Are quasars powered by Supernovae?
Graeme Ing, 2006 Project 32, HET604
Abstract
In this paper, I
make a comparison of two of the leading models explaining quasar/AGN activity,
that using a central supermassive black hole as the energy source and that using
a starburst region with a high frequency of supernovae and compact supernova
remnants. In order to make the comparison, I summarise the observed properties
of quasars, and then describe how each model explains these properties. In
conclusion, I discuss the shortcomings and similarities of the models and state
that either model adequately supports quasar observations, and that further
research may show that the reality is a hybrid of both models.
1. Introduction
Quasars (Quasi
Stellar Radio Sources) are extra-galactic objects that despite their incredible
distances from Earth, shine with the luminosity of billions of times that of
the sun, making them the brightest objects in the universe. The earliest discovered
were “radio-loud”, radiating so strongly at radio wavelengths that they stood
out as bright beacons on radio maps of the sky. It was later determined that
there are almost ten times as many radio-quiet quasars as loud ones.
Once it became obvious that they
existed at the nucleus of remote galaxies, quasars were added to the family of
Active Galactic Nuclei (AGN), even though in many cases they so outshone their
host galaxy that it remained undetected until the advent of more modern
telescopes. The popular model of AGN postulates a supermassive black hole at
the centre of an active galaxy as the source of energy for the AGN or quasar.
A group of scientists led by Roberto
Terlevich developed a competing model that explained quasar activity by means
of starburst regions and high levels of supernova activity.
In this paper, I outline the details
of each model and compare them to determine how well each explains the basic
properties of a quasar.
2. Quasar discovery
The discovery of
quasars came in the 1950’s during the use of radio interferometers to make radio-maps
of the sky. Strong spikes recorded in some of the radio spectrums could not be
accounted for by terrestrial interference. These spikes corresponded to tiny point-like
objects of less than 1 arcsecond (arcsec) in diameter, yet no optical component
to them could be observed. Radio astronomers were used to observing the broad
spectrum of “radio galaxies”- luminous elliptical galaxies with strong radio
signatures - yet these radio spikes appeared to be coming from star-sized
objects. Astronomers referred to these point sources as “radio galaxies with no
galaxy”. (Sparke & Gallagher 2005) In 1960, Matthews and Sandage finally located
the optical component to radio source 3C48, which turned out to be a faint 10th
magnitude blue “star”. Analysis of its spectrum proved difficult leaving them
to conclud that 3C48 was a stellar object within our Milky Way galaxy. A decade
later, measurements of 3C48’s non-existent proper motion against the sky and
high recessional velocity made it extremely unlikely to exist within our galaxy.
In the early 1960’s, studies of
another point (or compact) radio source, 3C273, positioned it to within 1
arcsec of a 13th magnitude star. A visible line, or jet, extended
from the object. The Dutch astronomer, Schmidt, studied the unusual emission
lines in its spectrum, and discovered that they represented the familiar Balmer
emission line sequence but red-shifted 15.8%. According to Hubble’s Law, this
placed 3C273 at a distance of ~640 million parsecs (Mpc), with a recessional
velocity of ~44,000 kms-1, and it became very clear that 3C273 was an extragalactic object. (Freedman
& Kaufmann 2001) With a reasonably accurate assessment of its distance, it
became clear that the observed 13th magnitude “star” had an absolute
magnitude of -27, making it one of the brightest objects in the sky. With no means of knowing what these highly
luminous radio objects were, astronomers labeled them Quasi Stellar Radio
Sources, or quasars.
3. Quasar properties
In order to compare
the two popular models of quasar formation and behavior it is first necessary
to summarize the basic properties of a quasar and pose a series of questions
that each model must answer.
A typical quasar has a luminosity of ~1010-1012 that of the Sun, or more than 100,000 times
the brightness of the entire Milky Way, and significantly brighter than the most
luminous galaxies. Quasars are highly luminous at optical, infrared,
ultraviolet and x-ray wavelengths, but contrary to early quasar observations,
are not necessarily very luminous at radio wavelengths. The amplitude of quasar
x-ray emissions indicates that the core material is as hot as 100,000K, much
hotter than any known stellar object. (Freedman & Kaufmann 2001) If quasars
are not stars, what are they and why are they the brightest objects in the
universe?
