This AGN textbook includes phenomena based on new results in the X-Ray domain from new telescopes such as Chandra and XMM Newton not mentioned in. Beckmann, Volker; Shrader, Chris R. Affiliation: AA(APC), AB(NASA/GSFC, USRA). Publication: Active Galactic Nuclei, ISBN pages. Authors:Volker Beckmann (1), Chris R. Shrader (2) ((1) Francois Arago and encourage research in the field of Active Galactic Nuclei (AGN).
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Skip to main content. Log In Sign Up. Volker Beckmann and Chris Shrader: Probing the Innermost Regions 5 1. Bondi Accretion and the Eddington Limit 46 3. Probing the Scale of the BLR 73 3. Emission, Dynamics and Morphologies 77 3. Probing the Central Engine 5. Dust Near and Far 5. Where It All Began 5.
The Obscured Inner Disk 5. How Did They Form? Active Galactic Nuclei —! Our understanding of them is roughly summarized in Chapter 1. Such a summary will be insufficient, but the goal is to provide an overview of the topic for the newcomer in the field. In Chapter 2 we take a quick tour through the radia- tive processes which are common in AGN and give the relevant formulas in order to make the arguments in the book understandable.
Chapter 3 then discusses our understanding of what mechanisms drive the AGN emission, and what the main elements are, such as the black hole itself, the accretion disk, the broad and narrow line regions, outflowing jets and absorbing material. The different types of AGN are discussed in Chapter 4, including an attempt to explain all different types by the most simple model possible.
In Chapter 5 we take a look at AGN in different ener- gy bands, from the radio to the gamma-ray domain, and examine the overall energy output for beamed and nonbeamed sources. In Chapter 6 we will discuss what one can learn from variability studies of AGN.
Up to that point we deal with the central engine and its closest surroundings. In Chapter 7 we will then have a look at in what types of galaxies and galaxy clusters supermassive black holes reside and how the AGN is influenced by the host galaxy and how in turn the central engine might affect the star formation in the surrounding medium. In order to understand the role of AGN in the Universe, we briefly discuss the current cosmological model in Chapter 8 and show how AGN can be used as tools for cosmological studies.
We then turn in Chapter 9 to the ultimate question of where quasars come from, how they might be formed, and how they evolve.
This also includes the aspects of AGN density evolution in time and how this might depend on the type of AGN or the en- ergy range we study. Chapter 10 summarizes the open issues remaining in AGN research, that is, the big questions which still lack a satisfying answer. We hope that the reader will find that stimulating — and that she or he through their own research will contribute to further progress in this thriving field of astrophysics.
The literature about AGN is full of acronyms and abbreviations. Njclei list the most important ones and those which are used yalactic this book starting after the Preface.
Finally, the physical and astronomical constants applied in this book talactic listed. Throughout the book we use cgs units rather than SI, as it is still common practice in astrophysics. As for any such undertaking as writing a text book covering a broad scientific topic, we heavily relied on the experience and publications of a large number of! Many of those publications are listed in the bibliography. Some of the textbooks we had at hand when compiling this book, were OsterbrockPe- tersonKembhavi and NarlikarKrolikDe Youngand of course Rybicki and Lightman Throughout the book we point the read- er to review articles on certain topics, and give some bibliographic references for further reading.
Here we have put an emphasis on recent publications over more established ones. The idea is that the reader will usually find references to earlier work in the most recent publications in a field. We would like to apologize to all the colleagues whose work we have not mentioned or have not given the proper weight in this book.
This book would not have been possible to write without the help, advice of and discussions with many colleagues. Volker thanks his mother for pointing out that NGC hosts the most massive black hole known to mankind.
Thus the extragalactic hypothesis spread rapidly in the scientific community, although it was still not completely accepted as true.
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One main difficulty was the fact that some of the nebulae were actually of galactic origin, such as planetary nebulae and glob- ular clusters. A significant step forward was the compilation of a large catalog of some nebulae assembled by William Herschel in the late eighteenth and ear- ly nineteenth century. Spectroscopic observations by Vesto Slipher of nebulae in the early twentieth century revealed that some of these show redshifted lines indicating they are moving relative to the Milky Way at velocities exceeding the escape velocity of our Beckmqnn Slipher, The issue, whether some of the observed nebulae are actually extragalactic, was finally settled in the s.
For example, he noticed a Doppler shift in M31 due to its rota- tion and absorption by dust similar to what was observed in our Galaxy.
Using the Mt. Becmkann are variable stars with characteristic light curves which allow to determine their absolute brightness. Using the distance modulus of several Cepheids in these nearby galaxies, Hubble confirmed the large distance of these objects, although again underestimating nulcei distance by a factor of! Based on his observations, he also established a system of classifying galaxies, the so-called Hubble sequence Hubble,and laid the starting point for cosmology assuming an expanding Universe Hubble, The first evidence that some galaxies were hosting some additional strongly emit- ting component in their center was found ga,actic Carl Seyfert in the s.
