Do any particles in AGN jets escape the galaxy?

Do any particles in AGN jets escape the galaxy?

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I have read, at for example, that whole stars can be ejected from certain galaxies. "These are thought to have been part of a binary star system that broke apart as it approached a supermassive black hole. As one star was captured, and the other was pushed away at a velocity exceeding the escape velocity of the Galaxy."

My question is about the jets that can form at the poles of the rotating supermassive black hole: "At the rotation axis of the supermassive black hole, matter from the accretion disc can be pushed away at the speed of light, creating jets that can extend for thousands of light-years."

At thousands of light-years away, is this matter still gravitationally bound to the galaxy? (If not, maybe some particles can go around in a big loop that takes a while.)

Yes, definitely.

While some matter returns to the galaxy as a so-called "galactic fountain" (e.g. Biernacki & Teyssier 2018), some material is ejected at super-escape velocity, becoming part of the intergalactic medium.

This is one of the mechanisms responsible for polluting the intergalactic medium (IGM) with metal-rich gas (i.e. elements heavier than helium). A recent observation of this is presented by Fujimoto et al. (2020).

The IGM itself is too dilute to form stars, and hence metals, itself, but is nevertheless observed to contain quite a lot of metals, usually seen as absorption lines in the spectra of background quasars (e.g. Songaila & Cowie 1996; Aguirre et al. 2008). These must be blown out from the galaxies, either by stellar feedback (through radiation pressure, cosmic rays, and supernova feedback), or by AGN activity (see also Germain et al. 2009).

Note however that although material can reach relativistic velocities, "pushed away at the speed of light" is just a tad too fast. Only massless particles can travel at the speed of light, but if you're massless it isn't really a big achievement - photons do that all the time, which is why we see the galaxies in the first place.

Astronomy Without A Telescope – Blazar Jets

Polar jets are often found around objects with spinning accretion disks – anything from newly forming stars to ageing neutron stars. And some of the most powerful polar jets arise from accretion disks around black holes, be they of stellar or supermassive size. In the latter case, jets emerging from active galaxies such as quasars, with their jets roughly orientated towards Earth, are called blazars.

The physics underlying the production of polar jets at any scale is not completely understood. It is likely that twisting magnetic lines of force, generated within a spinning accretion disk, channel plasma from the compressed centre of the accretion disk into the narrow jets we observe. But exactly what energy transfer process gives the jet material the escape velocity required to be thrown clear is still subject to debate.

In the extreme cases of black hole accretion disks, jet material acquires escape velocities close to the speed of light – which is needed if the material is to escape from the vicinity of a black hole. Polar jets thrown out at such speeds are usually called relativistic jets.

Relativistic jets from blazars broadcast energetically across the electromagnetic spectrum – where ground based radio telescopes can pick up their low frequency radiation, while space-based telescopes, like Fermi or Chandra, can pick up high frequency radiation. As you can see from the lead image of this story, Hubble can pick up optical light from one of M87‘s jets – although ground-based optical observations of a ‘curious straight ray’ from M87 were recorded as early as 1918.

Polar jets are thought to be shaped (collimated) by twisting magnetic lines of force. The driving force that pushes the jets out may be magnetic and/or intense radiation pressure, but no-one is really sure at this stage. Credit: NASA.

A recent review of high resolution data obtained from Very Long Baseline Interferometry (VLBI) – involving integrating data inputs from geographically distant radio telescope dishes into a giant virtual telescope array – is providing a bit more insight (although only a bit) into the structure and dynamics of jets from active galaxies.

The radiation from such jets is largely non-thermal (i.e. not a direct result of the temperature of the jet material). Radio emission probably results from synchrotron effects – where electrons spun rapidly within a magnetic field emit radiation across the whole electromagnetic spectrum, but generally with a peak in radio wavelengths. The inverse Compton effect, where a photon collision with a rapidly moving particle imparts more energy and hence a higher frequency to that photon, may also contribute to the higher frequency radiation.

Anyhow, VLBI observations suggest that blazar jets form within a distance of between 10 or 100 times the radius of the supermassive black hole – and whatever forces work to accelerate them to relativistic velocities may only operate over the distance of 1000 times that radius. The jets may then beam out over light year distances, as a result of that initial momentum push.

Shock fronts can be found near the base of the jets, which may represent points at which magnetically driven flow (Poynting flux) fades to kinetic mass flow – although magnetohydrodynamic forces continue operating to keep the jet collimated (i.e. contained within a narrow beam) over light year distances.

Left: A Xray/radio/optical composite photo of Centaurus A - also not technically a blazar because its jets don't align with the Earth. Credit: X-ray: NASA/CXC/CfA/R.Kraft et al. Submillimeter: MPIfR/ESO/APEX/A.Weiss et al. Optical: ESO/WFI. Right: A composite image showing the radio glow from Centaurus A compared with that of the full Moon. The foreground antennas are CSIRO's Australia Telescope Compact Array, which gathered the data for this image.

That was about as much as I managed to glean from this interesting, though at times jargon-dense, paper.

On the Origin of Galactic Jets

An extraordinary cosmic laboratory 21 million light-years away is providing radio astronomers their best opportunity yet to decipher the mysteries of the ultra-powerful “engines” at the hearts of many galaxies and quasars. An international research team using the National Science Foundation’s Very Long Baseline Array (VLBA) and Very Large Array (VLA) radio telescopes has peered deeply into the core of the galaxy NGC 4258, learning important new information about the mysterious region from which high-speed jets of subatomic particles are ejected. The scientists announced their findings today at the American Astronomical Society meeting in Toronto, Ontario.

