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An Introduction to Active Galactic Nuclei

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Oct 15, There are some very complicated issues of galaxy formation. Unfortunately, here is the same problem as with the stars. The origin of galaxies remains unclear, in spite of huge activity in the field. What the "formation" means? It means that we have the material that is assembling into galaxies. Report Block. Jeans instability on steroids. Oct 16, All very well to use the term "star formation" when all current observations confirm that stars do not form all by themselves - none has been spotted so far. The physics of the Jeans limit in conjunction with magnetic field repulsion simply prevents stars from forming all by themselves.

Hence the need for seeding with dark matter and now as in this article a relief is sought in the tensions created by SMBHs. That just leaves the first stars unaccounted for. And hence the first galaxies. And hence all the rest of the stars and galaxies. As I have been saying for years. As, generally, bluer host galaxy colours imply higher SFRs e. As the classification of radio AGN often relies on non-radio properties, such as the optical emission line properties or multi-wavelength proxies Sect. Thus, one of the most severe selection effects in the radio band is the contribution of SF-related processes to the total radio power 7 especially given the limited angular resolution available in many radio continuum surveys.

Detailed high angular resolution radio continuum studies often reveal a mixture of SF- and accretion-related radio emission in non-jetted HEGs e. Such high-angular resolution studies are, however, still limited to rather small samples, and to-date the main source of radio emission in the most powerful AGN i. While agreement exists that radio emission in jetted quasars is powered by SMBH accretion-related processes e.

Padovani et al. Hence, for a full, quantitative assessment of the evolution of the AGN radio LF separated into the two types out to high redshift and over a broad luminosity range a combination of surveys with various areal coverages is needed, with access to a robust AGN classifier, and methods to isolate AGN-related radio emission in low radio luminosity AGN, that can be uniformly applied throughout the entire redshift range considered. Studies such as those discussed in the previous section are becoming feasible only now, and will be invigorated with the onset of the Square Kilometre Array SKA 11 , offering an observing window between 50 MHz and 20 GHz extending well into the nanoJy regime with unprecedented versatility, in combination with contemporaneous projects over the entire electromagnetic spectrum such as, e.

A revolution has in fact started in radio astronomy, which has entered an era of large area surveys reaching flux density limits well below current ones. This, amongst other things, will revolutionise AGN studies. Identifying AGN in these new radio surveys, however, will not be straightforward and will require many synergies with facilities in other bands e.

IR studies have had a strong impact on our understanding of AGN structure, their evolution through cosmic time, and their role in galaxy evolution. In Sect. Finally, in Sect. The basis of this paradigm is the presence of dust surrounding the accretion disk on scales larger than that of the broad line region BLR , with an inner boundary set by the sublimation temperature of the dust grains Barvainis In type 2 AGN the dust obscures the line of sight towards the accretion disk and the BLR and therefore only narrow emission lines can be observed in the optical spectrum e.

Antonucci and Miller ; Antonucci , although see Elitzur and Netzer for a discussion about possible real type 2 AGN where the difference is not caused by dust obscuration; see also Sect. Observations of the strength of the silicate feature at 9. A number of authors have studied the fraction of lines-of-sight that are obscured by the dusty torus, either by comparing the relative fraction of type 1 and 2 AGN at a given redshift, or by modelling the SED of individual objects. However, a single number does not englobe the diversity of AGN in Nature.

Additionally, some authors have found a significant variance in the amount of dust in AGN e. At rest-frame NIR wavelengths, where the AGN emission has a local minimum at the cross-over between the dropping accretion disk emission and the rising dust emission, the stellar 1. As the stellar emission drops steeply longward of the 1. Spaceborne telescopes are, therefore, better suited for the identification of large AGN samples.