Whilst
the first quasars were discovered by their radio strength, most subsequent
quasars were discovered optically or via x-ray, and are much weaker radio
sources. In fact, 90% of quasars are “radio-quiet”, with radio emissions <1%
of the power of the radio-loud variety. (Sparke & Gallagher 2005) Many
quasars exhibit tightly collimated jets of material streaming away from the
quasar in opposite directions, visible at optical and radio wavelengths.
Figure 1: Jet - optical wavelengths
(Biretta, J., NASA/NRAO)
For a radio-loud quasar, these jets are large
and strong radio emitters, but are small or non-existent for a radio-quiet
quasar. The jets transport material out of the quasar at supersonic velocities,
creating pressure shock waves when they collide with the material in the
interstellar medium (ISM). After slowing to subsonic velocities, the material disperses
into the ISM up to several hundred thousand parsecs from the quasar nucleus,
creating large “radio-lobes” of ionized material similar to those in a radio
galaxy. However, the central core and jets of a radio-loud quasar can emit
10-100 times the power of a typical radio galaxy. (Sparke & Gallagher 2005)
Radio emission from
the jets is due to synchrotron radiation, a non-thermal process by which highly
charged electrons spiral along a strong magnetic field within the jet and emit
photons. Synchrotron energy requires a constant and powerful energy source to
fuel it. What lies within the core of a quasar to generate sufficient energy?
The opening angle of a jet is typically <15°, (Demory)
and the jet remains highly collimated for thousands of parsecs. What quasar
process creates and collimates the jets?
Quasars exhibit long-term variability, changing their luminosity by as
much as 100% over a period of months or years. The period of variability of an
object depends largely upon its size. To possess a period of less than a year
limits the object to one light year in diameter. Quasars then are definitely
not galaxies or even star clusters, but range in size from less than the
diameter of our solar system to several times its diameter. Quasars are also
micro-variable at x-ray wavelengths, with x-ray emissions changing several
percent in minutes to hours. What are the possible causes of long-term
variability and short–term x-ray micro-variability?
Close analysis of a quasar spectrum shows
very broad emission lines. Such wide emission lines are caused by Doppler
broadening, due to a high rotational velocity of the radiating source, as much
as 10,000 kms-1.
Further, the strength of the Hydrogen (H) emission lines is usually evidence of
large quantities of high-energy particles that stimulate significant numbers of
H atoms into an ionized state. Quasar emission lines are highly red-shifted;
for example the Lyman-alpha emission line, normally in the ultraviolet (UV) is often
red-shifted far into the infrared. (For a z=5.0 quasar) Two major “bumps” are
evident in the spectrum (See Figure 2), one toward the blue/UV end, referred to
as the “big blue bump” though it is more of a rise than a bump, and gives the
quasar its typical blue colour; the other bump lies in the far infrared. What
internal quasar process causes these spectral bumps and the strong, broad
emission lines?
Figure 2: Quasar spectrum
(Freedman & Kaufmann
2001)
Whilst 3C273 has a red
shift of z=0.16, quasars have been discovered as distant as z=6.4. Using a
Hubble constant H0 of
70 km/s/Mpc (WMAPWeb 1999), this
object is >~27 gigaparsecs distant. The closest known quasar is 3C405, also
known as the radio galaxy Cygnus A at z=0.056. (~230 Mpc) (Freedman &
Kaufmann 2001)
The Sloan Digital Sky Survey of 2005 catalogued over 76,000 quasars.
