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