He obtained spectra of six galaxies, showing high-excitation nuclear emission lines veckmann on a normal star-like spectrum Seyfert, He also noticed that some galaxies showed broad emission lines, while others exhibited only narrow ones. The nature of the strong emission from the center of some galaxies remained nucleii mystery. A common hypothesis was the assumption that a large number of stars would pro- duce the observed features.
A step closer to current understanding was the idea that in the center of these galaxies resides a stellar type object of very large mass, which then would emit mainly by accretion processes of a surrounding disk of gas Hoyle and Fowler, The hypothesis that there might exist objects in the Universe whose gravity would be sufficient to trap even light was discussed first by John Mitchell 2 in the late eighteenth century Mitchell, The concept of the black hole was ignored though in later years, as light was considered to be made of massless particles with no interaction with a gravitational field.
When Albert Einstein formulated the general relativity theory the possible existence of black holes was shown to be a solution for the gravitational field of a point mass and of a spherical mass by Karl Schwarzschild Only when solutions had bedkmann be found to explain phenomena like AGN, and the fact that massive stars had to collapse in- to a black hole Oppenheimer and Volkoff,was the existence of black holes accepted beckmznn a continuously growing fraction of the scientific community.
It 2 Mitchell wrote: The field was now open to study the physics involved in the accretion phenomenon, to nudlei and explain AGN emission throughout the electromagnetic spectrum, and to study the distribution in space, the origin, the evolution and fate of these elusive objects.
 The AGN phenomenon: open issues
Furthermore, the centers of these broad emission lines did not cor- respond to the laboratory wavelengths of any known atomic species and certainly not to the well known hydrogen Balmer series or other common lines known to be of astrophysical origin. This dilemma was resolved in the s leading to the basic AGN paradigm described in the previous section of a distant and highly lu- minous object powered by a massive, accreting black hole.
The deep gravitational potential of the black hole was responsible for the dynamical broadening of the observed lines and for radiatively efficient accretion leading to the extreme lumi- nosities. The line identification dilemma was solved with the realization that the distances involved were of such magnitude that the cosmological expansion of the Universe redshifted atomic emission lines to the observed values including some high-ionization UV lines were.
Fast forwarding ahead several decades, it became evident that these broad emis- sion line spectra could be exploited as a diagnostic of the physical conditions in the environment ambient to the central black hole.
This led to a dramatic revision of our basic understanding of AGN. Additionally, this knowledge has been used to cross-calibrate alternative black hole mass estimation methods and to bet- ter constrain physical models of the broad-line emission media as virialized gas clouds in photoionization equilibrium with the central engine radiation field.
Another distinguishing observational feature of some AGN is the presence of narrow, nonvariable forbidden emission lines. The similarity to nebular line emis- sion in our galaxy was noted and some of the atomic physics and computational formalism developed and to study those objects was employed Osterbrock, There were some significant differences between the galactic nebulae and the AGN as well.
In particular, the AGN narrow lines required a broad-band ionizing con- tinuum extending far bluewards of the stellar radiation fields responsible for pho- toionizing the galactic nebulae.
The dynamics of the inner AGN region can also be probed in cases using the narrow lines. These types fea- tures have been interpreted in the context of kinematics, such avtive biconical outflows Kraemer et al. Another observational aspect of AGN that was evident in early observations was activve the continuum spectral distribution was very distinct from an integrated stel- lar continuum characteristic of normal galaxies Oke and Sargent, Observa- tionally, AGN were comparatively very blue.
In fact, radio-quiet AGN would later be identified and bec,mann primarily by performing multicolor imaging of sky re- gions, sorting the results in color-color plots, and performing spectroscopic follow- ups on the blue excess subpopulation thus identified Green et al.
The big blue bump spec- tral component was a positive flux excess relative to an underlying nclei con- tinuum. It exhibited curvature that suggested a thermal origin. This was interpret- ed as the first observational evidence for the presence of an accretion disk Malkan and Sargent, athus lending support to the basic paradigm of Lynden-Bell The quest to further corroborate this basic paradigm and to gain a deeper fundamental actibe of putative AGN accretion disks was the driver be- hind many subsequent observational campaigns and theoretical efforts during the decades which followed.
Among the most prominent, roughly spherical, examples in galactuc catalog was object number 87, thus its desig- nation as M Probing the Innermost Regions 5 Figure 1.
Note that this is a simplified view and not to scale. Graphic courtesy of Marie-Luise Menzel. There were relatively few of examples of these highly collimated, bipolar out- flows or jets from the centers of AGN until the observational techniques of radio interferometry matured in the decades subsequent to the Second World War. There are now AGN subclasses, which are believed to be related to each other within the ggalactic of the unification scenarios described in subsequent chapters.
Our general understanding of the main components of an AGN is shown in Fig- ure 1. The schematic representation distinguishes between sources which display a jet and are therefore bright in the radio band, and those which do not show strong radio dominance and in which case one assumes no or weak jet emission. Probing the Innermost Regions During the same decade of the s when the basic AGN paradigm was being de- veloped, the first cosmic X-ray source, known as Scorpios X-1, was discovered using!
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