The majority of nearby (low-red shift) radio-quiet quasars are hosted predominantly
by spiral galaxies, whilst high-red shift and radio-loud quasars tend to be
found at the nucleus of elliptical galaxies. We also observe that a large
number of quasar host galaxies show evidence of interactions, collisions or
mergers. Could this provide a clue to the origins and evolution of the quasar? Analysis
of quasar distribution makes it clear that quasars are more common at larger red
shifts, and were thus more common in earlier times. According to Sparke &
Gallagher (2005), there were 1000 times more quasars at z=2 than now, and Figure
3 illustrates that this relationship continues with increasing z numbers. Why
was the early universe richer in quasars?
Figure 3: Number of quasars per cubic gigaparsec (Gpc) vs. red
shift z
(Sparke & Gallagher 2005,
original Hewitt, P.)
We now know that
quasars are the very active nuclei of distant galaxies. Early observers were
unable to detect the faint quasar host galaxy due to the luminosity of the quasar
itself. Quasars now belong to the Unified Model of Active Galactic Nuclei
(AGN), a model that attempts to explain the similar properties of quasars, Seyfert
galaxies, radio galaxies and BL Lacertae objects. The unified model proposes
that the internal process and energy source of these objects is the same, and
that it is the angle at which we view them, be it end on to the jets or
perpendicular to the jets, that determines the type of AGN object that we
observe. Other AGN types are outside the scope of this paper, other than to
comment that the models discussed below can be applied to AGN in general and
not quasars alone.
4. The Supermassive Black Hole model
The most favoured
model of quasar activity utilizes the existence of a supermassive black hole (BH)
at the centre of the active galaxy to power the quasar. There is extensive observational
evidence for supermassive BHs in many nearby galaxies as well as our own, and
they probably exist at the centre of most if not all galaxies, so why do we not
observe a quasar at the heart of every galaxy?
Unlike a stellar black hole, formed
from the death and subsequent gravitational collapse of a massive star, a
supermassive black hole has grown to extremely large mass due to the
availability of infalling material. From its position at the centre of a galactic
nucleus, the supermassive BH benefits from the natural infall of stellar
material and gas due to gravity, galactic mergers and interactions, and can
grow to masses of 108
to 109 solar masses. We
have gained most of our knowledge of supermassive BHs by studying one that
lurks at the centre of our own galaxy.
Whilst we cannot observe a
supermassive BH directly (since light cannot escape its surface), it is usually
possible to observe the accretion disk that surrounds most active supermassive
BHs. As material spirals into a black hole under gravity it will naturally
settle into a flattened disk due to centripetal forces, which we can observe as
a disk or torus of gas and dust. Within this rapidly rotating disk, friction and
turbulence heats up the material to as much as 106 K (EinsteinWeb), stripping it of kinetic
energy and orbital momentum and allowing it to fall further toward the black
hole. Doppler shifting can measure the rotational velocity of the accretion
disk – the edge of the disk approaching us will be blue shifted and the
receding edge will be red shifted. Velocities exceeding 10,000 kms-1 have been measured. After estimating the
mass of the accretion disk, we can calculate the central mass required to hold
such a high-velocity disk in place. An alternative method is to measure the
mass and kinematics of stars and clusters in close orbit around the black hole.
The supermassive BH at our own galactic centre masses 3 billion solar masses
within a diameter of ~1 light day, (Freedman & Kaufmann 2001) a size
consistent with that of a quasar.
The high temperatures within the
accretion disk could explain quasar core temperatures and luminosity. A single
star drawn into a supermassive BH of average mass can release up to 1047 J of energy, enough energy to fuel a quasar
for a year. (Freedman & Kaufmann 2001) It is quite possible that a
supermassive BH within a dense galactic nucleus with plentiful infall material has
the capability to power a quasar for tens of millions of years (Myrs).
How does the supermassive BH model
provide for the highly collimated jets and their emission of synchrotron
radiation? As shown in Figure 4, the infalling material accelerates towards the
inner edge of the accretion disk under gravity and loss of orbital momentum.