The new research provides significant quantitative support for a theoretical model for the origin of such jets first proposed in 1979.

NGC 4258 is the galaxy in which a warped disk of water molecules was discovered in 1994. That disk, observed in detail with the VLBA, was shown to be orbiting a central mass some 35 million times more massive than the Sun. That central mass, the astronomers believe, is a black hole. More recent studies of the disk and its surroundings have given astronomers their most detailed look yet at the heart of an active galactic nucleus (AGN), including the ability to pinpoint the exact center of the system, where the black hole resides.

The 1994 observations provided the best evidence to date for the existence of a black hole at the heart of a galaxy. Black holes, so dense that not even light can escape their gravitational fields, have long been suspected as the driving force behind the energetic central engines of AGNs. The fortuitous existence of the molecular disk in NGC 4258 has helped astronomers use the ultrasharp radio “vision” of the continent-wide VLBA to probe with unprecedented clarity into the heart of that galaxy’s central engine.

The researchers are: James Herrnstein, James Moran, and Lincoln Greenhill of the Harvard-Smithsonian Center for Astrophysics Philip Diamond of the National Radio Astronomy Observatory in Socorro, NM Mikoto Miyoshi of Japan’s Misusawa Astrogeodynamics Observatory and Naomasa Nakai and Makoto Inoue of Japan’s Nobeyama Radio Observatory. The work formed the basis of Herrnstein’s Ph.D. dissertation at Harvard University.

The extraordinary detail of the observations is made possible by the fact that the water molecules in the disk orbiting the black hole are amplifying microwave radio emissions in the same manner that a laser amplifies light. These natural amplifiers are called cosmic masers, and they produce bright targets for radio telescopes. Study of water masers at the center of NGC 4258 is what revealed the orbiting disk in 1994.

Further studies of the water masers in NGC 4258 now have allowed the research team to deduce the exact location of the object orbited by the disk. In addition, new observations of the galaxy’s center show radio emission the astronomers believe traces the inner parts of the high-speed jets. Combined, these new observations allow measurement of the distance between the black hole and the innermost observable portions of the jets.

Such measurement is extremely important, because the standard theoretical model, proposed in 1979 by Roger Blandford of Caltech and Arieh Konigl of the University of Chicago, makes a clear prediction that all detected radio emission will be offset from the central engine generating the jets. The new radio observations of NGC 4258 are the first to show the exact location of the core of an AGN, and thus the first to allow measurement of the offset between the core and the detected emission closest to it.

Significantly, the offset measured in NGC 4258 is fully consistent with the quantitative prediction made by the model of Blandford and Konigl.

“There has been a lot of speculation about the relationship between radio jets and black holes over the years,” said Herrnstein. “But this measurement precisely pins down the geometric relationship between them in this object.”

In addition to these measurements, the research team also has recorded the movement of individual maser regions within the orbiting disk. Such motion was expected, and helps further confirm the fact that the masers are indeed part of a disk orbiting a black hole. These motions are seen in masers within the part of the disk closest to our line of sight, where orbital motion would be most evident to us. The masers observed at the edges of the disk (as seen from Earth) do not show any such measurable proper motion over time.

Moran notes that “Although the period of rotation of the megamaser disk is about 800 years, the movement of the masers during the two years of observations was about 60 microarcseconds, equivalent to a motion of about one millimeter seen at a distance of 3,000 kilometers. Being able to witness the disk turning at such a great distance is very exciting.”

Another benefit will come from combining the measurements of the proper motions with measurements of the Doppler shift in the radio emission from the masers at the disk’s edge. These two pieces of information allow the astronomers to calculate the distance to NGC 4258 with greater precision than before. This distance calculation will not be subject to many of the uncertainties that plague other extragalactic distance measurements, and thus will help calibrate the still-uncertain cosmic distance scale for other galaxies.

The researchers still are refining their calculations of the distance, but expect to arrive at a figure accurate within 5 percent.

“Such precision is possible because of the well-understood dynamics of the system,” said Greenhill. “It is a purely trigonometric method, independent of the normal hierarchy of extragalactic distance indicators.”

The galaxy NGC 4258 also is known as Messier 106, and is visible in moderate-sized amateur telescopes in the nighttime winter sky of the northern hemisphere, near the Big Dipper.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc. The Harvard-Smithsonian Center for Astrophysics is operated by the Harvard College Observatory and the Smithsonian Astrophysical Observatory.

Powerful Black Hole Jet Explained

While we may never know what it looks like inside a blackhole, astronomers recently obtained one of the closest views yet. The sightingallowed scientists to confirm theories about how these giant cosmic sinkholesspew out jets of particles travelling at nearly the speed of light.

Ever since the first observations of these powerful jets,which are among the brightest objects seen in the universe, astronomers have wonderedwhat causes the particles to accelerate to such great speeds. A leading hypothesissuggested the blackhole's gigantic mass distorts space and time around it, twisting magneticfield lines into a coil that propels material outward.

Now researchers have observed a jet during a period of extremeoutburst and found evidence that streams of particles wind a corkscrew pathaway from the black hole, as the leading hypothesis predicts.

