Agn Vs Starburst’

THE GREAT AGN DEBATE

‘AGN VS STARBURST’

Determining the origin of the prodigious amounts of energy in the centres of many galaxies is still an ongoing research area. There are two main (competing) theories. The first is that a supermassive black hole (SBH) resides in the nuclei of all "active" galaxies (ie. the Unified Model in which all active galaxies have a SBH surrounded by a gaseous accretion disk inside a dusty torus). Gravitational energy from infalling matter on to the SBH is released and emitted via high energy collimated jets. Alternatively rapid and massive star formation (starbursts) could also provide much of the luminosity in such galaxies. Such a scenario is strongly proposed for LINER (Low Ionisation Nuclear Emission Regions) and ULIRGs (Ultra Luminous Infrared Radio Galaxies) and some suggest at least some starburst properties in all AGN.

Artist’s conception of a AGN

www.astrochix.com 1 TABLE OF CONTENTS

Two competing theories…...... 3 An introduction to Active Galactic Nuclei...... 3 Radio Quiet AGN ...... 3 Radio Loud AGN...... 4 Supermassive Black Holes (SMBH) as a energy source for AGN ...... 5 Supermassive Black Holes ...... 5 Supermassive Black Holes as AGN...... 6 Unified Model of AGN ...... 7 Introduction to Starbursts as energy source for AGN...... 8 Starburst Galaxies ...... 8 Starbursts as AGN ...... 8 The 1999 Debate ...... 10 Connection between black holes and starbursts ...... 11 Summary ...... 11 Bibliography...... 13 Further Reading...... 14 Image Credits ...... 15

www.astrochix.com 2 Two competing theories… 1. Since the early 1980’s there have been two competing theories to explain the origin of the prodigious amounts of energy in the centres of many galaxies. The AGN Model, states that the radiation from the AGN is the result of accretion of material into a supermassive black hole (SMBH) at the centre of the host galaxy. The Starburst Model postulates the ‘activity to be caused by one or several violent star formation events in the central regions of galaxies’ (Terlevich 1986, p.505). An introduction to Active Galactic Nuclei 2. An Active Galactic Nuclei (AGN) is an extremely luminous, yet compact region at the centre of a galaxy. The standard model of an AGN states that material close to the central black hole forms an accretion disc, which is pulled inwards causing the accretion disc heat up. AGN emit electromagnetic radiation over the radio, infrared, optical, ultra-violet, x-ray and/or gamma ray wavelengths. The radiation from the accretion disc excites material close to the black hole and this radiates via emission lines. Part of the AGN’s primary output may be obscured by interstellar gas and dust close to the accretion disc, but this is re-radiated at some other waveband, most likely the infrared. Figure 1 - Streaming out from the centre of the galaxy M87 like a cosmic searchlight is one of nature's most amazing phenomena, a black-hole-powered jet of electrons and 3. ‘Central massive black holes are now other sub-atomic particles travelling at nearly the speed of thought to reside in virtually all galaxies with a light. hot spheroidal stellar component’ (Marconi 2003, p.21). Some accretion discs produce jets of fast collimated outflows that emerge from close to the disc. The mechanism which produces the jets is not yet well understood. There is no one way of confirming the observational signature of an AGN. There are two main types of AGN, Radio-quiet and Radio-loud, which are each broken up into further sub-types.

Radio Quiet AGN 4. Seyfert galaxies were discovered by Carl Seyfert in 1943, who noticed small objects with a highly luminous nucleus. Seyfert galaxies typically reside in spiral galaxies, and are characterised by extremely bright nuclei, and spectra which have very bright emission lines of hydrogen, helium, nitrogen, and oxygen. Seyferts were classified as Type 1 or 2, depending upon whether the spectra show both narrow and broad emission lines (Type 1), or only narrow lines (Type 2). The narrow and broad components are believed to both originate from the accretion disk, but in Type 2 Seyferts it is believed that the broad component is obscured by dust and/or by our viewing angle on the galaxy. Rowan-Robinson (1977, p.645) found that Seyfert Type 2 have excess dust emission relative to the optical continuum, he concluded that ‘The distinction between Type 1 and Type 2 Seyferts is caused by strong dust obscuration in the latter’. Fernandes et al (2001, p81) state they ‘have detected unambiguous signatures of young massive stars within ~ 300pc of the nucleus in

www.astrochix.com 3 30% - 50% of Seyfert 2’s by means of high quality optical and, wherever possible, UV spectroscopy.’