Conservation of momentum dictates that the material closer to the inner edge
wants to move outwards through the disk. The point at which this outward
pressure balances the inward-pulling gravity defines the inner edge of the
disk. As fresh material continues to spiral inward, the pressure at the inner
edge eventually reaches a critical level at which the material is expelled. The
path of least resistance is perpendicular to the disk and so the material is
ejected as plasma at relativistic speeds (close to the speed of light) above
and below the disk, resulting in the jets observed in many quasars.
Figure 4: Pressure gradient in a cross-section through an
accretion disk surrounding a supermassive black hole (red is highest pressure)
(Freedman & Kaufmann
2001, original Hawley, J.F., Smarr, L.L.)
Since the hot plasma
in the disk is ionized its motion creates a magnetic field, but because there
is a velocity gradient across the width of the accretion disk, the magnetic
field becomes twisted and wrapped around, much like the magnetic field within a
star. This twisted magnetic field moves with the ejected plasma outward into
the jets. As high-energy plasma particles move within the field, they create synchrotron
radiation that is so easy to detect in a radio-loud quasar. (Freedman &
Kaufmann 2001; Sparke & Gallagher 2005) The collimation of the jets over
thousands of parsecs is harder to explain, though one possibility is that
collimation is due to the gyroscopic effects of the rotating disk. (Rees 1984)
According to the supermassive BH
model, the x-ray and UV emission of a quasar originates largely from the high
temperature plasma in the inner part of the disk, and partly from the
high-energy particles in the jets. This UV emission produces the “blue bump” in
the quasar’s spectrum, whilst IR radiation originates mostly from the dust in
the disk and produces the infrared “bump”. The cooler, outer edge of the disk
probably radiates at optical wavelengths, as do the cooler portions of the jet.
(Sparke & Gallagher 2005) Many studies have been made of the spectra of
accretion disks, e.g. M84, M87, and NGC 7072, and they appear to be consistent
with a typical quasar spectrum. The width of the emission lines is the result
of Doppler broadening due to the high rotational velocity of the accretion
disk.
De Vries, Becker, White et al. (2005)
proposed two possibilities for the long-term variability of quasars utilizing
the supermassive BH model: microlensing and disk instabilities. Microlensing is
a line of sight phenomenon that allows a massive object lying between a quasar
and Earth, e.g. a galaxy or galaxy cluster, to act as a gravitational lens bending
quasar emissions to focus them temporarily such that their luminosity appears
dimmer or brighter to observers. This focusing effect can last from days to
years until the Earth and Sun move sufficiently through space to move out of
the focal point of the gravitational lens. Their other, preferred, explanation
pointed to instabilities in the accretion disk at the heart of the quasar. Such
instability may be due to an irregular infall of material or the infall of an
entire star cluster or group of stellar objects, creating “flares” in the disk.
The energy released is more than enough to account for quasar luminosity
variations. Modeling of accretion disk stabilities is considerably more complex
in reality and dependent upon the thickness of the disk, and we still lack
sufficient knowledge to explain such instabilities. (Hawkins & Taylor 1997) Gaskell and Klimek (2003) support the likelihood
of an intrinsic cause of variability, rather than one external to the quasar
such as microlensing. They state that in many cases quasar luminosity at UV wavelengths
varies by factors of 30+ in less than a year, a change so fundamental that it
could only be due to changes in the quasar’s energy source.
The existence of flares in the
accretion disk can also explain quasar x-ray micro-variability. Another
possibility is that during the impact and tearing apart of a large stellar
object with the accretion disk, shock waves travel in both directions around
the disk to impact on the far side. These effects may be part of the disk
instabilities to which de Vries et al. were referring.
How can this model explain the greater density of quasars in earlier epochs?
We know that supermassive BHs have dormant and active periods and likely cycle
between them. An AGN or quasar is only possible during an active period when the
BH has available material upon which to feed. For a supermassive BH to generate
enough energy to feed a quasar of luminosity 1012, it typically consumes all available
material within 100 Myrs, and leaves behind a supermassive BH of 108 – 109 solar masses. (Sparke & Gallagher 2005)
This matches the observed mass of most supermassive BHs and there are few, if
any, of masses significantly greater than 109, providing evidence that these BHs probably
do not feed for extended periods. Once the supermassive BH at the core of a galaxy
falls dormant, so will the quasar. It is possible that quasars were predominant
in earlier epochs because there was more material in a galactic core to feed
upon than in recent times. In the first billion years, universal expansion was
considerably less than today, providing a greater density of material in a
specific volume.