"We got an unprecedented view of the inner portion ofone of these jets and gained information that's very important to understandinghow these tremendous particle accelerators work," said Boston University astronomer Alan Marscher, who led the research team. The results of the studyare detailed in the April 24 issue of the journal Nature.

The team studied a galaxy called BL Lacertae (BL Lac), about950 million light years from Earth, with a central black hole containing 200million times the mass of our Sun. Since this supermassive black hole's jetsare pointing nearly straight at us, it is called a blazar (a quasar is oftenthought to be the same as a blazar, except its jets are pointed away from us).

The new observations, taken by the National ScienceFoundation's Very Long Baseline Array (VLBA) radio telescope, along with NASA'sRossi X-ray Timing Explorer and a number of optical telescopes, show materialmoving outward along a spiral channel, as the scientists expected.

These data support the suggestion that twisted magneticfield lines are creating the jet plumes. Material in the center of thegalaxy, such as nearby stars and gas, gets pulled in by the black hole'soverwhelming gravity and forms a disk orbiting around the core (the material'sinertia keeps it spiraling in a disk rather than falling straight into theblack hole). The distorted magnetic field lines seem to pull charged particlesoff the disk and cause them to gush outward at nearly the speed of light.

"We knew that material was falling in to these regions,and we knew that there were outbursts coming out," said University of Michigan astronomer Hugh Aller, who worked on the new study. "What's reallybeen a mystery was that we could see there were these really high-energyparticles, but we didn't know how they were created, how they were accelerated.It turns out that the model matches the data. We can actually see the particlesgaining velocity as they are accelerated along this magnetic field."

The astronomers also observed evidence of another phenomenonpredicted by the leading hypothesis ? that a flare would be produced whenmaterial spewing out in the jets hit a shock wave beyond the core of the blackhole.

The Top Ten Signs that you have found an AGN

Yes, yes, what we call an AGN is just the consequence of a supermassive black hole at the center of a galaxy. But we can't really see the black hole itself directly, so -- how can we recognize an AGN?

Let's look at a list of observational evidence for AGN .

Bower et al., ApJ 492, 111L (1998) measured the motions of gas in the nucleus of M84 and found a clear signature of rotation.

If you can't see honest-to-goodness Keplerian motion, at least you can see evidence for very high velocities.

Most AGN also display narrow emission lines, much less broad than the broad emission lines (not surprisingly). The material producing this light must be farther from the black hole, in order to move more slowly.

The trouble is -- not all narrow emission lines in galaxies are due to AGN, or, more specifically, to phenomena associated with a supermassive black hole. Regions of star formation can also produce emission lines, and some galaxies, the "starburst galaxies", exhibit very large amounts of star formation.

Fortunately, if one can measure the properties of some specific emission lines, one can distinguish between light created by starburst regions from light created near AGN.

Figure taken from Groves and Kewley, "Pathways Through an Eclectic Universe", ASPC 390, 283 (2008)

If you see a linear feature, or pair of features, extending outward from the nucleus of a galaxy, there's little doubt that a big black hole lives nearby. Take a look at M87, for example.

Figure courtesy of Adam Block , Mount Lemmon SkyCenter and University of Arizona .

Image courtesy of H. Marshall (MIT) et al., CXC, NASA, F. Zhou, F. Owen ( NRAO), J. Biretta (( STScI), E. Perlman ( UMBC) et al.

However, you must be a little careful: there are jets which don't derive from supermassive black holes. Right here in our own Milky Way Galaxy, for example, the variable star SS433 produces a jet of its own, as these radio images show.

Figure taken from a very nice exercise written by Irving Robbins

So, no, the material in the jets is not moving faster than light . just very, very close to it.

Some galaxies show pairs of gigantic regions of radio emission far from the center of the galaxy in opposite directions.

Figure courtesy of the NRAO Image Gallery and NRAO / AUI / NSF

Compare the radio emission to the visible extent of the galaxy:

These so-called "radio lobes" are regions of the intergalactic medium which have been struck by the high-energy particles in the jets.

The spectra of AGN are strange -- even if one ignores the broad and narrow emission lines. Look at the shape of the continuum for an AGN .

and for an ordinary galaxy:

Because AGN emit so much light at short wavelengths, their optical colors stand out from those of ordinary stars and galaxies. This makes them easy to find in a big survey: just look for objects with unusual, blueish colors.

Figure taken from Finlator et al., AJ 120, 2615 (2000)

Note, however, that at high redshifts, AGN and quasars will move to different regions of any color-color diagram.

The cause of the "big blue bump" is thermal emission from an accretion disk around the central supermassive black hole. Since the "effective" temperature of this disk is something like a few hundred thousand Kelvin, the peak of the emission lies in the UV.

Thermal emission from the accretion disk ought to produce soft X-rays -- those with energy around 1-5 keV. Lots of other types of sources can produce soft X-rays, too, such as very young protostars and the warm ionized medium, so one must be careful in using X-ray emission to identify AGN. Below is the Spectral Energy Distribution (SED) for Markarian 421.

Figure taken from Abdo et al., ApJ 716, 30 (2010)

The innermost regions of the accretion disk can produce "hard" X-rays, with energy exceeding 5 keV. In a few cases, we can see radiation from iron atoms -- the K-alpha line, emitted when an electron drops from the L shell to the K shell in the atom -- -- which shows the effects of special and general relativity. Here's an illustration of the theory,

and here's an example of the data.

