Active Galaxies and Quasars
Limits of the Observable Universe

posted Apr 18, 2001, jeb
revised May 1, 2001, jeb

Outline

Beyond the Local Realm


Seyfert Galaxies


Radio Galaxies


BL Lac Objects


The Central Engine of an Active Galaxy


Quasi-stellar Objects


Active Galaxy Evolution

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Beyond the Local Realm

Some galaxies are more luminous (and more active) than normal galaxies
• These are known as Active Galaxies
  • M87
    • Central galaxy in the Virgo cluster (Virgo A)
      • 60 Mly away
    • more
  • Centaurus A
    • Closest radio galaxy
      • 15 Mly away
    • more

The most energetic active galaxies can emit hundreds or thousands of times more energy per second than the Milky Way

The nature of the energy emitted from a normal galaxy differs from that of an active galaxy
• Plot of wavelength dependence
• A normal galaxy has a spectrum that looks like a collection of stars following the blackbody curve

An Active Galaxy will have large emission of electromagnetic radiation in the infrared, radio, ultraviolet, and X-ray
• That is large compared to a normal galaxy like the Milky Way

An Active Galaxy will be characterized by
1. higher than normal luminosity
2. non-thermal emission
3. rapid variability
4. peculiar appearance
   • nucleus and large structures
5. jets from nucleus
6. emission lines broad or narrow
7. polarized emission

Synchrotron Radiation
• When charged particles accelerate they emit electromagnetic radiation
  • Radio transmitter
  • Magnetic fields bend the paths of charged particles
    => acceleration => radiation

Types of Active Galaxies
• Seyfert Galaxies
• Radio Galaxies
• BL Lacs?
• Quasars (Quasi-stellar Objects)
  • very distant objects

A small percentage of all galaxies are active galaxies
• Most of the active galaxies are radio galaxies

The fraction of galaxies which are active galaxies increases with increasing distance from the Milky Way
• galaxies were generally more active in the past

Seyfert Galaxies

Seyfert Galaxies are spiral galaxies with extremely bright nuclei
• Nuclei 10,000 times as bright as the Milky Way

Seyfert Galaxies emit a lot of infrared and radio energy
• This is not "star-like"
  • That is, not like blackbody radiation

Seyfert Galaxies have broad emission lines
• Atoms emit photons of specific colors
  • The emission spectra of some well-known elements
• If the atoms are moving rapidly, the lines will be broadened by Doppler Effect
• The atoms can be moving rapidly if the galaxy is hot, or if it is rotating rapidly
• Therefore, we conclude, the Seyfert Galaxies are hot or rotating rapidly

Seyfert Galaxies have small nuclei (few light years)
• The irregular variations of a particular Seyfert galaxy's luminosity over a period of two decades.
  • Because this Seyfert, called 3C 84, emits most strongly in the radio part of the electromagnetic spectrum, these observations were made with large radio telescopes.
  • The optical and X-ray luminosities vary as well

Photographs of a Seyfert galaxy (NGC 5728) from the ground and from Earth orbit.
• This object is about 130 million light years (40 Mpc) away (one of the closest active galaxies known)

Radio Galaxies

Most active galaxies are Radio Galaxies
1. Compact radio galaxies
   • radio emission is same size or smaller than the optical image
   • core-halo radio galaxies
     • Radio contour map of a typical core-halo radio galaxy superimposed on an optical image
       • The radio emission from such a galaxy comes from a bright central nucleus, or core, surrounded by an extended, less intense halo
     
     
2. Extended radio galaxies
   • radio emission is larger than the optical image
   • lobe radio galaxies
     • Centaurus A with giant radio-emitting regions extending a million parsecs or more beyond the central galaxy
       • The lobes cannot be imaged in visible light and are observable only with radio telescopes
       • In this image, the lobes are filled with false color to indicate decreasing intensity from red to yellow to green to blue
     
     

The giant elliptical galaxy M87 (also called Virgo A)
• image is different at different wavelengths
• (a) A long optical exposure of its halo and embedded central region
• (b) A short optical exposure of its core and an intriguing jet of matter, on the same scale as (a)
• (c) A radio image of its jet, on a somewhat expanded scale compared with (b)
  • The red dot at left marks the bright nucleus of the galaxy; the red and yellow blob near the center of the image corresponds to the bright "knot" visible in the jet in (b)
• (d) A near-infrared image of the jet, at roughly the same scale as (c)