There seems to be doubt about how
massive black holes could have reached in the early universe. At the time of
the earliest known quasar (z=6.4) the universe was only 800 Myrs old. This is
very little time for the near-uniform primordial gas cloud to condense into
denser clumps and galaxies, and then reach the extreme levels of gravitational
collapse required to form a supermassive black hole capable of powering a
quasar.
The observational evidence that
quasars are more predominant in interacting galaxies can also be explained by
the greater supply of material available to the supermassive BH by a galactic
merger or collision.
5. The Supernovae
model
In the mid-1980’s, Roberto Terlevich and Jorge Melnick proposed a model to explain quasar behavior without a supermassive BH as a central mass. Instead, it relied upon the activity of starburst regions and supernovae.
A starburst region is marked by higher than normal star formation. Such activity may take place during the birth of an elliptical galaxy, or in the aftermath of a galactic collision or merger, when such an event deposits vast quantities of gas and dust into the galactic nucleus. Figure 5 compares the stellar birthrate of spiral and elliptical galaxies, illustrating the burst of star formation in an elliptical in the first 100-1000 Myrs. In a starburst region up to 300 solar masses of stars may be created in a single year, compared to the 2-3 solar masses in a normal spiral. (Freedman & Kaufmann 2001)
Figure 5: Stellar birthrate in elliptical and spiral galaxies
(Freedman & Kaufmann 2001,
adapted from Silk, J.)
A starburst region consists of newly formed, hot
According to Terlevich, Melnick & Moles (1987), a quasar is the result of a starburst region within a dense galactic nucleus, such as a newly formed elliptical. During the first 3 Myrs, star formation is normal for an HII region, and within this period the most massive stars (>60 solar masses) move from burning hydrogen to burning helium in their cores. Terlevich et al. propose the appearance of “Warmers” during this period. A Warmer is a category of Wolf-Rayet star that is blowing away its outer material via strong stellar winds. This severe mass loss exposes the helium-burning core with temperatures of 50,000-150,000K, which can irradiate the area, particularly at x-ray wavelengths. (Cid Fernandes 1997; HarvardWeb 1998) If these temperatures are transferred to the ISM within the galactic nucleus, they would be in line with observed quasar temperatures. After 4 Myrs, the first massive stars supernova (type Ib) and their shock waves photoionize material in the ISM, creating thermal radiation chiefly at optical and x-ray wavelengths.
Over the next 4 Myrs, the supernovae activity peaks and the region, though optically dim, becomes very luminous with non-thermal radio emission. As each spherical supernova shockwave moves outward from its stellar core, it forms a bubble or shell known as a supernova remnant (SNR). The combination of high numbers of SNR and strong stellar winds from the Warmers creates a high-pressure region within the starburst region. The SNR bubbles combine to form a hot (~107 K) “superbubble”, also known as a Superwind, continually pushing outwards against the ISM and remainder of the HII region. Due to resistance along the densest galactic plane, the superbubble can only expand perpendicular to the plane, much the same as the hot plasma from an accretion disk around a supermassive BH, and so the material at the outer edge of the SNRs is ejected outward along the galactic poles to form jet-like structures. (Terlevich 1992) (See Figure 6) It is difficult to explain how this ejected superwind could remain collimated in the form of the observed jets, and why it does not spread out into the Intercluster Medium as suggested by Figure 6.