In what follows, we focus solely on selection using space-based broad-band photometry, as they account for the great majority of MIR identified AGN, although most implications and many of the caveats also apply to ground-based and to spectroscopic observations. We explore those further in Sect. Spitzer only criteria. The left and right panels show the case of a shallow and deep survey, respectively. MIR AGN identification is considerably less sensitive to obscuration of the central engine by dust compared to optical identification, as dust opacity is lower at longer wavelengths and is, therefore, better for the selection of obscured AGN than optical identification, although its sensitivity to obscured AGN decreases with increasing redshift due to the K-correction e.

As discussed in Sect. The main advantage of the MIR over the X-rays is that the integration times needed for AGN identification are much shorter, and hence allow for faster survey speeds. However, MIR identification is affected by contaminants and biases that are only marginally relevant to X-ray or optical selections. In the next sections we discuss these issues, which need to be taken into account when drawing statistical conclusions about the AGN population from MIR selected samples.

However, there are a number of different populations that can mimic the colours of AGN in these bands and will affect most selection criteria, although the extent will depend on each specific selection. Such galaxies can appear as contaminants in shallow and deep observations see previous section. As they are uncommon and the co-moving volume is low enough at the respective redshift range, they are typically only a minor contaminant. At those redshifts the 1. In addition to extragalactic contaminants, there are a number of Galactic sources that can mimic the colours of AGN in the MIR, such as brown dwarfs or young stellar objects.

Broad-band MIR AGN selection primarily relies on the detection of the hot dust emission at low and intermediate redshifts.

Monster Lurking in the Galactic Core - Active Galactic Nuclei

Therefore, AGN with low hot dust emission relative to that of their host could escape identification, especially if they reside within luminous hosts. Although such analogues may not necessarily exist, they might occur more often at lower luminosities. A clear case among the observational evidence e. Such objects would be missed systematically by all MIR selection criteria at low redshifts.

The luminosity of the spheroidal component of a galaxy correlates with the mass of its SMBH at least at relatively low redshifts: e. Such a bias has been discussed by, e. Quantifying this bias is difficult, as it strongly depends on the selection function being used, but it needs to be taken into account and the complete selection function needs to be modelled in order to be able to give a physical and statistical interpretation of results based on MIR-selected AGN. Such methods, however, offer little gain compared to MIR and optical selection.

Populations of heavily reddened AGN have also been found by means of MIR photometry, often in combination with optical observations. The unambiguous observations of the silicate feature at 9. At the same time, however, IRS observations indicated that in some cases the source of obscuration resides in the host rather than the torus e.

A number of techniques have also been developed to model the observed MIR spectra and constrain the AGN and starburst contributions see e. Although MIR spectroscopy has had a great impact on our understanding of AGN, the number of objects studied through these techniques is limited when compared to photometric studies, as spectroscopic observations require significantly longer integration times.

The upcoming generation of ground-based giant telescopes will significantly expand upon the current NIR and MIR capabilities, as most of them will have significant focus on these wavelengths. Observations with the JWST will probe with high angular resolution a number of targets that are not accessible from the ground, likely having a major impact in our understanding of AGN.

Euclid , 28 expected to launch in , will have a 1. Both telescopes will also obtain NIR slitless grism spectroscopy in their survey areas. Through their unique combination of area and depth, both surveys will probe AGN activity during the formation of the first galaxies in the Universe. This section discusses the selection and properties of optically-selected AGN as contrasted with investigations at other wavelengths. The focus here is on the more luminous subsets that would typically be classified as quasars or Seyfert 1 galaxies.

Even though optical surveys are not ideal for probing obscured AGN, we discuss how they can guide our search for them.

The bias towards unobscured sources in the optical is partially mitigated, however, by an increase in information content for the sources that are identified—in the form of physics probed by the combination of optical continuum, absorption, and emission. We discuss the physical mechanisms behind optical emission in Sect. How next-generation surveys such as LSST can bridge the evolution of luminous quasars to lower-luminosity AGN that are typically better probed at other wavelengths will be addressed in Sect.