5. Radio-quiet quasars/ and QSO (Quasi Stellar Object) are more luminous versions of Seyfert Type 1 galaxy. They show strong optical continuum emission, X- ray continuum emission, and broad and narrow optical emission lines. Some astronomers use the term QSO for this object reserving 'quasar' for radio-loud objects, while others use the terminology radio-quiet and radio-loud quasars. QSO host galaxies can be spirals, irregulars or ellipticals: there is a correlation between the quasar's luminosity and the mass of its host galaxy, so that the most luminous quasars inhabit the most massive galaxies.

6. Ultraluminous InfraRed Galaxies (ULIRG), as defined by Sanders and Mirabel 12 (1996) is Lir > 10 L SM appear to show signs of recent and/or continuing interaction and disruption, usually from galactic mergers and collisions. Sanders and Mirabel (1996) show that ULIRGs ‘appear to be ~2 times more numerous than optically selected QSO, the only other previously known population of objects with comparable bolometric luminosities’. Although many ULIRGs are starburst galaxies, indicators of AGN activity increase with luminosity. Arp 220 is the closest ULIRG to Earth and as a result has been the subject of much study. Most of its energy output appears to be caused by a massive starburst, the result of the merging of two smaller galaxies. Recent observations revealed more than 200 huge star clusters in the central part of the galaxy, the most massive of which contains enough material to equal about 10 million suns (Hubblesite). Observations by the Chandra and XMM- Newton satellites have shown that Arp 220 probably includes an active galactic nucleus (AGN) at its core.

7. A Low-Ionization Nuclear Emission-Line Region (LINER) is a type of galactic nucleus that is defined by its spectral line emission. These systems show weak nuclear emission-line regions, and no other signatures of AGN. There is debate if they are actually AGN, if so they constitute the lowest-luminosity class of radio-quiet AGN. Radio Loud AGN 8. Radio Loud AGN are types of active galaxies that are very luminous at radio wavelengths. The radio emission is due to the synchrotron process (electromagnetic radiation, generated by the acceleration of ultra relativistic charged particles through magnetic fields). Radio loud host galaxies are almost always, large elliptical galaxies. These galaxies are can be detected at large distances, making them valuable tools for observational cosmology.

9. Radio-Loud Quasars (Quasi Stellar Radio Source) are extremely powerful and distant AGN. Initially after their discovery there was some confusion as to their origin, however now the scientific community generally accepts that a quasar is a compact region that surrounds the central SMBH of a host galaxy. Quasars behave much like radio-quiet quasars; however they appear to show emissions from a jet. The first quasars were discovered with radio telescopes in the late 1950s, recorded as radio sources with no corresponding visible object. Further investigation found that many had unusually small angular sizes, which on a photographic plate would appear as the image of a star. When the spectra of these ‘stars’ were located it was

www.astrochix.com 4 discovered that their spectra was unlike any known stars or galaxies. These objects remained a mystery for some timed until John Bolton suggested that ‘the objects were so far away that their light was greatly red shifted’ (Jodrell_Astro). Unfortunately Bolton’s theory was not generally accepted at the time.

10. In 1962 research showed that quasar 3C 273 was ‘receding at a rate of 47,000 km/s’ (Jodrell_Astro). This discovery revolutionized quasar astronomy, encouraging other astronomers to find redshifts from the emission lines from other radio sources. The term quasar was coined by Chinese-born U.S. astrophysicist Hong-Yee Chiu in 1964. In 1969, Donald Lynden-Bell, proposed that quasars could derive their power from material falling into black holes at their cores.