Figure 6:
Gas density in an ejected superwind. (red is highest pressure)
(http://www.sr.bham.ac.uk/research/sbsim.html)
Over the next 50 Myrs, the less massive stars supernova (type II) and their shock waves contribute to an ISM already compressed and denser from earlier supernovae and stellar wind activity. The remnants from these supernovae referred to as compact supernova remnants (cSNR), distinguished by the impact of their shock waves upon a dense ISM. These cSNR may number as many as 100 at any one time (Terlevich et al. 1993) within the tiny space of the galactic nucleus, which is orders of magnitude more supernovae than usually seen within an entire galaxy. Compact supernovae remnants are the driving force in Terlevich’s supernovae model, separating a normal starburst region from the high-energy starburst region capable of powering an AGN or quasar. The starbursting core of an elliptical is expected to possess an observed diameter of 0.1 to 1 arcsec at z=2, (Terlevich 1992) which is consistent with observed quasar sizes.
Broad emission lines are visible within the spectrum of these regions, largely due to both the photoionization of the ISM by the cSNR and the stellar winds from the Warmers. (Cid Fernandes 1997; HarvardWeb 1998) This intense activity radiates very strongly at optical, UV and x-ray wavelengths, at the luminosities observed in quasars. (Terlevich 1992) These emissions take place in a very small area, resulting in our observation of a point-like object, much as a quasar is, and are not to be confused with the diffuse and expansive radiation from starburst galaxies.
The strong UV radiation results in a rise to the blue-end of the region spectrum in much the same way as the “blue bump” in a quasar spectrum. The far-infrared “bump” is due to the large quantities of dust found within a starburst region. Predominantly this dust is the result of continual and extensive mass loss by the more massive stars, particularly those identified as Warmers. This warm dust is so abundant in these regions that starburst galaxies are some of the brightest IR objects in the universe. (Freedman & Kaufmann 2001)
Evidence in the 1990’s by Conti, Dopita and others suggests that Warmers are rarer than earlier suspected and may not lead to the extensive photoionization that Terlevich and Melnick first thought. This sheds doubt on the part played by Warmers in the early stages of the supernovae model. (Cid Fernandes 1997)
There are two credible explanations for long-term quasar variability: Type II supernovae radiate most of their energy at UV and x-ray wavelengths. The initial and highly luminous flare of such a supernova can last for several weeks, with periods of 10-40 days typical. (Terlevich et al. 1987) One such flare would be detectable against the luminosity of the entire starburst region as a fluctuation. In addition, the cSNR’s from these same type II supernovae expand unevenly due to density variations in the ISM, and cool swiftly and erratically; both effects causing luminosity variations at many wavelengths, including x-ray. (Terlevich et al. 1993)
The prime cause of radio emission in the supernovae model is the supernovae remnants themselves. Electrons move outward from the nucleus with the shockwave, to become the source of synchrotron radiation at radio wavelengths. This is a well-observed phenomenon of even local supernovae, e.g. the Crab Nebula or 3C144. Considerable criticism has been leveled at this model, claiming inadequate evidence to explain the intensity of radio-loud quasars using supernovae. One piece of evidence came with the discovery of radio-supernovae several hundreds times more luminous than Cassiopeia A, also known as 3C461, and known for many years as the strongest extrasolar radio source. In reply to these criticisms, Terlevich calculated the supernovae rate required to generate the radio output of a quasar, and compared this to the supernovae rate required by his model to explain the non-radio luminosity - which turned out to be 1 supernova to generate 1011 luminosity in the earlier years of his model. (Terlevich et al. 1987) As can be seen from Figure 7, this relationship is equal, proving that the model could explain radio output rates via supernovae alone. Radio emission from the jets and lobes could be explained by the ejection of synchrotron radiating cSNR material out of the galactic plane as described above.
Figure 7:
Predicted supernova rate vs. supernova rate required to explain quasar radio
luminosity
(Terlevich et al. 1987)
Without the infall of material into a supermassive BH, how does Terlevich account for the mass of material required to generate the numbers of simultaneous supernovae his model demands? Terlevich observed a number of mature elliptical cores to compute their luminosity function. He then extrapolated his data to predict the properties - and hence luminosity function - for young, newly formed ellipticals that fit the host galaxies for his supernovae-model quasars. Surprisingly, his predicted core luminosity plot matched almost exactly with the luminosity plot of quasars between z=2 and z=3. He therefore concluded that only a tiny percentage (5%) of total galactic mass, i.e. its core, is required to fuel a starburst region with the same luminosity as a typical quasar. This was compelling evidence that a supermassive BH was not required to power a quasar.
