Space Phenomenon

1.6 Quasars and Distant Objects.
Quasar From Quasi-Stellar Radio Source:

Quasars are extremely massive objects believed to contain the mass of entire galaxies focused into extremely dense regions (see singularity). As such quasars are the most luminous objects in the known universe. Their name is a vulgarism of quasi-stellar-objects. To discuss them fully it is necessary to appreciate something about their distance, coupled with the effect distance and speed have on the frequency of light received.

 

Image From NASA homepage http://www.nasa.gov/newsinfo/research.html

The story of Quasar discovery really needs to begin with the work in 1912 of Vesto M. Silpher who discovered that most of the galaxies are receding from our own. Edwin Hubble and Milton Humason later established that the amount of redshift increases with the faintness of the galaxy. This correlation became known as Hubble’s law. A redshift is due to the relative velocity of separation between a receding source in the galaxy and the earth along a line connecting those two points. To understand redshift and the doppler effect it is necessary to be familiar with the basic principles that govern the electromagnetic spectrum. The waves obey the general form:

c = vλ

where 

c is the speed of light essentially a constant 2.997 925 *108 ms-1 

v is the frequency of the radiation

λ is the wavelength of the radiation, that is the distance between successive troughs or peaks on the wave.

 

The change is wavelength of a receding target is given by the formula:

D l =V/C l

The recession of a source weakens the amount of radiation received by altering the number of photons and the total energy of the individual photons.

Silpher showed that eleven out of fifteen observed spiral nebulae now known to be galaxies had red shifts. The ratio is actually far higher with only relatively few galaxies within the local group showing blue shifts. Therefore most Galaxies are receding and it appears that the more distant a galaxy the greater it's speed of recession. For objects as distant as quasars there speed of recession is so great that relativistic effects come into the equation. For speeds approaching that of light the redshift is related to distance and velocity by:

Fv = (1+V/C/1-V/C) 1-V/Z.

This is very important because as we look at greater distances we also look at greater velocities. As we move out of the local-group, we begin encountering very old objects that are receding exceptionally fast. That is, with speeds that tend towards c. Quasars are among these distant and old objects and they are also among the most luminous objects in the known universe. The first quasar was discovered in 1962 and was 3C-273 in Virgo. The 3C prefix used here and later refers to the Third Cambridge Catalogue. 3C 273 is the nearest quasar to our own Galaxy, it has a red shift z, of 0.158 (Moore 1995) which from Hubble’s law gives

z=Hd/c or z = L-Lo/Lo

A distance using an approximation of the speed of light as 2.997 925 *105 kms-1and a Hubble constant approximation of 42 kms-1 Mpc-1 we would obtain a distance of. 1127.79 Mpc. For these kinds of distances, the Hubble Law relationship is very useful. The most distant quasar known is PC 1247 + 3406 where z = 4.897. (Roy and Clarke (b) 1988). It now seems likely that BL Lacertœ objects are the same objects as Quasars just with BL Lacertœ objects orientated so we look down the ejecta jet. (Roy and Clarke (b) 1988)In 1963 Maarten Scmidtat Palomar Observatory in California measured the redshift of a faint object 3C273. The readings were so high that they were neither due to gravitational or pure recession but were coupled with the expansion of the universe.

Quasars are one of the main targets of the Sloan Digital Sky Survey, which began in May 1998. The survey is a joint effort by a number of institutions and is based at the Apache Point Observatory near Sunspot, New Mexico. Some of the more recent work was published in December 1998. The team expect to examine as many as 100,000 quasars during the survey and hope to learn more about the origins of these phenomena and other aspects of the universe. The ultimate goal of the survey is to map one-fourth of the sky and produce a three-dimensional picture of the universe 100 times larger than previous renderings. The survey data will give researchers a large-scale view of the universe to help test various theories about how it evolved.

http://encarta.msn.com

Singularity Image From NASA homepage http://www.nasa.gov/newsinfo/research.html

Unfortunately, the key in these equations is the Hubble constant, after Edwin Powell Hubble (1889-1953). Estimates of the constant based on nearby objects such as Cepheids within the Virgo cluster using the Hubble Space Telescope tend to give a larger value than Sunyaez-Zel’dovich effect which looks further out into the universe. The Sunyaez-Zel’dovich measures distortions in the background radiation by clusters of galaxies. Gustav Tammann who favours a lower number for the Hubble Constant describes the reasons for attributing a lower number because of inherent errors in the calculations of objects at great distances. The Malmquist Effect states that as one looks further away one will only see the brighter galaxies giving a false impression of average brightness with distance. A number other than 42 for the above equation could cause the answer to be greatly changed. Alan Sandage and Gustav Tammann have given a value of H at 50km/s Mpc (±10 percent). Their method uses five distance measurements and they accept there could be systematic error in each. Unfortunately research by Marc Aaronson, John Huchra and Jeremy Mould have come up with an answer of 90 km/s Mpc (±5 percent). The debate still continues with the Hubble Constant and though recent research points to a lower rather than a higher value until this is resolved it is difficult to use the red shift as an absolute distance measurement. (P. Teerikorpi, 1997)