Let's look at that SED for Mark 421 again. The peak on the right represents photons with extremely high energies: gamma rays. Not all AGN are observed to produce such gamma rays, but some do. What physical process could produce photons with such high energies?

Figure taken from Abdo et al., ApJ 716, 30 (2010)

The answer is inverse Compton scattering. Recall that AGN have jets full of relativistic particles flying out from their centers.

Some AGN vary in luminosity by large factors over just a few days, or even a few hours. Look at 3C273, for example, in X-rays:

Figure taken from Kataoka et al., MNRAS 336, 932 (2002)

Figure taken from Kataoka et al., MNRAS 336, 932 (2002)

Some AGN radiate light which is polarized -- a very unusual situation. The study of optical polarization by Smith et al., ApJ 663, 118 (2007) indicates that the polarized light is concentrated in the nucleus, as one would expect .

Radio interferometry can probe the polarization on scales fine enough to show its spatial structure.

Figure taken from Gabuzda, in "High Energy Blazar Astronomy", ASPC 299, 99 (2003)

Light from the jets is produced by synchrotron radiation, as energetic electrons spiral around a magnetic field. The magnetic field lines impose an order on the radiation (or at least a portion of it), causing the electric field vectors to be aligned. If we can see the jets directly, we may expect to detect polarized light.

But there's another way that AGN can produce polarized light, in a more indirect manner. Consider the "unified model" of an AGN. (You didn't think you could escape this picture, did you?)

Figure taken from the Essential Radio Astronomy course at NRAO and based on the original figure of Urry and Padovani, PASP 107, 803 (1995)

If the observer views the system from the poles of the torus, he can see the broad-line region and perhaps even the accretion disk itself. If the observer views the system from edge-on, the torus blocks his view of those regions. However, some of the light from the inner regions may fly out along the poles, then scatter off material farther away and bounce toward the observer. In other words, some light from the inner regions may reach the observer, but only after being scattered. The scattering process will polarize the light, so any of this radiation from the "hidden" portions of the system ought to be polarized.

Figure taken from Smith et al., MNRAS 359, 846 (2005)

And, voila, when we observe some edge-on AGN, we do see polarized light which follows the expected pattern in wavelength and polarization fraction.

  1. High velocity, circular gas motions
  2. Broad emission lines
  3. Narrow emission lines (of the right sort)
  4. Jets (big ones)
  5. Radio lobes
  6. Wierd optical/IR colors -- the "big blue bump"
  7. Soft and hard X-ray emission
  8. Gamma-ray emission
  9. Variability on a range of time scales
  10. Polarized light

For more information

Copyright © Michael Richmond. This work is licensed under a Creative Commons License.

How to Escape from a Black Hole

Avery E. Broderick/University of Waterloo/Perimeter Institute

Black holes have a bit of an image problem. That’s to be expected from an immense remnant of a stellar explosion with billions of times the mass of the sun and a gravitational pull so powerful, not even light can escape. Anything that ventures too close gets swallowed whole, never to be seen again. Or so the popular thinking goes. But there’s a dramatic exception to that ironclad rule: all over the cosmos, galaxies with black holes at their center produce powerful energy jets, or blasts of superheated gas and dust that erupt from the very matter swirling down into the hole and travel outward for hundreds of thousands of light-years.

Astronomers have cataloged thousands of such energy jets over the decades, but what they’ve never been able to figure out is what powers them. How can material that effectively circles the galactic drain suddenly wrest itself free, and with such titanic force? Now, thanks to a study by an international team of astrophysicists that was published in the journal Science, there appears to be an answer — one that helps explain not only how the galactic pyrotechnics are produced but also how galaxies themselves grow and expand.

What astronomers — with a little help from Albert Einstein — already understand is that every black hole is surrounded by what’s called an event horizon, a threshold at which matter reaches a point of no return. It may be impossible to see the black hole itself, but with the right instruments you can detect the matter at the last moment before it disappears and, in effect, measure and mark the presence of the hole by the very absence it produces. Material at the event horizon forms a so-called accretion disk, a concentrated swirl of dust and gas that orbits the hole at nearly the speed of light, gradually feeding itself inward. It’s at that point that, well, something happens to produce the jets. But what?

To find out, a team led by Sheperd Doeleman, an astrophysicist at MIT’s Haystack Observatory, focused on a jet bursting from a black hole at the center of the M87 elliptical galaxy, 54 million light-years from the Milky Way. That jet, studied since the early 1900s and among the closest within viewing range, also happens to emanate from a black hole with a highly visible event horizon — mostly because M87 ranks among the sky’s brightest deep-space objects, meaning there are plenty of light emissions reflecting off the debris in the accretion disk.

That doesn’t mean the disk can be studied with any detail, however. Black holes are very small objects on a cosmic scale, and 58 million light-years is still 58 million light-years. To sharpen their resolution, Doeleman and his team thus used a method known as Very Long Baseline Interferometry (VLBI), in which multiple radio telescope dishes collect wave emissions from different perspectives and later align them into measurement data, much as the mirror and lens on a standard telescope aligns light waves into an image. “It’s a specialized thoroughbred technique which gives us the highest amount of detail of anything available to astronomers,” says Doeleman. For their study, they used data from radio dishes in Arizona, California and Hawaii, combining them in such a way that the observatories acted as a single, massive instrument with a resolution 2,000 times that of the Hubble Space Telescope. That revealed a lot.