11. Blazars and OVV AGN. A blazar is a very compact and highly variable energy source located at the centre of a galaxy hosting a SMBH, and is among the most violent phenomena in the universe. Blazars can be divided into two types, BL Lac Objects and Optically Violent Variable (OVV) quasars. The name ‘blazar’ was originally coined in 1978 by astronomer Ed Spiegel to denote the combination of these two classes. Blazars are AGN with a relativistic jet that points towards Earth, we observe almost down the jet facing us, which explains the rapid variability and compact features of both types of blazars. Supermassive Black Holes (SMBH) as a energy source for AGN Supermassive Black Holes 12. SMBH have a mass of between 105 and 1.8x 1010 solar masses. Most, if not all galaxies, including the Milky Way, are thought to host supermassive black holes at their centers. It’s generally considered that they are responsible for powering AGN. The Rev John Michell is credited with being the first to conceive the idea of an object massive enough to prevent light from escaping, in 1783. Which is a remarkable feat, as Donald Lynden-Bell noted ‘Michell’s imagination invented supermassive black holes; we are lucky to live in the era when they were discovered’ (2006). In 1964 Edwin Salpeter published a paper describing how a massive object would accrete materiel from the ISM and the resultant release of energy. The term ‘black hole’ wasn’t coined until 1967, by physicist John Wheeler who claims he picked up the term from anther person.

13. The discovery of pulsars by Jocelyn Bell in 1969 contributed to the acceptance of the existence of black holes. ‘The clear evidence of the existence of neutron stars – which had been sorely viewed with much skepticism until then – combined with the presence of a critical mass above which stability cannot be achieved, made the existence of stellar mass black holes inescapable’ (Ferrarese and Ford 2006, p.13)

14. There is strong observational evidence for the existence of SMBH. The UCLA Galactic Center Group provided evidence that Sagittarius A* is a supermassive black hole residing at the centre of our galaxy. Our galactic central black hole is calculated to have a mass of approximately 4.1 million solar masses (Naeye, 2008). In April 2008, Valtonen et al declared that they had identified the largest known supermassive black hole, located in OJ 287, weighing in at 18 billion solar masses

www.astrochix.com 5 Supermassive Black Holes as AGN 15. Since Lynden-Bell’s seminal paper in 1969 it has been argued that AGN must be powered by accretion onto massive black holes. In the standard model of AGN, cold material close to the central black hole forms an accretion disc which transports matter inwards, causing the accretion disc to heat up. The radiation from the accretion disc excites material close to the black hole and this radiates via emission lines. Many accretion discs produce jets, twin highly collimated and fast outflows that emerge from close to the disk.

16. Rees (1984) speculated that an AGN or quasar is part of the formation process of a SMBH. His reasoning was similar to Terlevich model, in that compact supernova remnant activity in a starburst region creates a highly compacted ISM whose highly photoionised state exhibits itself as a quasar, and provides sufficient material to condense into a supermassive black hole. Lynden-Bell (2006, p.2) notes that after the initial black hole accretion disk theory of quasars many believed that radio galaxies and quasars have jets perpendicular to their disks, however it wasn’t until 1995 that ‘such disks around black holes were definitively confirmed’.

17. The SMBH argument is compelling, as to date observational evidence points to SMBH as the source of AGN. Arguments include:  The prodigious output of quasars requires that AGN be very efficient and powerful. The tremendous power emitted by quasars, up to 1048 erg s-1 provides yet another indirect argument in favour of this picture.  The rise of the velocity dispersion of stars towards the centres of galaxies and steep gradients in stellar velocities across the galactic nuclei (attributed to rotation) provide evidence in favour of massive, non-stellar, objects at the centres of a the galaxy.  Variability; changes often seen over days in the optical and over hours in the X- ray spectrum, strongly suggest extremely compact active regions. Smaller, but significant, optical variability over periods of tens of minutes in BL Lacertae objects, have been detected by Miller et al (1989). Relativistic beaming effects could allow for the variations observed at a distance apparently to be faster than those seen by an observer in close proximity to the SMBH.  Broad emission line regions of quasars and Seyferts often correspond to velocities of greater than 5000 km s-1, implying that they are due to a large number of small clouds. Narrow emission lines (velocity of 300-1500 km s-1) are normally assumed to originate well outside the compact core, and it has been demonstrated that a large fraction of galaxies contain unusual LINERS frequently to exhibit broad wings around their narrow lines, supporting the claim that SMBHs are extremely common in galactic nuclei  Several types of radio observations strongly support the SMBH idea. The alignment between small and large scale jets imply a good ‘memory of direction for a substantial time (up to 106 yr), long term stability is attributed to the spin axis of a SMBH. In addition very rapid bulk motion is likely to occur where a deep relativistic gravitational well is available, in the case of AGN, almost certain to be a SMBH.