Many models, usually assuming a geometrically thin, optically thick accretion disk, have been developed in order to explain this emission e. Baldwin The features described in the previous section are the basis of optical AGN identification, be it photometric or spectroscopic. Broad band photometry is sensitive to the presence of broad emission lines in the various filters as a function of redshift, as they alter the otherwise very typical colours of the AGN that separate them from the stellar locus see, e. Finally, we are generally reliant on optical spectroscopy to provide confirmation of a source as an AGN and to determine its redshift, while the presence of narrow emission lines in the spectra of galaxies and their ratios are indicative of the presence of an AGN e.

The problem with the optical band, as compared to, say, the hard X-rays, is that bright optical sources are not necessarily AGN. The same is true for the radio Sect. Number of quasars as a function of time. The dashed line and triangles give the number of quasars in the largest heterogeneous samples to date. Paradoxically, many optically unobscured AGN are missed by optical surveys. These are objects whose colours put them in or close to the stellar locus.

Lower-luminosity AGN are also a challenge for imaging-only optical surveys like the Dark Energy Survey [DES 30 ] and LSST 31 for the reasons noted above: without spectroscopy, it is difficult to distinguish a normal galaxy from an active galaxy. Yet this is the population that we most need to probe, especially for comparison to X-ray and MIR samples.

It does not matter whether the optical obscuration is by a smooth torus or a clumpy one see Sect. This result is due to a combination of effects: the host galaxy is not always obscured and both strong emission lines and scattering can result in non-negligible optical flux producing unusual or even AGN-like colours, which can cause them to be identified as potential type 1 sources despite them being type 2.

Active Galaxies

However, while the optical may be missing a crucial component of the AGN zoo in terms of obscured AGN, it more than makes up for that loss in terms of information content of those AGN that are detected. Moreover, the information content in the continuum, emission lines, and absorption lines from optical spectroscopy is particularly rich. These correlated features may be indicative of differences in the hardness of the spectral energy distributions e. Leighly Arguably the best example of where optical provides additional information content and makes up for selection effects is in our ability to utilise BH mass scaling relations to estimate the masses of the BHs powering quasars e.

Vestergaard and Peterson Again, this process requires optical spectroscopy. However, beyond that redshift other broad emission lines need to be used. As the sample size of high-redshift quasars with both optical and IR spectroscopic coverage grows, corrections to this scheme might help bringing the BH masses into alignment e. One of the shortcomings of the wide, but shallow SDSS work was that it generally only probed the bright end of the LF, whereas narrow, but deep X-ray surveys were better able to probe the faint end.

Recent works, however e. What is needed here are both deeper surveys in the optical and wider surveys in the X-ray Sect. But even without new data there is a decade of observations that could be incorporated into an updated bolometric LF. Improving our knowledge of the quasar LF has consequences beyond the study of quasars: it also has important consequences for reionization in the early Universe. It is important to understand that the bias in the optical band is towards the high luminosity end of the AGN distribution, i. In part that is because that is the population that optical surveys themselves are biased to.

These new optical surveys will help complete a multi-wavelength bridge that will allow AGN astronomers to more fully sample luminosity-redshift space across the full electromagnetic spectrum, as illustrated by LaMassa et al. We discuss in this section X-ray-selected AGN. The physical mechanism behind X-ray emission is examined in Sect. In this review we define the X-ray band as covering the energy range of 0.


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The primary reasons for this are the following: 1 X-ray emission from AGN appears to be near universal; 2 X-rays are able to penetrate through large column densities of gas and dust particularly at high X-ray energies ; 3 X-ray emission from host-galaxy processes are typically weak when compared to the AGN see also Sect.

However, thermal X-ray emission due to the inner regions of the accretion disk can also be produced at the lowest X-ray energies e. The X-ray emission is then modified due to the interaction with matter in the nuclear region e. The relative strength of these components can vary quite significantly from source to source, mostly due to differences in the geometry and inclination angle of the torus to the line of sight, leading to a broad range of X-ray spectral shapes. A large number of X-ray observatories have been launched since the first pioneering rocket flights of the s see Giacconi for a review.