M87’s event horizon, the researchers learned, is about the size of our solar system. The matter that produces the jets appears to come from an orbital position near the innermost edge of the accretion disk, about 5.5 times as distant as the horizon itself. That seems remote, but according to Einstein’s gravitational theories, it’s the last possible point at which matter can move in a stable orbit, because space time is distorted near a black hole. It’s also the birthing ground for the jets, possibly because magnetic fields embedded in the material that’s circling near the hole become twisted, carrying energy away in the form of an electromagnetic blast that is filled with charged particles — the very charged particles that emit the radio waves the scientists collected from Earth for their study.

The M87 jet’s tight orbit fits only one theoretical model of black-hole dynamics, one that suggests that gravity from the swirling accretion disk can rotate a black hole over time, causing both to spin in the same direction and drawing the innermost orbit into the range where the astronomers found the M87 jet. That supports years of conjecture that black holes are anything but motionless. Says Doeleman: “The black hole has to be spinning to explain those measurements.”

Although the study centers on a single jet, the ramifications extend across the galaxy, since the energy blasts broadly distribute matter and energy, feeding and disrupting star formation. Astronomers therefore hope their next look at the jets’ launch pad will be even more detailed. They plan on expanding their telescope array to include radio dishes worldwide, increasing the sensitivity of their virtual telescope by a factor of 10 and possibly leading to images rather than just measurements. As good as the high-speed energy jets are at escaping black holes, avoiding astronomers’ prying eyes will — with luck — prove much more difficult.

Gravitational lenses as high-resolution telescopes

3.3 Relativistic jets

Relativistic jets are beams of plasma launched in the vicinity of accreting SMBHs. There are two competing theories on the origin of the jet power. The first proposes that jets are powered by the gravitational energy of accreting matter that moves toward the black hole, where jets may either be launched purely electromagnetically [81,82] or as the result of magnetohydrodynamic processes at the inner regions of the accretion disk [61,83] . The second theory utilizes the rotational energy of a rotating black hole [84] .

The relativistic motion of plasma in jets leads to multiple effects of the special theory of relativity including relativistic boosting, time dilution, and apparent superluminal motions. The radiation emitted by jets is Doppler boosted toward the observer by D n [85–87] . The Doppler factor is defined as

where β = v ∕ c is the velocity of moving plasma, v , in units of the speed of light c , and θ o b s is the angle to the line-of-sight with the observer. The exponent n combines effects due to the K correction [88] and the Doppler boosting caused by relativistic aberration, time dilation, and the solid angle transformation [50] . In the calculations presented in this review, the index n = 4 is assumed.

The emission experiences strong relativistic boosting when the jet is pointed close to the line-of-sight ( < 2 0 ∘ ). Relativistic beaming changes the apparent beam brightness, as a result, only the side of the jet pointed toward the observer is visible, and the resulting extremely luminous object is called a blazar. Features used in blazar classification include the presence of a compact radio core, with flat or even inverted spectrum, extreme variability (both in timescale and in amplitude) at all frequencies, and a high degree of optical and radio polarization [89] .

Non-thermal emissions produced by a relativistic jet dominates the broadband spectrum of blazars. The spectral energy distribution (SED) of blazars is characterized by two broad spectral components. A low-energy component extends from the radio up to optical/UV/X-rays is produced by the synchrotron radiation of relativistic electrons. The high-energy component extending from X-rays to gamma-rays, according to recent interpretations, is produced by inverse-Compton (IC) radiation with a possible source of seed photons, being either the synchrotron radiation, the broad line region (BLR), or the dusty torus (DT).

Blazars are divided into two classes: flat spectrum radio quasars (FSRQs) and BL Lac objects FSRQs are distinguished by the presence of broad emission lines, which are absent or very weak in BL Lac objects. The high-energy component of FSRQs is usually much more luminous than the low-energy one. The high-energy component of BL Lac objects results from the Comptonization of synchrotron photons. The luminosity at the peak of the high-energy component is comparable or lower than the synchrotron peak luminosity [90] .

Relativistic jets of blazars provide environments to accelerate particles to velocities close to the speed of light. The two most popular processes used to explain particle acceleration in relativistic jets are internal shock scenario [91,92] , and reconnection of magnetic field [93,94] The internal shock scenario assumes an instability in the central engine, which results in ejection of shells of plasma [95] . The shells with inhomogeneous velocity or mass distribution “catch up”, a nonelastic collision occurs, and particles are accelerated through the first-order Fermi mechanism - a process in which particles scatter between the upstream and downstream regions of shocks to gain energy [96–98] . Acceleration of particles in shocks is commonly used to model non-thermal phenomena in the universe using Monte Carlo test particle simulations [e.g., 99] and semianalytic kinetic theory methods [100–102] .

The observed spectral energy distribution (SED) of blazars can be well reproduced with shock scenario [103–106] . However, the first-order Fermi mechanism requires relatively long timescales of the order of days to sufficiently accelerate particles. Observation of blazars show variability down to (sub-)hour time scales [107,108] , challenging the shock scenario.

