Physics
Quasars
Quasars are extremely luminous and distant active galactic nuclei that emit massive amounts of energy across the electromagnetic spectrum. They are powered by supermassive black holes at the centers of galaxies and are among the most energetic objects in the universe. Quasars are important for studying the early universe and the evolution of galaxies.
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10 Key excerpts on "Quasars"
eBook - PDFCosmic Discovery
The Search, Scope, and Heritage of Astronomy
- Martin Harwit(Author)
- 2019(Publication Date)
- Cambridge University Press(Publisher)
These observations, which first became possible around 1960, established the surprisingly high surface brightness of the Quasars at these wavelengths and provided accurate positions which also brought optical astron- omers into the search. Since these early observations, Quasars have been studied in great detail. Their light emission varies over months and years; the radiation may be partly polar- ized; powerful X-ray emission is observed; and many Quasars show families of optical absorption spectra, each family corresponding to a different velocity shift. Although first discovered at radio wavelengths, Quasars need not necessarily be bright radio sources. In 1965 Allan Sandage began finding Quasars that were not notable radio emitters and the term radio quiet quasar was coined. Like all Quasars, these objects are highly luminous. 168 The most fundamental questions on Quasars are still not properly under- stood: Are these sources really as distant as their red shifts indicate? Or are they more local red-shifted objects? If they are truly distant, they are fantastically bright. If they are local, their high red shift would have to be explained. Quasars remain a puzzle more than fifteen years after their discovery, although evi- dence is gathering that each quasar is associated with a galaxy or cluster of 138 Discoveries galaxies that shows the same red shift. Previously these galaxies could not be detected since the quasar was so much brighter and outshone nearby galaxies. More recent results, however, suggest that the quasar may represent a bright eruption involving many stars in a galaxy and, if verified, will clearly confirm a cosmic distance for Quasars. A class of sources that have many of the properties of Quasars is represented by the archetype BL Lacertae which used to be considered a variable star. The BL Lacertae objects are compact sources with high surface brightness.
eBook - ePub- Jayant V. Narlikar(Author)
- 2012(Publication Date)
- Dover Publications(Publisher)
A view held by many astrophysicists is that Quasars are linked to galactic nuclei in an evolutionary process. We have seen, for example, how the nucleus of M 87 is considerably brighter than its outer parts. There are other types of galaxies, known as Seyfert galaxies (see, for example, Fig. 1.7), in which the contrast between a bright nucleus and fainter surroundings is even more marked than it is in M 87. The Quasars may be one step further on this sequence so that if they are located at large distances we only see the bright central nucleus and nothing of the faint periphery (if it exists at all). This is the establishment view and it is not difficult to extrapolate from this and argue that, as in M 87, Quasars may also contain supermassive black holes serving as energy machines. Indeed, since the mid-1970s, the idea that a quasar is powered by a supermassive black hole has gained considerable popularity. Once it was decided that a supermassive black hole is responsible for quasar luminosity, the theoretical bandwagon could move in two directions. In one direction lay the problem of explaining how a supermassive black hole could form in the first place, while the other direction faced the problem of finding a process which can convert the black hole’s gravitational energy into radiation energy. Some progress has been made on both these fronts. Various scenarios have been constructed leading to the formation of a supermassive black hole. For example, a gigantic cloud may straightaway collapse to a black hole. Or it may condense into a star cluster which evolves through star collisions, gas depletion, and disruptions into a supermassive star which then becomes a black hole. Another method makes use of the formation of massive (~ 100 M) stars which become supernovae and leave behind neutron stars or stellar mass black holes which may subsequently form the supermassive black hole
eBook - PDFThe Cosmos
Astronomy in the New Millennium
- Jay M. Pasachoff, Alex Filippenko(Authors)
- 2019(Publication Date)
- Cambridge University Press(Publisher)
Similar emission lines have been seen in x-ray binary systems in which the compact object is likely to be a black hole (see the discus- sion in Section 14.7). Such lines, in both active galaxies and x-ray binaries, are now being analyzed in detail to detect and study pre- dicted relativistic effects such as the strong bending of light and the “dragging” of space-time around a rotating black hole. 17.4 WHAT ARE Quasars? The idea that Quasars are energetic phenomena at the centers of galaxies is now strongly supported by observational evidence. First of all, the observed properties of Quasars and active galactic nuclei are strikingly similar. In some cases, the active nucleus of a galaxy is so bright that the rest of the galaxy is difficult to detect because of contrast problems, making the object look like a quasar; special techniques are needed to reveal the galaxy (■ Fig. 17–16). This is especially true if the galaxy is very distant: we see the bright nucleus as a point-like object, while the spatially extended outer parts (known as “fuzz” in this context) are hard to detect because of their faintness and because of blending with the nucleus. In the 1970s, a statistical test was carried out with Quasars. A selec- tion of Quasars, sorted by redshift, was carefully examined. Faint fuzz (presumably a galaxy) was discovered around most of the Quasars with the smallest redshifts (the nearest ones), a few of the Quasars with intermediate redshifts, and none of the Quasars with the larg- est redshifts (the most distant ones). Astronomers concluded that the extended light was too faint and too close to the nucleus in the distant Quasars, as expected. In the 1980s, optical spectra of the fuzz in a few nearby Quasars revealed absorption lines produced by stars, but the vast majority of objects were too faint for such observations. In any case, the data strongly suggested that Quasars could indeed be extreme examples of galaxies with bright nuclei.