The majority of the results presented here have been obtained from the most sensitive X-ray observatories in operation, all of which employ grazing-incidence optics to focus X-ray photons and achieve high sensitivity at high spatial resolution: Chandra , XMM-Newton , and NuSTAR. The low background, high spatial resolution, and good collecting area allows Chandra to detect sources three orders of magnitude fainter than previous-generation X-ray observatories. Since an increase in redshift leads to an increase in the rest-frame energies probed within a given observed-frame band, the effect of absorption is less significant at high redshift than low redshift.

Other wavebands that are less sensitive to the effects of heavy obscuration e. However, the unambiguous identification of the signatures of CT absorption and the measurement of absorbing column densities requires X-ray observations. This is most effectively achieved from broad-band X-ray spectral fitting i.

For AGN with low observed X-ray luminosities e. The dominant host-galaxy phenomenon at X-ray energies is commonly referred to as X-ray binaries see Fabbiano and Remillard and McClintock for reviews. The emission from X-ray binaries is due to mass accretion onto a degenerate star neutron star or BH from a companion star in a binary system: X-ray binaries are sub-classified into low-mass X-ray binaries LMXBs and high-mass X-ray binaries HMXBs , depending on the mass of the companion star.

Active Galactic Nuclei (AGN) / Supermassive Black Holes

To accurately identify or characterise the AGN requires taking account of both of these X-ray binary components. Another potential component of non-AGN contamination at X-ray energies is emission from hot gas, either from the host galaxy or a galaxy cluster. The dashed bins indicate column density upper limits and the dashed lines indicate the typical adopted column density threshold between X-ray absorbed and unabsorbed AGN. Overall there is also good agreement between the optical and X-ray signatures of absorption e.

The overall consistency between the X-ray and optical absorption indicators provide some of the strongest observational support for the basic unified AGN model e. Antonucci ; Urry and Padovani However, the clear disagreements between absorption indicators for a small subset of the AGN population e. Time-series observations have shown that the X-ray and optical spectral properties of AGN can vary on relatively short timescales as a result of changes in the absorbing column density along the line of sight e.

Therefore, some of the occasional disagreements between absorption indicators are due to non-simultaneous X-ray and optical observations; however, AGN variability does not explain the differences for all cases and some AGN appear to genuinely depart from the basic unified model see also Sect. Space density versus redshift for AGN selected across a wide range in X-ray luminosity. Blank-field cosmic X-ray surveys have provided some of the most detailed and sensitive constraints on the evolution of the AGN population.

The fraction of X-ray absorbed AGN is found to be a function of luminosity, with a decreasing fraction towards higher X-ray luminosities, and appears to also increase with redshift e. The redshift evolution in absorption may be a consequence of the luminosity dependent X-ray absorbed AGN fraction shifting to higher luminosities at higher redshifts e. These results suggest that the covering factor of the obscuring material i.

We now look towards the scientific gains that can be anticipated from several future X-ray facilities; see Sect. Athena has many key scientific aims but, from the point of view of this review, two of the main advances will come from the following: 1 excellent-quality X-ray spectroscopy both in terms of unprecedented sensitivity and spectral resolution , to elucidate the physics of AGN activity e.

XIPE is dedicated to undertake temporally, spatially, and spectrally resolved X-ray polarimetry. Therefore, XIPE will provide unique physical insight on the geometry and connections between these different regions Goosmann and Matt This applies also to most types of extragalactic objects, including the non-jetted AGN detected in large numbers in IR, optical, and X-ray surveys thanks to the radiation resulting from accretion onto the central SMBH Sects.

The green line represents the expected emission from a typical blazar host galaxy. The SEDs here and in Fig. The strong and highly variable non-thermal radiation from the jet encompasses the entire electromagnetic spectrum, but is not dominant in the optical-UV and the soft X-ray bands where most of the emission is due to accretion onto the SMBH and to the BLR.