Magnetic reconnection was proposed as a more likely candidate that shocks for explaining short variability timescales observed in the jet emission. During an event of magnetic reconnection, the annihilation of field lines of opposite polarity transfers the field energy to the particles. It is still under debate if shocks or magnetic reconnection accelerate particles in relativistic jets [109] . Both mechanisms are based on assumptions that the energy dissipation is happening at small distances, ∼ parsecs from the central engine. The recent observations show evidence that variable emission can be produced more than a dozen of parsecs from the central engine, which challenges both scenarios of particle acceleration.

Jets transport energy and momentum over even megaparsec distances [110] . Radio interferometry resolves the details of complex jet structure that includes hotspots and blobs [111–114] . Improved angular resolution of current X-ray satellites demonstrates that the high energy emission from jets also form structures as large as hundreds of kpcs [115–117] . At gamma rays, the technology is inadequate to resolve the sources. However, the short variability timescales, < 1 day, suggest that the sources of the gamma-ray radiation during a flare is of the order of 10 −3 parsec [118] . To explain the observed rapid variability and to avoid catastrophic pair production in blazars, models assume that the γ -rays are produced in compact emission regions moving with relativistic bulk velocities in or near the parsec scale core [119] . However, recent detection of sub-TeV emission from FSRQs suggests that the blazar zone can be located several parsecs away from a SMBH [120] . It remains unclear whether the radiation source is the same at all energies. The source of radiation may be close to the base of the jet or it may originate from blobs of plasma moving along the jet at relativistic speeds.

Black holes and the quasar connection

Before Hubble, quasars were considered to be isolated star-like objects of a mysterious nature. Hubble has observed several quasars and found that they all reside at galactic centres. Today most scientists believe that super massive black holes at the galactic centres are the "engines" that power the quasars.

Prior to the launch of Hubble a handful of black hole candidates had been studied but the limitations of ground based astronomy were such that irrefutable evidence for their existence could not be obtained. Black holes themselves, by definition, cannot be observed, since no light can escape from them.

However, astronomers can study the effects of black holes on their surroundings. These include powerful jets of electrons that travel huge distances, many thousands of light years from the centres of the galaxies.

A stream of electrons ejected from the centre of galaxy M 87.

Matter falling towards a black hole can also be seen emitting bright light and if the speed of this falling matter can be measured, it is possible to determine the mass of the black hole itself. This is not an easy task and it requires the extraordinary capabilities of Hubble to carry out these sophisticated measurements.

The disk around the black hole at the centre of galaxy NGC 7052.

Hubble observations have been fundamental in the study of the jets and discs of matter around a number of black holes. Accurate measurements of the masses have been possible for the first time. Hubble has found black holes 3 billion times as massive as our Sun at the centre of some galaxies. While this might have been expected, Hubble has surprised everyone by providing strong evidence that black holes exist at the centres of all large galaxies and even small galaxies. Hubble also managed not only to observe the jets created by black holes but also the glowing discs of material surrounding a supermassive black hole.

Furthermore, it appears that larger galaxies are the hosts of larger black holes. There must be some mechanism that links the formation of the galaxy to that of its black hole and vice versa. This has profound implications for theories of galaxy formation and evolution and is an ongoing area of research in astronomy.

One big question which remains is why most galaxies in our cosmic neighbourhood, including the Milky Way, appear to have a dormant black hole which is not funnelling in large amounts of matter at present.

"Hubble provided strong evidence that all galaxies contain black holes millions or billions of times heavier than our sun. This has quite dramatically changed our view of galaxies. I am convinced that Hubble over the next ten years will find that black holes play a much more important role in the formation and evolution of galaxies than we believe today. Who knows, it may even influence our picture of the whole structure of the Universe. "

Duccio Macchetto
ESA astronomer, Head of the Science Policies Division, STScI

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Do any particles in AGN jets escape the galaxy? - Astronomy

Radio synthesis maps have shown jets in hundreds of AGN, on scales from subparsec to megaparsecs. The continuity of jets in direction indicates that the central generator has a memory over millions of years, and disk structures provide a natural way to control the direction of the jets. There is a vast literature on the collimation and production of jets I will mention only a few points here.

How fast are they? Structures of jets can indicate their Mach numbers (with respect to the external medium), but not immediately their absolute velocities. Some sources look as if the jets are rather slow and flexible, while others look like highly relativistic blowtorches. We do not even know for sure whether we are seeing a phase or group velocity when motions can be measured. Strong evidence for relativistic bulk motions comes from superluminal sources, in which the projected speed of motion (always outward from the core) of distinct blobs is 1-10c. A natural explanation is (backwards) time dilation in material approaching us at

0.9c the Doppler boosting of this material would make these objects bright, so that the boosted sources were observed first. The emitter stays only slightly behind its earlier radiated wavefronts, so the projected motion is quite rapid (see Superluminal Radio Sources, ed. Zensus and Pearson, Cambridge 1987). The governing equations reflect the relativistic Doppler dilation and boost effects. If we consider the projected separation between a stationary core and a blob moving away from it at a rate c &beta at an angle &theta to our line of sight, the apparent transverse velocity will be

which has a maximum value vmax

&gammac. The apparent jet/counterjet ratio R (for physically identical jets) becomes

where &xi is related to the spectral index &alpha by 3-&alpha for a confined blob and 2-&alpha for a continuous jet. The relativistic &gamma factor does not appear in the ratio because it is identical for both components, so that the geometric factors alone are left. The data show v = 1-10 c for superluminal sources (and subluminals also exist, mostly for nearby and fairly low-power objects like M87 and Cen A).