The term quasar is unfortunate because further research has shown that only 1 in 200 emit radio noise. We can use a common astronomical aid, that of brightness fluctuations, to measure their distance. A "typical" quasar measures 1 light day in diameter, not too different from the actual size of our solar system. However even though the body appears to be small at a given time it radiates an energy level some 1000 times that of the light from every star in our Galaxy. This is broken if a Galactic star undergoes super nova implosion/explosion as discussed elsewhere. It is likely that within the small area is a singularity of around 100 million solar masses. Such a massive mass warps space-time so as to swallow material from near by galaxies at the rate of 1 solar mass a year.

One of the most important factors to be remembered when discussing this phenomena is that a quasar is so distant it is also ancient. Observations using the HST in 1995 showed what appeared to be companion galaxies very close to the Quasar being distorted by the gravitational field. It is possible that the Quasar is the initial form of a galaxy before baryonic matter clusters to form what we see today. (Gribben 1996).

Emitted Radiation and Expelled Matter 

Astronomers have proposed two distinctly different mechanisms for galactic dynamos. The first was the proposed by Martin J. Rees of the University of Cambridge and Roger D. Blandford, now at the California Institute of Technology. During the early 1970s, the two sought to explain the prodigious luminosity--thousands of times that of the Milky Way--and the spectacular "radio jets" (highly focused streams of energetic material) that stretch over millions of light-years from the centers of some hyperactive young galaxies known as quasars. They suggested that an ultramassive black hole--not much larger than the sun but with perhaps a million times its mass--could power a quasar.

Black holes are not the only engines that are theorised to drive violent galactic events. Some galaxies apparently undergo short episodes of rapid star formation in their cores: so-called nuclear starbursts. The myriad new stars produce strong stellar winds similar to the Ttauri stage that all main sequence stars experience during initial instabilities before they move onto the main sequence proper. As these early stars are both massive and hot (classes O and B) they burn their fuel very rapidly producing short stable lifetimes and far more frequent of supernovae in the stellar clusters. The fast-moving gas ejected from the supernovae strikes the background interstellar dust and gas and heats it to millions of degrees.

The pressure of this hot gas forms a cavity, similar to a liedenfrost barrier whereby the cooler material forms an insulating shell trapping the hot material in the inner cavity. As the bubble expands, more cooler gas and dust accumulates in the dense outer shell at the edge of the bubble, slowing its expansion. The transition from free flow inside the bubble to near stasis at its boundary gives rise to a zone of turbulence that is readily visible from Earth. If the energy injected into the cavity is large enough, the bubble bursts out of the galaxy's gas disk and spews the shell's fragments and hot gas into the galaxy halo or beyond, thousands of light-years away from their origins.

Roberto Terlevich of the Royal Greenwich Observatory and his collaborators have led the most recent research aimed at determining whether starbursts alone can drive the outpourings of hot gas characteristic of active galaxies. In 1985 Terlevich and Jorge Melnick, now at the European Southern Observatory, argued that many such galaxies contain unusual stars they dubbed "warmers" believed to be extremely hot stars with temperatures higher than 100,000 degrees Celsius and very powerful stellar winds. The development of warmers is attribute. Terlevich and his colleagues contend that their model explains the spectra and many other properties of certain active galaxies.

 Both the starburst and the black-hole explanations appear plausible, but there are important differences between the two based on energy efficiency that may reveal which one is at work in a given galaxy. Objects pulled towards a singularity prior to crossing the Schwarzchild radius will accelerate to a very high speed and will collide with other matter. Due to the fact that velocity of the matter tends towards that of light, the kinetic energy available is comparable to the rest-mass energies (E=mc^2). The efficiency  of the energy conversion is based on the

A black hole can convert as much as 10 percent of the infalling matter to energy. Starbursts, in contrast, rely on nuclear fusion, which can liberate only 0.1 percent of the reacting mass. As a result, they require at least 100 times as much matter, most of which accumulates as unburned fuel. A black holes efficiency is dependant on its angular momentum, which is one of the few properties not lost when matter becomes a part of the singularity. Due to the twisting of space time at the horizon black holes are forbidden from rotating at a high speed. Their maximum efficiency though is the conversion of 42% of infalling matter into energy. This would contrast with a mere 6% for a static black hole and 0.1% for a novae/starburst. Over the lifetime of a starburst-powered quasar, the total mass accumulated in the nucleus of the galaxy could reach 100 billion times the mass of the sun, equivalent to the mass of all the stars in the Milky Way galaxy.