Active galactic nucleus

The SED of blazars see Figs. The low energy component, peaking between the IR and the X-ray band, is generally attributed to synchrotron radiation produced by relativistic electrons moving in a magnetic field. In leptonic models e. In hadronic scenarios e. In this case blazars would also be neutrino emitters from the decay of charged pions extending their SEDs outside the electromagnetic spectrum into newly explored multi-messenger scenarios, which might even include cosmic rays CRs e.

These instruments operate in space and are characterized by a very large FoV thousands of square degrees; e. The small overlap between the two bands between 50 and GeV allows for inter-calibration between space and ground-based observatories. This is particularly important as this is where spectral breaks occur in the most extreme astrophysical sources.

The size of the point spread function and the effective area of Fermi -LAT strongly depend on energy, resulting in a sensitivity limit that significantly depends on the intrinsic source spectrum. Red and black points represent extragalactic and Galactic plus unidentified sources respectively. These observatories are well suited to carry out long-integration surveys of large parts of the sky, with typical output being fluxes averaged over long integration periods or the discovery of strong flares in bright objects.

The main lists of objects in this energy band detected by IACTs are available on-line 42 as interactive tables that are updated periodically as new sources are detected. At present they include approximately sources distributed in the Galaxy as shown in Fig. These samples do not represent uniform surveys of the VHE sky as they only include sources detected during pointings of known sources, often during flaring states. Only a fraction of the high Galactic latitude sky has been observed so far.

It includes sources, many more that those detected by IATCs. The 2WHSP sample includes sources and is available on-line. The complex broad-band SEDs of blazars result from the superposition of many spectral components, such as the double humped non-thermal emission, light from the host galaxy, the BLR, and the accretion onto the SMBH. This mix, combined with different viewing angles and a wide range of maximum particle acceleration energies leads to SEDs with largely different shapes, causing very strong selection effects when looking at blazars in widely separated regions of the electromagnetic spectrum.

To understand the intrinsic population properties of blazars is therefore essential to control all the selection biases. Construction should start in , with the first telescopes on site in The comparison of this plot with that of Fig. We discuss in this section variability-selected AGN. The future of this field is discussed in Sect. AGN display erratic, aperiodic flux variability over a wide range of timescales from years to minutes. The distribution of AGN variability power i. In other words, for example, much faster variability is observed in the X-ray band than in the optical band, where the same variability amplitude is reached only over longer timescales Fig.

The minimum timescale of variability measured in a given waveband provides us with an estimate of the linear size of the source component emitting in that waveband e. Terrell The X-ray band is where some of the most rapid hours-minutes , largest-amplitude flux variations are measured. This variability is thought to originate in the innermost regions of the accretion flow corona and inner disk.

Moreover, it is responsible for driving at least part of the variability from the outer accretion disk, observed at longer wavelengths UV and optical. Processes occurring in the jet can also contribute to the observed AGN variability e. These variations may ultimately originate from stochastic instabilities within the accretion flow Malzac However, whether and how accretion-driven variability is transferred into the jet is not definitely known. In the following we will focus on discussing variability directly associated with the accretion process. Note that one can indirectly probe AGN variability on much longer timescales by studying powerful past events in nearby galaxies through studies of extended emission line regions.

The structure of an active galactic nucleus

The availability of long-term monitoring and continuous sampling X-ray observations has allowed us to make significant progress towards a good characterization of X-ray variability in low-redshift AGN. These studies played a key role in our general understanding of AGN variability. In recent years, dedicated ground-based campaigns e.

These studies have been revealing properties significantly different from those characterizing X-ray variability e. These lags are too short to be explained by some form of outwards diffusion in the flow. Therefore, heating from a central X-ray source is considered the most plausible mechanism. Indeed, the derived lag amplitude-wavelength dependence is consistent with that expected from reprocessing of X-rays in a standard disk e. In this scenario the lags are dominated by the light crossing time from the X-ray source to the region of the disk emitting at a given wavelength, with longer red wavelengths coming from larger radii, thus producing longer lags.