This would also explain how starbursting regions of sufficient density could have formed in the very early universe. The pre-galactic clumps would have been considerably smaller than the size of today’s galaxies, but denser. It is likely that two colliding or merging clumps could provide the small amount of mass necessary to fuel a starburst region and supernovae-driven quasar at high z numbers.
However, Terlevich’s model requires a starburst region of coeval high-metallicity stars, and the spectra of distant quasars contain emission lines for heavier elements, just as the model expects. What was the source for such high-metallicity material in the very early universe, when the first stars were still burning hydrogen and helium?
6. Comparative Notes
Both of the models
discussed above can provide suitable explanations for the observed properties
of quasars, and clearly both have stood the test of time and research by a
variety of astronomers. There is no doubt that the supermassive BH model is
more popular, probably due to the considerable research and observation made of
supermassive BH’s and accretion disks. The supermassive BH model easily
explains quasar properties, perhaps with the exception of jet collimation
(which the supernovae model cannot either), and the likelihood of the formation
of such compact and massive objects in the early universe (< 1 billion years).
Indeed, the observed spectra of several accretion disks are extremely
compatible with those of observed quasars.
In comparison, there are several
potential issues with the supernovae model, namely jet collimation, x-ray
micro-variability and the high rates of supernovae required. Whilst there has
been considerable study of both starburst regions and supernovae and their
remnants, few astronomers have applied this knowledge to the formation and
evolution of quasars.
The two models are very similar in
many ways. They both explain the jet and radio-loud phenomena by means of
material ejected perpendicular to the densest galactic plane, and synchrotron
radiation at radio wavelengths. They both attempt to explain long-term
variability by thermal and density instabilities; be it the accretion disk in
the supermassive BH model, or the compact supernova remnants in Terlevich’s. In
this case, microlensing can be ignored as an external factor that affects both
models equally. Galactic interactions seem to be a factor in both models,
providing both an influx of material to fuel the quasar, and a potential
explanation for its short lifetime.
It is possible that quasars form from both models independently, or from a combination of both. A hybrid model, using both supermassive black holes and supernovae within starburst regions has become popular in the last decade. Such a model theorises that a supermassive BH and accretion disk powers the quasar, creates the jets and explains the radio-loudness of quasars as due to synchrotron radiation within these jets. Further, it proposes that the rapid x-ray variability is due to accretion disk instabilities, but uses the supernovae model to explain the high optical and UV luminosities and short quasar lifespan. In this model, the starburst region fuels the formation (or mass increase) of the supermassive BH.
Rees (1984)
speculates that an AGN or quasar is part of the formation process of a
supermassive BH, rather than the other way around. His reasoning follows
similar lines to the Terlevich model, in that cSNR activity in a starburst
region creates a highly compacted ISM whose highly photoionized state exhibits
itself as a quasar, and provides sufficient material to condense into a
supermassive black hole. Because the supermassive BH is a “by-product” of the
quasar, this model could explain the greater numbers of observed supermassive
BH than quasars, as well as the short lifespan of a quasar.
7. Conclusion
Evidence for, and
research into the supernovae model continues to increase, as does our ability
to probe into the cores of distant quasars with greater resolution; and whilst
the supermassive black hole model provides an excellent explanation of the
quasar process, I expect that we will find that it is not that simple. Starburst
regions and high numbers of compact supernovae remnants may play an important
role in “igniting” a quasar and perhaps supplying a dormant supermassive black
hole with sufficient material to allow it to commence feeding and continue to
power the quasar.
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WMAPWeb: http://hubblesite.org/newscenter/newsdesk/archive/releases/1999/19/text/