The more mass located near the nucleus, the more rapidly the orbiting stars must move. Recent ground-based near-infrared observations have revealed the presence of a dark compact object with a mass two million times that of the sun at the center of our own Milky Way. And recent radio-telescope findings have revealed an accretion disk with an inner radius of half a light-year spinning rapidly around a mass 20 million times that of the sun at the center of a nearby spiral galaxy called NGC 4258.

Cosmic rays of exceptional power have reached Earth from the outermost edges of the Universe, say an American physicist and a German astronomer. They have studied the five most energetic cosmic rays ever observed and conclude that they came from five extremely distant quasars.

 Current research into the cosmic rays emitted by very distant objects such as quasars, is suggesting a new form of particle may be created/emitted by their outbursts. Further to the workbegun in 1997  (This Week, New Scientist, 9 August 1997, p 18), an American physicist and a German astronomer have reported conclusions on the nature of exceptionally energetic cosmic ray events that, if true, strengthens theories that there may be a new subatomic particle

 “No known particle could have survived such a long journey,” says Glennys Farrar, one of the researchers, at Rutgers University in New Jersey.

The rays emitted by quasars may travel for over a billion years before being received on Earth, these particles while travelling through the ISM will collide with photons and lose energy. This gradual loss of energy means that particles with an energy greater than 1020 electronvolts should not theoretically be able to travel more than 150 million light years and any that originate farther away should not reach Earth. Yet Farrar and Peter Bierman of the Max Planck Institute for Radio Astronomy in Bonn have studied data on the rays, which all have energies greater than 1020 electronvolts. All appear to be coming from quasars more than 150 million light years away.

“In fact,” says Farrar, “one quasar, known as 1204+281, is about 12 000 million light years away, approximately 80 per cent of the way to the edge of the Universe.”

 

 Figure showing detected rays in area of restricted energy levels (from New Scientist 29 Aug 1998)

 It is difficult to explain how the cosmic rays could have crossed such distances to Earth. Farrar believes that the rays may contain a new particle. The particle is predicted by some so-called "supersymmetric" theories, which attempt to unify gravity with other forces of nature.

The particle is called the S0--a peculiar hybrid consisting of a normal up, down and strange quark bound to a gluino. In supersymmetric theories, every known particle has a "superpartner" and the gluino is the superpartner of the gluon. "An S0 would be neutral and less likely to interact with the big-bang photons," says Farrar. "It could easily travel across thousands of millions of light years."

All five quasars in the study are of a type known as "compact radio-loud". Such quasars are the most powerful sources of energy in the Universe.

“If any objects can accelerate cosmic ray particles to 1020 electronvolts and beyond, these quasars can,” says Farrar.

"It's a very exciting possibility," says David Hough of Trinity University in San Antonio. "We're talking about pieces of quasars--objects at the very edge of the Universe--actually falling to Earth."

It could be a coincidence that the rays seem to be coming from the five quasars, admits Farrar. "But we calculate that the chance of that happening is less than half of one per cent due to the rarity of the type of quasar involved." Farrar and Bierman have submitted a paper to the journal Physical Review Letters.

New Scientist, 29 August 1998

Gamma Ray bursts With Optical Counterparts

GRB

Constellation

X-ray

Transient

Peak Radio Strength

(microjanskys)

Red Magnitude

Redshift (z)

970228

Ori

Yes

 

25.2

 

970508

Cam

Yes

550

25.7

0.835

971214

UMa

Yes

 

25.6

3.418

980326

Pyx

No

 

25.3

 

980329

Aur

Yes

300

25.4

 

9805119

Cep

Yes

300

26.2

 

980613

UMa

Yes

 

24.5

 

980703

Psc

Yes

1,050

22.6

0.966

When examining radiation arriving at the earth, some of the highest energy readings are produced by Gamma Ray Bursts. It is becoming likely that these phenomenon may be produced by the debris expelled from a supernova in a hyperfast jet, according to a Princeton astronomer. His theory would also explain why some pulsars fly through space much faster than normal stars. Gamma-ray bursters, which exist at the edge of the Universe, are the source of the most powerful blasts of energy known. Though the energy emissions have always presented a problem for researchers, the discovery last year of X-ray and optical “afterglows” associated with these bursts suggests that they might be the fading fireballs of some kind of stellar explosion.