However, the availability of long X-ray observations and simultaneous optical monitoring has allowed the extension of these studies to longer timescales, revealing that on month-to-year timescales variations in the optical are more intense than in the X-rays e. This behaviour cannot be explained by X-ray reprocessing only, but it requires some additional source of intrinsic disk variability dominating on long timescales Sect. However, the sole analysis and modelization of the PSD has not led to a clear identification of the underlying process. Indeed, most of the originally proposed models such as shot-noise models: e.

This relation indicates that the absolute amplitude of X-ray variability rms, e. Such findings argue against models such as additive shot-noise models invoking the presence of independent flares or active regions. These models assume the emergence of local perturbations of disk parameters triggering variations of the accretion rate, with long timescale perturbations produced at larger radii. If the timescale of the perturbations is longer than the radial diffusion time, the perturbations can propagate inward, combining multiplicatively with perturbations produced at smaller radii, and reach the innermost zones where most of the energy is released e.

These models can explain the presence of large amplitude X-ray variability on a wide range of timescales, even orders of magnitude longer than the viscous-timescale of the compact X-ray emitting regions of the flow. This component would explain the excess long-timescales optical variability detected in some AGN Sect. However, if due to the slower, viscous propagation in the flow the large lags can be easily recovered in standard disk-corona geometries e.

Variations of photoelectric absorption, e. However, this is unlikely to be responsible for the bulk of it, as several arguments and observational evidences rather favour an intrinsic origin. Extragalactic surveys are typically characterized by uneven sampling, with large gaps between consecutive cycles. This makes PSD techniques unsuitable for studying variability in these datasets.

The identification of variable sources requires assessing whether the variability observed between observations exceeds the variations due to statistical fluctuations. Thus, the ability to detect significant variability in single sources depends on the error of each measurement. At a given source flux and exposure, this in turn depends on the collecting area of the detector and the background contribution, which affects the uncertainty on the flux measurements e. Figure reproduced from De Cicco et al. Some accretion discs produce jets of twin, highly collimated , and fast outflows that emerge in opposite directions from close to the disc.

The direction of the jet ejection is determined either by the angular momentum axis of the accretion disc or the spin axis of the black hole. The jet production mechanism and indeed the jet composition on very small scales are not understood at present due to the resolution of astronomical instruments being too low.

The jets have their most obvious observational effects in the radio waveband, where very-long-baseline interferometry can be used to study the synchrotron radiation they emit at resolutions of sub- parsec scales. However, they radiate in all wavebands from the radio through to the gamma-ray range via the synchrotron and the inverse-Compton scattering process, and so AGN jets are a second potential source of any observed continuum radiation. There exists a class of 'radiatively inefficient' solutions to the equations that govern accretion. In this type of accretion, which is important for accretion rates well below the Eddington limit , the accreting matter does not form a thin disc and consequently does not efficiently radiate away the energy that it acquired as it moved close to the black hole.

Radiatively inefficient accretion has been used to explain the lack of strong AGN-type radiation from massive black holes at the centres of elliptical galaxies in clusters, where otherwise we might expect high accretion rates and correspondingly high luminosities. AGN are a candidate source of high and ultra-high energy cosmic rays see also Centrifugal mechanism of acceleration. There is no single observational signature of an AGN. The list below covers some of the features that have allowed systems to be identified as AGN. It is convenient to divide AGN into two classes, conventionally called radio-quiet and radio-loud.

Radio-loud objects have emission contributions from both the jet s and the lobes that the jets inflate. These emission contributions dominate the luminosity of the AGN at radio wavelengths and possibly at some or all other wavelengths. Radio-quiet objects are simpler since jet and any jet-related emission can be neglected at all wavelengths.

AGN terminology is often confusing, since the distinctions between different types of AGN sometimes reflect historical differences in how the objects were discovered or initially classified, rather than real physical differences. See main article Radio galaxy for a discussion of the large-scale behaviour of the jets. Here, only the active nuclei are discussed. Unified models propose that different observational classes of AGN are a single type of physical object observed under different conditions.