eBook - PDF- Tony Hey, Patrick Walters(Authors)
- 1997(Publication Date)
- Cambridge University Press(Publisher)
10 The Big Bang, black holes and unified fields One exciting property of Quasars was the variation observed in their optical brightness.This is evident not only from recent experiments but also from old photographs which inadvertently included Quasars. A good example is the quasar 3C 279, which flared up in brightness by a factor of about 25 in both 1936 and 1937. Some Quasars have been seen to alter in intensity on timescales as short as a week or 1ess.The significance of this vari- ability is that it allows us to place a limit on the size of the power source at the heart of Quasars. Suppose the source varies in intensity over a period of one week. Light travelling to us from the back of an object one light week in diam- eter arrives one week after light from the front.The fact that we see a varia- tion in one week then means that the source must be less than one light week across.This distance is only about ten times the size of our solar system - for a source more powerful than one hundred galaxies! Quasars are one example of objects now known as 'active galactic nuclei', or AGNs.The most popular explanation for the enormous power generated by such objects is that it comes from a supermassive black hole, but the details of such theories are still somewhat speculative. Two applications of general relativity After some eighty years, during which general relativity has been subjected to more and more precise tests, it seems reasonable to assume that it is correct, and therefore that we can use the theory to uncover other new physics. In this section, we describe two such uses of general relativity: the first is connected with the bending of light; and the second involves the discovery of gravita- tional waves.We begin with the bending of light.
eBook - ePubGeneral Relativity: The most beautiful of theories
Applications and trends after 100 years
- Carlo Rovelli(Author)
- 2015(Publication Date)
- De Gruyter(Publisher)
Quasars were seen to be an extreme part of the more general phenomenon of Active Galactic Nuclei (AGN). The centres of a few percent of all galaxies appear to have some non-stellar activity in the form of bright broad emission lines, a nonthermal radio source or an X-ray source. Quasars occur when the AGN outshines the host galaxy in the optical band. AGN of lesser power are known as Seyfert galaxies or just low luminosity AGN as the power drops. Jets of highly-collimated relativistically outflowing plasma are found in about 10% of AGN. An example outflow in one of the nearest AGN is shown in Figure 6.. Unusual emission spectra at the centres of some galaxies had been known as a phenomenon since about 1908, and the first jet was reported in M87 by Heber Curtis in 1918 [12]. Little follow-up work was done before the 1960s, apart from Carl Seyfert’s PhD thesis in the 1940s [64]. By the end of the 1980s most astronomers considered that AGN were powered by accretion onto black holes, but a minority still favoured some other explanation, such as multiple supernova outbursts. The arguments for black holes were strong but circumstantial. A major observational problem was that black holes are by definition difficult to observe directly. Quasars appear to be associated with a past phase in the evolution of the Universe, when it was 1–5 billion years old (mostly at redshifts of 2–4). They are now relatively uncommon compared with that era. (3C273 is a rare quasar at low redshift: low is relative here as it was moderately high when discovered in the early 1960s.) The enormous powers involved mean that the accretion rates were high and the mass doubling time of the black holes could have been a few 100 million years. Radiation pressure would have restricted the inflow to below a limit first deduced for stars by Eddington, known as the Eddington limit
eBook - PDF- P R Wallace(Author)