www.astrochix.com 6 18. There is strong observational evidence for the existence of SMBH as AGN. The UCLA Galactic Center Group provided evidence that Sagittarius A* is a supermassive black hole residing at the centre of our galaxy. Our galactic central black hole is calculated to have a mass of approximately 4.1 million solar masses (Naeye, 2008). In April 2008, Valtonen et al declared that they had identified the largest known supermassive black hole, located in OJ 287, weighing in at 18 billion solar masses. Unified Model of AGN 19. In 1977 Michael Rowan-Robinson published On the Unity of Activity on Galaxies, which united quasars, radio galaxies, and Seyfert galaxies into a single activity in galaxies. In 1978 Martin Rees published a paper which discussed how quasars are fuelled from their surroundings, the supply of gas, the nature of the inflow and the radiation process. Along with Roger Blandford Rees, in 1978 published a paper on their interpretation of strong double radio sources, discussed their ‘beam’ model for blazars, and the properties of compact and variable components in quasars and AGN. Figure 2 – Blazar (viewing directly into the jet)

Quasar Seyfert Type 1

Seyfert Type 2

Radio Galaxy

Black Hole

Accretion Disk

Torus of neutral gas and dust Radio Jet

20. AGN unified models attempt to explain the variety of subtypes due solely to differences in the orientation of the central object. This theory predicts that the

www.astrochix.com 7 differences that we see are caused by the angle of the system, ie the plane in which the accretion disc, the outer parts of the broad line region and the molecular torus all lay. One main proponent of the unified model is Robert Antonucci, who concluded ‘…it now appears that much of the variety in AGN types is just the result of varying orientation relative to the line of sight’ (1993, p.473). Figure 2 depicts an AGN, which will show different characteristics depending on the line of sight. Introduction to Starbursts as energy source for AGN Starburst Galaxies 21. A significant number of galaxies show the signature of recent large-scale star formation activity, higher than the usual formation rate seen in most galaxies. Generally galaxies undergo a period of star formation after a collision or close encounter between two galaxies. ‘The tidal forces between the two galaxies disrupt gas and cause it to fall inward, greatly accelerating the normal process by which interstellar clouds collapse and form stars’ (Weaver, 2007) When quasars we discovered in 1963 scientists struggled to explain these bright and very distant objects. One of the first theories to gain momentum was the starburst theory, which claims that massive bouts of star formation at the centres of these galaxies explain their brightness.

22. These galaxies are known as ‘starburst galaxies’. They are generally characterized by strong H II-region-type emission-line spectra, due to O and B-type stars, and relatively strong radio emission, due to recent supernova remnants. In some cases, the starburst appears to be confined to an unresolved region at the galactic centre, which often looks like an AGN. Starburst Galaxies can be categorised in a number of ways; Starburst Galaxies (SB), which form stars at a rate of a few 10s of solar masses per year; Luminous InfraRed Galaxy (LIRG) which form stars at a rate of 10s to 100s of solar masses per year; Ultra Luminous InfraRed Galaxy (ULIRG) which form stars at a rate of 100s to 1000s of solar masses per year. Starburst galaxies are not common in the local universe, LIRGs and ULIRGs are even rarer. However, in the early universe, all three types of galaxies were more common. Starbursts as AGN 23. Starburst nuclei can sometimes match Seyfert galaxies and some quasars in terms of their total power output. Starburst nuclei usually radiate most strongly in the infrared portion of the electromagnetic spectrum. ‘This infrared emission comes from dust grains that have been heated to temperatures of several tens to several hundreds of degrees Kelvin by the ultraviolet light produced by the hot young stars’ (Astro_web). Star formation may begin in the interstellar medium of a galaxy, where there are dense clouds of dust and gas. If an interstellar cloud is massive enough that the gas pressure is insufficient to support it, the cloud will Figure 3: LH95 Stellar nursery in undergo gravitational collapse. However other events Large Magellanic Cloud may occur to trigger star formation, including;  Molecular clouds may collide with each other,

www.astrochix.com 8  A nearby supernova or other phenomena (eg superwinds) may send shocked matter into the cloud at very high speeds; and  Galactic collisions can trigger massive starbursts of star formation as the gas clouds in each galaxy are compressed and agitated by tidal forces.