The currently favoured unified models are 'orientation-based unified models' meaning that they propose that the apparent differences between different types of objects arise simply because of their different orientations to the observer. At low luminosities, the objects to be unified are Seyfert galaxies. The unification models propose that in Seyfert 1s the observer has a direct view of the active nucleus. In Seyfert 2s the nucleus is observed through an obscuring structure which prevents a direct view of the optical continuum, broad-line region or soft X-ray emission.

The key insight of orientation-dependent accretion models is that the two types of object can be the same if only certain angles to the line of sight are observed. The standard picture is of a torus of obscuring material surrounding the accretion disc. It must be large enough to obscure the broad-line region but not large enough to obscure the narrow-line region, which is seen in both classes of object. Seyfert 2s are seen through the torus. Outside the torus there is material that can scatter some of the nuclear emission into our line of sight, allowing us to see some optical and X-ray continuum and, in some cases, broad emission lines—which are strongly polarized, showing that they have been scattered and proving that some Seyfert 2s really do contain hidden Seyfert 1s.

Infrared observations of the nuclei of Seyfert 2s also support this picture. At higher luminosities, quasars take the place of Seyfert 1s, but, as already mentioned, the corresponding 'quasar 2s' are elusive at present. If they do not have the scattering component of Seyfert 2s they would be hard to detect except through their luminous narrow-line and hard X-ray emission.

Historically, work on radio-loud unification has concentrated on high-luminosity radio-loud quasars. The large-scale radio structures of these objects provide compelling evidence that the orientation-based unified models really are true. However, the population of radio galaxies is completely dominated by low-luminosity, low-excitation objects. These do not show strong nuclear emission lines—broad or narrow—they have optical continua which appear to be entirely jet-related, [24] and their X-ray emission is also consistent with coming purely from a jet, with no heavily absorbed nuclear component in general.

Most likely, they form a separate class in which only jet-related emission is important. At small angles to the line of sight, they will appear as BL Lac objects. In the recent literature on AGN, being subject to an intense debate, an increasing set of observations appear to be in conflict with some of the key predictions of the Unified Model, e. Therefore, one cannot know whether the gas in all Seyfert 2 galaxies is ionized due to photoionization from a single, non-stellar continuum source in the center or due to shock-ionization from e. The two classes of populations appear to differ by their luminosity, where the Seyfert 2s without a hidden broad-line region are generally less luminous.

The covering factor of the torus might play an important role. Some torus models [37] [38] predict how Seyfert 1s and Seyfert 2s can obtain different covering factors from a luminosity- and accretion rate- dependence of the torus covering factor, something supported by studies in the x-ray of AGN. While studies of single AGN show important deviations from the expectations of the unified model, results from statistical tests have been contradictory. The most important short-coming of statistical tests by direct comparisons of statistical samples of Seyfert 1s and Seyfert 2s is the introduction of selection biases due to anisotropic selection criteria.

Studying neighbour galaxies rather than the AGN themselves [45] [46] [47] first suggested the numbers of neighbours were larger for Seyfert 2s than for Seyfert 1s, in contradiction with the Unified Model. Today, having overcome the previous limitations of small sample sizes and anisotropic selection, studies of neighbours of hundreds to thousands of AGN [48] have shown that the neighbours of Seyfert 2s are intrinsically dustier and more star-forming than Seyfert 1s and a connection between AGN type, host galaxy morphology and collision history.

Moreover, angular clustering studies [49] of the two AGN types confirm that they reside in different environments and show that they reside within dark matter halos of different masses. The AGN environment studies are in line with evolution-based unification models [50] where Seyfert 2s transform into Seyfert 1s during merger, supporting earlier models of merger-driven activation of Seyfert 1 nuclei. While controversy about the soundness of each individual study still prevails, they all agree on that the simplest viewing-angle based models of AGN Unification are incomplete.

Seyfert-1 and Seyfert-2 seem to differ in star formation and AGN engine power. While it still might be valid that an obscured Seyfert 1 can appear as a Seyfert 2, not all Seyfert 2s must host an obscured Seyfert 1.