Now Renyue Cen of Princeton University in New Jersey is suggesting that these intense bursts of energy might come from a supernova that is expelling material far faster in one direction than in others. In a paper to appear in Astrophysical Journal Letters Cen speculates that some unknown process sweeps a path free of protons and neutrons, allowing the neutrinos formed within the exploding star to escape. Some of the neutrinos decay into electrons and positrons, forming a jet traveling at about 99.9994 per cent of the speed of light.

The electrons would emit light (radiation in a visible wavelength) as they interacted with magnetic fields. However their relativistic velocities would bring a doppler effect into play which would boost this radiation to gamma-ray frequencies, producing the burst that we observe. Cen’s theory would also explain why some pulsars are moving at up to 500 kilometres per second--tens of times faster than ordinary stars.

“A supernova jet propels a pulsar in the opposite direction just like the exhaust of a rocket,” says Cen.

“There is good reason to believe that at least one gamma-ray burster is associated with a supernova and I'm not surprised if high-velocity pulsars are associated with supernovae too,” says Andy Fabian of the University of Cambridge.

Proof could come if scientists observed a gamma-ray burst and a supernova going off together. But this will not be easy. “Unfortunately, supernovae are extremely faint and hard to spot at the typical distance of gamma-ray bursters,” says Cen. Cen has also theorised that all supernovae may produce superfast jets, but that we only see them as a gamma-ray burst when the jet points our way. Since several supernovae are known to go off every century in our Galaxy, he believes that every few hundred million years the Earth could be inline with the orientation of a supernova jet.

“The effect on life on Earth would be catastrophic and might well trigger mass extinctions,” says Cen.

Scientists have observed two very distinct forms of gamma-ray bursts. The first has very energetic or "hard" gamma rays. In the region of one to three bursts of this type are observed every day, always from different directions in the sky. It is still not precisely known what causes such periods of activity, however it is known that they originate from some of the most distant regions of the Universe ad  possibly from an extreme type of supernova, or colliding neutron stars or black holes.

Studying even higher-energy cosmic rays are very difficult if not impossible to observe due to the requirement of extremely large ground-based detectors. These detectors would need to overcome the problem of low flux by watching enormous effective areas for months or years to gather the required data. The information, however, must be extracted from cascades of secondary particles, electrons, muons and gamma rays, initiated high in the atmosphere by an incoming cosmic-ray nucleus. Such indirect methods can only suggest general features of the composition of a cosmic ray on a statistical basis, rather than identifying the atomic number of each incoming nucleus. 

When a photon's energy becomes large enough, it creates an avalanche of particles on penetrating the atmosphere. These particles then emit optical light that can be detected on the ground by large mirrored collectors such as Whipple in Arizona. Whipple currently detects particles of energy 300 GeV or higher. If it is upgraded, as planned, to VERITAS (Very Energetic Radiation Imaging Telescope Array System), the array will detect particles of energy as low as 100 GeV, closing the gap with the satellite data.

 

   

   Schematic View of The Creation of a Particle ‘Shower’ in the Atmosphere

  Although the Earth’s atmosphere prevents primary cosmic rays and gamma rays from reaching the sea level or even to the height of the tallest mountains there is a technique for the study of cosmic rays from the ground level. As the primary particles strike the atmosphere, their energy is transferred through subsequent collisions to a cascade of particles. As seen in figure 3. A gamma ray enters the atmosphere and splits into an electron-positron pair within approximately one interaction length. That is for particles interacting electromagnetically, such as gamma-rays, the interaction length is normally referred to as a radiation length. Within the next interaction length, the electron and positron will each radiate one bremsstrahlung photon which, in turn, will interact in the next radiation length, and so forth. The multiplication event stops when the particles fall below a critical energy ec. For an electromagnetic shower, that is one consisting mainly of photons, electrons and positrons, the critical energy is reached when the cross-section (that is a measure of the probability that a particular interaction will take place within a given area), for bremsstrahlung becomes smaller than for ionisation. In addition to the electromagnetic components the cascades will also include hadrons and muons

(Bergström and Goobar 1999)

As well as the heavy bursts there are also soft gamma-ray repeaters which are a much rarer component. The outbursts come from just a few places in the sky, although more than one burst is seen from each source, hence the name "repeater". Three SGRs were discovered in 1979, a fourth in 1997 and a fifth in 1997, work has continuued since then and the numbers are increasing. Their gamma rays are less energetic than those of the other type hence the name “soft”. Though the name may seem inappropriate given that they radiate as much energy as the Sun does in a year every second..

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