24. As the molecular cloud collapses it breaks into smaller pieces and as temperature and pressure increase the fragments coalesce into rotating spheres of gas. If the original cloud is massive enough it is possible that the cause of the perturbation may trigger the accretion of matter in a number of different locations, forming a star cluster.

25. Although the SMBH as a power source for AGN has been popular, not everyone has accepted the idea. Roberto Terlevich, is a sceptic, and points out that ‘most active galaxies are of the radio quiet variety and they can be explained by conventional astrophysics, without recourse to exotic ideas about black holes’ (Clark, 1993). Terlevich, Melnick, Boyle, Moles and Filippenko were some of the first astronomers to put forward credible theories which may explain how the energy created to power an AGN comes from bursts of star formation and supernova explosions in young galaxies. Terlevich and collaborators explored ‘possibilities such as warmers and compact Supernova Remnants’ (Fernandes 1996, p.1). Terlevich (1986) argues that ‘for at least a substantial fraction of nuclei currently classified as ‘active’, the black-hole hypothesis is unnecessary’. They suggest that the luminosity of young stars is comparable to the luminosity of an AGN.

26. The beginning of the Starburst AGN Model, commenced in 1985 when Terlevich and Melnick proposed that the AGN was a cluster of metal rich, young massive ‘warmers’, stars reaching temperatures of ~ 150000K. However it soon became clear that ‘warmers’ were not as common as previously thought, and the model could no longer be based on these massive stars. In 1987 Terlevich, Melnick and Moles declared a quasar is the result of a starburst region within a dense galactic nucleus, such as a recently formed elliptical. 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 starburst core of an elliptical is anticipated to possess an observed diameter of 0.1 to 1 arcsec at z=2 (Terlevich 1992) which is consistent with observed quasar sizes.

27. This model was critisised roundly, with the complaint that there was inadequate evidence to explain the intensity of radio-loud quasars using supernovae. In response to 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 - Figure 4 – Artists conception of an AGN which turned out to be 1 supernova to generate 1011 luminosity in the earlier years of his model.

www.astrochix.com 9 (Terlevich et al. 1987, p509). Terlevich concluded that the model could explain radio output rates via supernovae alone.

28. One of Terlevich and Boyle’s arguments for Starbursts as an energy source for AGN is that high redshift quasars have large amounts of metals, which means that there must have been a significant amount of star formation, explaining the starburst model. In 1993 Terlevich and Boyle claimed that they could replicate the shape, amplitude and evolution of the QSO luminosity function of a galaxy via the starburst model. They contend that giant ellipticals went through a period of intense star forming activity around z=2. Even though the starburst theory explained a number of AGN properties it has been subjected to criticism from several fronts. Unresolved issues include:  Similarities between radio quiet and radio loud AGN  Rapid X-Ray variability  Absence of a Lyman edge  Stellar features in the UV  Broad Fe lines 29. In 2006 Lipari and Terlevich published a paper that explored a unified scenario involving SMBH and starburst with outflow, that they concluded was capable of explaining most of the observational properties of, at least part of AGN.

The 1999 Debate In 1999 the Scientific Organising Committee (SOC) organised a debate between two teams of scientists on the question, what powers an ULIG, SMBH or starburst? Two teams were formed, the Blue Team (R.D. Joseph), pro starburst, the Green Team (D.B. Sanders), pro SMBH. Both teams examined the current literature and observational evidence for their side of the debate, and came to a conclusion. The Green Team found that:  ULIRGs represent an extremely important stage in galaxy transformation,  ULIRGs enrich the surrounding intergalactic medium via superwinds,  It appears that both starbursts and AGN are important to varying levels to the luminosity output of ULIRGs,  There is likely an evolutionary connection between the fuelling of starbursts and the fuelling of AGN,  ULIRGs may eventually evolve into optically selected UV excess QSOs,  Warm ULIRGs and objects with a high infrared luminosity are likely to be powered by an AGN, and  Dust obscuration could mask a dominant AGN in the majority of Radio Quiet QSOs. The Blue Team found that:  In the majority of cases ULIRGs are largely powered by starbursts,  Even where there is evidence for the existence of an AGN, it is not obvious that the AGN provides most of the energy output,

www.astrochix.com 10  Observational techniques in every spectral region provides evidence for the presence of starbursts in ULIRGs.  Where AGN are present they seem to be relatively insignificant in astrophysical terms. Apparently even scientists who are reading the same literature and reviewing the same observations can come to vastly differing conclusions. The seeds for a connection between SMBH and starbursts were sown.