This material is not always the conventional jet in an early stage Barthel has shown that radio galaxies of large projected size (i.e. presumably viewed 90° to the jet axes) can have superluminal motions, and proposed an intermediate model in which material is initially ejected over a broad cone angle. Only the tiny fraction coming near our line of sight is boosted enough to see at high angular resolution, and it is this fraction that would exhibit superluminal motion. On larger scales, structures in the M87 jet a kiloparsec from the core have been found to show transverse velocities of 0.3 - 0.5c (Reid et al 1989 ApJ 336, 112 Biretta et al. 1989 ApJ 342, 128) - so far the only direct evidence that something in large-scale jets is moving at high speeds. In M87, HST imaging shows individual features with apparent transverse speeds anywhere from 0.6-6c (Biretta et al. 1999 ApJ 520, 621). These make sense for &gamma

6 and a jet oriented within 19° to the line of sight, rather different than the visual impression shown in the image below (a rotated section of the Hubble Heritage picture). Light-time effects (Penrose rotation) work to make planar features within a relativistic jet appear more edge-on than they really are, probably important for the region around knot A in the M87 jet which is often thought to be an internal shock front.

This issue - relativistic motion producing apparent superluminal motion - is not unique to radio galaxies and quasars. There is a class of galactic superluminal sources, associated with strong gamma-ray emission and evidently generated by accretion onto compact objects in a genuinely small-scale counterpart to the extragalactic cases (Levinson and Blandford 1996 A&AS 120, 129). In these instances, the distance to the source is not in serious dispute, using the galactic rotation curve and velocities of foreground H I clouds to estimate where they must lie. For the best-studied ones, GRS 1915+105, and GRO J1655-40, we have both the apparent separation velocity and the core-lobe separation on each side, giving extra data to fit the velocities. In both cases, the intrinsic velocity is close to 0.9c, with angles to the line of sight of 8-20°. These are in turn reminiscent of SS433 with velocities of 0.26c in jets which are highly collimated, precessing in a binary system, and cool enough to emit optical line radiation.

Work on radio galaxies with jets and ionization cones shows that there really are two different levels of collimation - a broad ionization cone, perhaps produced by an obscuring torus, and the much narrower collimated jets inside this cone (see the ESO Extranuclear Activity workshop).

Tracking the features within small-scale jets has revealed interesting complications. The paths are not always radial to the nucleus, usually taken as the source with the flattest spectrum in ambiguous cases. This is based on the general principle that synchrotron spectra are flattened at lower frequencies by self-absorption, so the densest plasma will have a flat or inverted spectrum. An interesting case is 3C 345, in which emission features repeatedly appear off the core and follow fairly consistent nonradial paths.This can be seen in Fig. 1 of Zensus et al. (1995 ApJ 443, 35, courtesy of the AAS) in which a new component appears in late 1985, brightens, and moves outward changing its relative position angle in the process:

Such motions, and the pronounced wrapped filamentary structure seen in nearby jets such as M87 and Centaurus A, suggest an important role for motion in helical patterns. This is easier to understand if much of what we see isn't physical blobs but enhancements in particle emission (perhaps linked to injection of particles) coupled with relativistic beaming. These objects are the ones most often detected in gamma rays. Survey with VLBA and incidence of superluminal/compact structures.

An important theme in jet studies has been the notion that BL Lacertae objects are dominated by the relativistically beamed emission from jets seen almost exactly end-on. This makes sense from viewpoints of energetics and host-galaxy properties, and predicts well-defined relations between the observed BL Lac counts and the luminosity function of the parent population which make sense if the parent population consists of the numerous FR I radio galaxies. In these cases, the jet emission is so strongly boosted that it becomes quite difficult to learn anything else about the source. Extended emission around blazars, while requiring high dynamic range to see in the presence of the strong core, is typically of about the luminosity and extent we'd see from FR I lobes seen end-on, giving some additional credence to this picture (Antonucci et al. 1986 AJ 92, 1). Antonucci reviewed these issues extensively in the14th Texas Symposium, 1989 Ann. NY Acad. Sci. 571, 180).

The magnetic field is well-ordered in many jets, as shown by polarization measurements. Synchrotron radiation can be very highly polarized (50%) if the field is globally ordered, and some sources apprach this level. The electric vectors show clear structure and alignment an especially common pattern is for the field lines to be along the jet in the inner portions and transition to an azimuthal configuration farther out. This is seen in M87, as well as in PKS0521-36, for which I'll show my own 2cm observations (1986 ApJ 302, 296).

Many objects with jets, especially the powerful FR II radio sources with long and highly collimated jets, show hot spots - compact enhancements in brightness of the lobes. Cygnus A is a prime example. These may in turn have internal structure, and often have the flattest spectra (thus most energetic particle populations) in the extended lobes. They have been pictures as encounter surfaces between the jet flows and a mostly unseen surrounding medium, with compression of the magnetic field occurring and thus vastly increased emissivity. Some (such as Pictor A) have such high-energy electron populations that sychrotron emission continues through the optical into the X-ray regime.