- 1991(Publication Date)
- World Scientific(Publisher)
Finally, the variation in light output in short periods of time (from hours to weeks) tells us that these extraordinarily luminous objects are as-tonishingly small; light days or weeks, compared with 100,000 light years for typical galaxies. Theorists were presented with a problem which seemed insoluble in their framework. It took them nearly 15 years to come up with a plausible explanation. 10.3. Unravelling the Puzzle of the Quasars The size attributable to Quasars was clearly so small as to sug-gest a black hole. As we showed earlier, a black hole with the mass of a galaxy of 10 11 suns would have an event horizon of a radius of about 10 light days, which is comparable to the dimensions of a typical quasar. Could the quasar be a collapsed galaxy, or at least a Black Holes in Attrophytict 243 collapsed galactic core? As noted in our earlier discussion of missing mass, many galaxies appeared to have cores of extremely high den-sity, so that the hypothesis that they might collapse to form massive black holes is not outrageous. But of course there is a problem with this notion: how could there be such stupendous energy output from a black hole, or rather, from its immediate neighbourhood? Only at a conference in the summer of 1977 did a consensus begin to emerge in favour of a model that had been the focus of attention of several groups, whose calculations had shown that it could account for the energy production of Quasars. Imagine a supermassive rotating black hole (of a billion or so solar masses) surrounded by an accretion disc of rotating gas (Fig. 10.7). Close to the black hole the particles of gas will be moving at a rate which is a significant fraction of the speed of light; the gas will be very hot and, as a consequence, ionized. It will produce an enormously strong magnetic field perpendicular to the plane of the disc.
eBook - PDF- Lizhi Fang, Remo Ruffini(Authors)
- 1985(Publication Date)
- World Scientific(Publisher)
Existing observations seem to suggest that most Quasars are in spirals . More identifications are expected in the near future, especially through the space telescope. Therefore, it seems logical to conclude that at least some Quasars are more distant and more powerful members of type 1 Seyfert nuclei. The Einstein X-ray observatory recently revealed that Quasars are also fairly strong X-ray emitters^ 1 °'. Gamma rays have been detected from the most well-studied quasar, 3C 273 . Among radio-loud Quasars are objects ( 10?) which are optically violently variable (OVVs), with very high optical polarization (up to 30J). Radiation from a majority of Quasars is, however, relatively stable and not highly polarized (< 3?). The same applies to Seyfert nuclei. 3. BL Lac Objects BL Lac objects are similar to Quasars in various aspects, - the quasi-stellar appearance, power law continuum spectra, etc. A major difference between BL Lacs and Quasars i s the absence, or weakness, of optical emission lines in the former. The typical redshifts of BL Lacs are comparable to those of relatively close Quasars. Many background galaxies have been already identified with near-by BL Lac objects. They are all elliptioals( 11) BL Lacs are generally strong radio and X-ray emitters, highly variable and possess high degrees of optical and radio polarization 65 (up tcvlO - 30%). The apparent luminosity of BL Lacs and Quasars are comparable. 4. Radio Galaxies Radio galaxies are extragalactic radio sources which have been identified with galaxies. There are extended and compact components. Most of the strong extended sources have a linear double structure which straddles a giant elliptical galaxy or a quasar near the center. Compact components coincide with the nuclei of galaxies. The output of typical extended radio sources i s ~ 10 1 * 2 - 1 0 ^ ergs s 1 . The nuclear emission from the associated galaxy is weaker than Quasars.