Connection between black holes and starbursts 30. ‘The idea that AGN are somehow linked to starbursts was not sparked by a single earthshaking discovery but has evolved slowly’ (Weaver, 2007). Although AGN and starbursts are distinct phenomena, a number of observations and findings have suggested a connection:  Observations have shown AGN alongside starbursts. Although the galactic cores are filled with gas and dust, x-ray observations have been able to provide images of the centres of dust obscured galaxies.  Heckman and Kauffmann (2004) studied >23,000 AGN taking spectral proof of highly ionized oxygen as a sign of AGN and strong hydrogen absorption as a sign of a starburst region. They suggested that the more powerful the AGN, the more likely it was that the galaxy had experienced a major starburst not long ago.  AGN host galaxies aren’t the only galaxies to have SMBH in the centre. SMBH produce AGN only when there is enough material to fuel the SMBH.  AGN and star formation were more closely related when the universe was a 10th of it’s age. ULIRGs and radio galaxies were more prevalent. They both appear to host AGN and SB. Veilleux (2008, p.7) reported, ‘Starbursts often co-exist with actively accreting SMBH, but the starburst-AGN relation appears to be tighter at high luminosity rather than low-luminosity’.  X ray background radiation – studies have shown a population of AGN previously unseen by optical telescopes – one explanation is that the AGN were accompanied by SB which choked the galaxies with dust. 31. Since Terlevich’s original paper the starburst model has evolved to take into account current observations, and discoveries. It has evolved from a ‘pure’ starburst model, to one that links the existence of starbursts and AGN. Kauffmann and Heckman (2004, p.20) suggest ‘that the cosmic evolution of star formation and of AGN activity in the Universe is inextricably linked.’ They go on to say that it is not yet understood if the AGN are responsible for active star formation, or if AGN are simply responding to changes in the host galaxy, including starbursts.

Summary 32. Lipari and Terlevich (2006, p.3) propose an evolutionary unified scenario for SMBH and starbursts with outflow. They suggest ‘a SMBH is formed during the formation of a galactic core and an AGN powered by accretion, which is formed by the interaction between a nuclear starburst and the SMBH.’

www.astrochix.com 11 33. Weaver (2007) suggests that AGN and starbursts are causally related. She suggests that two scenarios are likely; that an AGN creates a starburst. As the SMBH influences its host galaxy, it pulls material into the centre, enabling rapid star formation. The second scenario is that a starburst creates a dense cluster of stars, within which normal stellar collisions occur. Over time massive stars are created, then die, becoming neutron stars or black holes. These objects would then coalesce, forming a black hole, which would gravitate towards the centre of the galaxy, absorbing other black holes, objects, gas and dust until it formed at SMBH at the centre of the galaxy.

34. Clearly all current research suggests that for most AGN starbursts is not the central source of energy, however they have been proven to exist in many galaxies, and in some cases are more powerful than the existing AGN, situations which show a strong link to SMBH. Most ULIRGs were thought to be powered mostly by starbursts, however there are indications that AGN activity increases with luminosity. Recent observations show a connection between LINER emission in LIRGs and star formation activity. The mid-infrared spectra of LIRGs with LINERs have been shown to look similar to the mid-infrared spectra of starburst galaxies, suggesting that some LINERs are powered by star formation activity. However, some mid-infrared emission has been detected in AGN, indicating that star formation may not be their only source of energy.

35. There are many people working on the AGN/starburst problem, many now appear to be in the AGN corner, with support for a ‘pure’ starburst model diminishing. As discussed there is now consensus that not only is there a connection between AGN and starbursts, but it’s likely that the host galaxy is reliant on both to develop, grow and power the AGN. The actual relationship or dependency has not yet been discovered, however it appears that continuing research will eventually uncover the true relationship.

www.astrochix.com 12 Bibliography

Antonucci, R. Unified Models for Active Galactic Nuclei and Quasars; Annual Reviews in Astronomy and Astrophysics 31 (1): 473–521, 1993.