At this point, there are only two important things we really don't understand about jets - how they get accelerated to begin with and how they manage to stay so well collimated. An overpressured, freely expanding jet would have much larger cone angles than we see even for highly relativistic motion, so such notions as magnetic confinement are attractive (especially since we know there's a significant field - because we see synchrotron radiation). A rought estimate (and fairly robust minimum value) for the field strength is the often-used equipartition value, which has equal energy density in particles and fields (which also almost exactly minimizes the energy density for a particular observed luminosity). Kellerman and Owen (in Galactic and Extragalactic Radio Astronomy, 2nd edition) give minimum-energy field Bmin = 1.5 × 10 -4 &theta 9/7 z -2/7 S 2/7 where the field is in gauss, the angular size &theta is in arcseconds, and flux density S in Jy. Typical values are 10 -3 -10 -4 gauss.

Many of the same considerations applicable to relativistic jets in AGN also seem to apply to gamma-ray bursts and their afterglows. The energy requirements become much more tractable if the luminosity is enhanced by beaming, and some of the afterglow light curves suggest that indeed the beaming is within a fairly narrow solid angle (instead of the isotropic but beamed emission we'd see from a relativistic fireball).

To briefly review properties of gamma-ray bursts:

A good set of overview reviews is included in the December 1995 PASP. Cosmic gamma-ray bursts were discovered serendipitously in 1965, while searching for terrestrial bursts which would indicate violations of the nuclear test-ban treaty. This happened when the Vela satellites were orbited one might suspect that there was a comparable Soviet program, but no public information seems to have been forthcoming. The first report in public was in 1973 the brief abstract continued to describe the state of our knowledge for 20 years: Bursts of high-energy radiation arrive randomly from unknown sources. Their durations range from shorter than 0.1 second up to minutes. Crude directional information eliminates obvious local sources. The directional accuracy of a single detector was (and remains) poor the best positions for bursts use time-of-flight "triangulation" from multiple detections including interplanetary spacecraft (for the stronger bursts, since only small hitch-hiker detectors can ride on probes designed for other purposes). Bursts last from a couple of seconds to two hours there is a wide variety of temporal structure, from smooth decays to highly structured quasiperiodic bursts. Before the launch of CGRO, it was widely believed that the bursts come from galactic neutron stars (from accretion events, starting with instabilities in accretion up to and including comet and asteroid impacts into the surface). However, with the surprising isotropy in distribution, cosmological models have gotten a new look. The major schemes here are of merging neutron-star binaries (note this is a guaranteed non-repeating event) or of some relative of a supernova outburst ("hypernova").

At cosmological distances, the energy release must be of order 10 52 ergs (Paczynski 1995 PASP 107, 1167). The arguments for a distant origin were originally quite general - isotropy plus log N -log S behavior, which to galaxy people fairly shout ``Cosmologically distant!". In this case, the behavior with flux implies that these objects occur at a rate increasing with cosmic time (which makes some sense given that the number of neutron stars and the number of coalescing binaries should grow with cosmic time). For these models, you do not expect repeating bursts, since the source is destroyed.

The major development was, of course, detection of optical afterglows around a few GRBs with well-determined positions. These turn out to be in galaxies at redshifts up to z=4 (starting with the report in IAU Circular 6588 on GRB 970228 and now a mainstay of the literature). Chandra data show Fe line emission for GRB 991216 at z

1, with line widths indicating that this surrounding material is expanding at about 0.1c. The implied abudances fit for recent supernova ejecta, reinforcing a connection between (some?) SN and GRBs.

It is virtually unavoidable that such powerful sources of high-energy radiation entail relativistic expansion of any material unfortunate enough to be involved. Even isotropic (spherical) expansion will involve beamed radiation, such that we would see radiation utterly dominated by material within a small angle &gamma on the spherical surface, with its apparent flux boosted by the same relativistic beaming factor as above. It is important to know whether the expanding is really isotropic or jetlike because this vastly changes the energetics of the whole explosion (by numbers of order 2&gamma²) as well as the physical picture. One often-discussed signature of jetlike structure would be a break in the fading of the afterglow. In general, as a jet cools and becomes less relativistic, one expects a fading enhanced by a decline in the beaming factor, which would undergo a slope transition when the beaming factor is comparable to the jet's cone angle. A spherical fireball would have no such abrupt transition. One way to see this is to note that initially, for a highly relativistic expansion, the jet and sphere will produce identical observed properties. Some afterglows (such as GRB 991216) do appear to show such breaks in their light curves, although others show inconsistent breaks at various frequencies or none at all during the few weeks they can typically be followed. Sari (in the 5th GRB Symposium volume from Huntsville, 1999, p. 504) points out that the best evidence for jets is in the bursts with the greatest calculated (isotropic) luminosity, another bit of support for tightly beamed radiation being important.

This year's most popular picture for GRBs involves some class of supernovae which produce relativistic jets. If a neutron star has just been formed, the energetics are appropriate, and in fact there were some puzzling speckle data on SN 1987A that might, in hindsight, have been showing us high-speed blobs leaving the scene. A promising interpretation has a supernova producing either a black hole or a "hot" neutron star surrounded by a very dense (and short-lived) accretion disk, so that some material escapes relativistically at its poles. « Accretion and outflow in AGN | Host galaxies of active nuclei » Course Home | Bill Keel's Home Page | Image Usage and Copyright Info | UA Astronomy