic of a JWST.


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b)Infrared High Angular Resolution Technology: VLTI/MIDI with PRIMA-FSU K-band tracking, lead: Pott (2012) VLTI/MATISSE (2ndgen. instrument, Co-I Th. Henning) (2016) LBT/LINC-NIRVANA (strategic instrument, PI T. Herbst) (2014)


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6 CONCLUSIONFor a long time, active galaxies held all the records for the highest-redshift

objects known, because of their high luminosity (either in the optical or the

radio): they still have a role to play in studies of the early universe, but it is now

recognised that by its nature an AGN gives a highly biased picture of the 'typical'

high-redshift galaxy.

As far as this project is concerned, we have tried to present a comprehensive

study of Active Galaxies, quantitative as well as qualitative. The study helped us

gain insight into one of the most mysterious phenomena observed in the universe

and has motivated us to continue our research into the same.


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REFERENCES

(1) Croton D.J., et al. (2006) The many lives of active galactic nuclei: coolingflows, black holes and the luminosities and colours of galaxies. MNRAS

365: 1128.(2) Sikora M., Madejski G.M., (1996) Learning about Active Galactic Nucleusjets from spectral properties of blazars. ApJ 484: 108117.

(3) Active Galaxies and Quasars Introduction and Glossary. URL:-http://www.astr.ua.edu/keel/agn/text.html.

(4) Vlahakis, N., Knigl, A., The Astrophysical Journal, 605, 656 (2004).(5) Active Galaxies and Quasars Introduction. URL:- imagine.gsfc.nasa.gov/

docs/science/know_l1/active_galaxies.html(6) Active Galaxy. URL:- www.astro.virginia.edu/~jh8h/glossary/

activegalaxy.htm(7) Ferrarese, L., Ford, H.C., (1996) Evidence for a massive black hole in theactive galaxy NGC 4261 from Hubble Space Telescope images and spectra.The Astrophysical Journal, v.470, p.444-459.

(8) Neufeld, D.A., (1995) The mass accretion rate through the masing

molecular disk in the active galaxy NGC 4258. Astrophysical Journal

Letters, v.447, p.L17-L20.

(9) Oliva, E., Origlia, L., Maiolino, R., and Moorwood A.F.M., Starbursts in activegalaxy nuclei: observational constraintsfrom IR stellar absorption lines.

URL:- http://arxiv.org/pdf/astro-ph/9908063.pdf.

active galaxies-a comprehensive study

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