eBook - ePubFundamentals of Radio Astronomy
Astrophysics
- Ronald L. Snell, Stanley Kurtz, Jonathan Marr(Authors)
- 2019(Publication Date)
- CRC Press(Publisher)
Such high redshifts implied that these objects were at cosmological distances from us (see discussion of cosmological redshifts in Section 1.5). These objects were, therefore, the most distant objects known at that time. They were so far away that not only would ordinary stars be impossible to detect but even whole galaxies would appear quite faint. Therefore, these quasi-stellar objects were inferred to be very luminous active nuclei of distant galaxies, whose stellar component was not detected in the glare of the bright core. Although they appear as faint as ordinary stars, taking into account their distances, the luminosities of these AGN can be as large as hundreds of times that of the entire Milky Way galaxy. Modern observational techniques permit the detection of the underlying host galaxy of a quasar despite the enormous glare of the core. Repeated imaging of Quasars with extremely high resolution radio observations using very long baseline interferometry techniques (see Volume I, Chapter 6) revealed significant outward motions (see discussion in Section 9.4.2), confirming that these objects contain radio jets. Hundreds of thousands of QSOs, with a wide variety of characteristics, have now been discovered. Quasars, as a rule, are found at large redshifts. In fact, one of the first Quasars discovered, 3C273, at z = 0.158, is considered a relatively low-redshift quasar. The number density of Quasars per unit volume is found to increase toward higher redshifts, peaking between redshifts of 1 and 2. Active galactic nuclei, in general, are more common at higher redshifts, typically with higher luminosities at higher redshifts. The redshift distribution depends on the particular defining AGN parameters and on the limitations of the survey, but the tendency for AGN to be more common in the past is definite in all studies. Figure 9.4 shows a histogram of redshifts of optically selected broad-emission line QSOs
eBook - PDF- John Bahcall, Jeremiah P. Ostriker, John Bahcall, Jeremiah P. Ostriker, John Bahcall, Jeremiah Ostriker(Authors)
- 2018(Publication Date)
- Princeton University Press(Publisher)
This recoil could displace the hole from the centre of the merged galaxy—it might therefore be relevant to the low-z Quasars that seem to be asymmetrically located in their hosts (and which may have been activated by a recent merger). The recoil might even be so violent that the merged hole breaks loose from its galaxy and goes hurtling through intergalactic space. As data accumulates, it should be feasible to pin down the luminosity functions and lifetimes of Quasars, and to correlate the masses of remnant holes in nearby galaxies to the morphology of their hosts. This subject should offer real oppor- tunities for fascinating analysis and modelling, with the aim of understanding the mechanisms within Quasars and how the quasar phase relates to the general process of galaxy formation. Quasars also already pose several other questions that can only be answered computationally. Being myself one of those whose cerebration is mainly done at about a milliflop without electronic aids, I suggest these with some diffidence. However, as computers advance from gigaflops towards teraflops, people like me will be relegated to the role of marginalised cheerleaders unless we can change our ways. I list below some fundamental problems that can be addressed by the technically strong student. Among these suggestions are: BIBLIOGRAPHIC NOTES 191 (i) Stars and gas in the central 100 pc of a newly-formed bulge. When does gas stop being able to form stars, and evolve instead into a supermassive object? (ii) Flow patterns and energy generation around a hole: the effects of Lense- Thirring precession on the flow pattern, the role of magnetic fields, and the pro- duction of relativistic jets. (iii) Tidal disruption of a star by a massive black hole.
eBook - PDFThe Curious History of Relativity
How Einstein's Theory of Gravity Was Lost and Found Again
- Jean Eisenstaedt(Author)
- 2018(Publication Date)
- Princeton University Press(Publisher)
The two Quasars turned out to have identical spec- troscopic characteristics (i.e., similar emission spectra), which was truly surprising. Both Quasars presented not only the same set of lines, but the intensity of each line was the same for both of them; moreover all lines had the same redshift, within the mea- surements’ margin of error. Was this a binary quasar, or only one quasar and its repeated image? The probability of distinguishing two objects with the same spectrum in such a tiny portion of the sky was practically zero, so it was concluded that even if the images were different, they were coming from one and the same object; in other words, it was a gravitational mirage. The phenomenon was also observed using radio waves, and the galaxy responsible for the deflection was eventually found, though not without effort: it was a very pale spot between the two images, located midway between the quasar and our own galaxy. But how could this happen? Several conditions must be met for this phenomenon to take place, but geometry (for it is ultimately a question of geometric optics) is crucial here. The deflector (typically a galaxy but it could also be a black hole) must not only be located on the trajectory of the light rays, but also at a certain specific distance from the source for the observer to receive the primary and/or secondary images. Depending on the configuration, the observer will receive a dou- ble image or a ring-shaped one, or more or less identifiable spots, often weakened but sometimes amplified. In cases where the source quasar presented luminosity “jumps,” astronomers were able to measure the different times light took to reach them (af- ter traveling through A or through B) and so obtained a detailed model of the phenomenon. This new technique allowed them to evaluate the quasar’s absolute distance in a particularly accurate way. The diagram in figure 15.3 shows an “optimal” configura- tion for a gravitational mirage to take place.
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