Astro_web: Astronomy Website; http://www.astro.bas.bg/galaxies/galaxies_files/agn.html (accessed 14 November, 1306)

Boyle, B., J., Terlevich, R., J., The Cosmological Evolution of the QSO Luminosity Density and of the Star Formation Rate, arXiv: astron-ph/9710134v1, 14 Oct 1997.

Clark, S., ‘Shine on, brilliant galaxy: What lies at the heart of the brightest galaxies in the Universe? An old theory could be making a comeback’, New Scientist, 18 September 1993.

Fernandes R., Heckman, T., Schmitt, H., Gonza-Lez Delgado, R.M., Storchi-Bergmann, T., Empirical Diagnostics of the Starburst-AGN Connection; The Astrophysical Journal, 558:81-108, Sep 01.

Fernandes, R., The Starburst Model for AGN, Past, Present and Future, arXiv: astro-ph/9608110v1, 16 August 1996.

Ferrarese, L., Ford, H., Supermassive Black Holes in Galactic Nuclei; Past, Present and Future Research, arXiv:astro-ph/0411247 09 Nov 2004.

Hubblesite: Hubble Website; http://hubblesite.org/newscenter/archive/releases/2006/26/full, (accessed 15 October 2008, 2130).

Jodrell_Astro: Jodrell Bank Website; http://www.jodrellbank.manchester.ac.uk/history/mk1quasars.html, (accessed 19 November 2008, 2135)

Kauffmann, G., Heckman, T.M., The Formation of Bulges and Black Holes: lessons from a census of active galaxies in the SDSS, arXiv: astro-ph/0406219v1, 08 June 2004.

Lipari, S., Terlevich, R., Evolutive Unification in Composite Active Galactic Nuclei, arXiv: astro- ph/0602090v1, 06 Feb 2006.

Lynden-Bell, D., Galactic Nuclei as Collapsed Old Quasars, Nature, Vol 223, Aug 16, 1969.

Lynden-Bell, D., Violent Star-Formation: Comments. Star Formation Through Time, ASP Conference Series, Vol 297, 2003.

Lynden-Bell, D., Magnetic Jets from Swirling Disks, Monthly Notices of Royal Astronomical Society, 1- 23, 05 February 2006.

Lynden-Bell, D., History of Nuclear Black Holes in Galaxies, Sackler Conference - Harvard 2006, Reminiscences on the History of an Idea.

Marconi, A., Hunt, L., K., The Relation Between Black Hole Mass, Bulge Mass, And Near-Infrared Luminosity, The Astrophysical Journal, 589:L21–L24, May 20 2003.

Miller, R.H., Carini, M.T., Goodrich, B.D., Detection of microvariability for BL Lacertae objects, Nature, 327-627, 1989 Naeye, R., Milky Way Central Monster Measured, Sky and Telescope, August 28, 2008

Weaver, K., The Galactic Odd Couple, Scientific American Special Edition, 15512991, Jan 2007.

Rowan-Robinson, M., On the Unity of Activity in Galaxies, The Astrophysical Journal, 213:635-647, 1 May 1977.

www.astrochix.com 13 Sanders, D., B., Mirabel, I., F., Luminous Infrared Galaxies, Annual Review of Astronomy and Astrophysics, 1996. 34:749–92.

Terlevich, R., Melnick, J., Moles, M., Starburst Models for AGN, Observational Evidence of Activity in Galaxies: Proceedings of the 121st Symposium of the International Astronomical Union, June 3-7, 1986, Kluwer Academic Publishers, Dordrecht, 1987.

Valtonen, M.J., A Massive binary black holes system in OJ287 and a test of general relativity, Nature, 452, 851-853, 17 April 2008.

Veilleux, S., AGN Host Galaxies, arXiv: astron-ph 0807.3904v2, 29 Jul 2008.

Alexander, D., M., The X Ray-Infrared/Submillimetre Connection and the Legacy Era of Cosmology, Astron. Nachr., AN 999, No. 88, 789-793, 2006.