Centaurus A
• One of the most massive and peculiar galaxies known
• Believed to be the result of a galaxy collision some 500 million years ago
• The pastel false colors mark the radio emission
• Note that the radio lobes emit no visible light

Cygnus A
• also appears to be two galaxies in collision
• On a much larger scale, this cosmic object displays radio-emitting lobes on each side of the optical image
• The optical galaxy in is about the size of the small dot at the center of the "radiograph"
• A thin line of radio-emitting material joins the right lobe to the central galaxy
• The distance from one lobe to the other is approximately a million light years

Two images of the active "head-tail" galaxy NGC 1265
• A radiograph, in false color
• and the same radio data, in contour form, superposed on the optical image of the galaxy
• This object is likely moving rapidly through space, trailing a "tail" behind as it goes
  • a little like a comet, but on a vastly larger scale

Perhaps the core-halo and the lobe galaxies are largely a matter of perspective
• A central energy source produces high-speed jets of matter that interact with intergalactic gas to form radio lobes
  • The system may appear to us as either a lobe or a core-halo radio galaxy, depending on our location with respect to the jets and lobes

BL Lac Objects

BL Lacertae was thought to be a variable star
• discovered in 1929

Now this object, and other similar objects (BL Lacs) are recognized as extragalactic sources
• great variability of light
• extremely distant (accounting for faint, star-like appearance)
• located at the centers of relatively normal elliptical galaxies

Full understanding not yet found
• link between radio galaxies and quasars?
• radio galaxies with the lobe pointed at us?
• something else?

The Central Engine of an Active Galaxy

The leading theory for the energy source in active galactic nuclei (and quasars)
• These objects are powered by material accreting onto a supermassive black hole
• As matter spirals toward the hole it heats up, producing large amounts of energy
• At the same time high-speed beams of gas may be ejected perpendicular to the accretion disk, forming the jets and lobes seen in many active objects
• Magnetic fields generated in the disk are carried by the jets out to the radio lobes, where they play a crucial role in producing the observed radiation

A combined optical/radio image of the giant elliptical galaxy NGC 4261, in the Virgo Cluster,
• A white visible galaxy at center, from which red-orange (false color) radio lobes extend for about 200,000 light years (60 kpc)
• The close-up photograph of the galaxy's core reveals a 300 light year diameter disk surrounding a bright hub that presumably harbors a black hole

Imaging and spectroscopic observations of M87 support the idea of a rapidly whirling accretion disk at its core
• An image of the central region of M87 shows its bright core and jet
• The magnified view of the core suggests a spiral swarm of stars, gas, and dust
• Spectral-line features observed on opposite sides of the core show opposite Doppler shifts
  • implying that the material there is coming toward us on one side and moving away from us on the other
• The strong implication is that an accretion disk spins perpendicular to the jet
  • and that at its center is a black hole having some 3 billion times the mass of the Sun

A network of radio telescopes has probed the core of the spiral galaxy NGC 4258
• NGC 4258 in the light of mostly hydrogen emission
  • Within the innermost 0.7 light year observations of Doppler-shifted molecular clouds (designated by red, green, and blue dots) show
    • They obey Kepler's third law perfectly
    • This reveals a slightly warped disk of rotating gas
    • At the center of the disk presumably lurks a huge black hole

Charged particles, especially fast electrons, emit synchrotron radiation while spiraling along magnetic field lines
The nature of synchrotron radiation is much different from blackbody radiation
• Variation in the intensity of thermal and synchrotron (nonthermal) radiation with frequency
• Thermal radiation, described by a blackbody curve, peaks at some frequency that depends on the temperature of the source
• Nonthermal synchrotron radiation, by contrast, is most intense at low frequencies
  • It is independent of the temperature of the emitting object

Quasi-stellar Objects

Quasars are radio sources not associated with an obvious visible source
• Source appears to be "innocent" star-like object
• But the star-like object has a very large redshift
  • The emission lines are shifted to the red
• This redshift comes from the expansion of the universe
  • cosmological redshift
• Hubble's Law then tells us that the quasars are at great distances

Redshifts
• If light is emitted with a true wavelength but appears to have a different observed wavelength due to the cosmological redshift, we say it has
  • redshift = ( observed wavelength - true wavelength ) / true wavelength

TABLE 25.1 Redshift and Recessional Velocity REDSHIFT v/c PRESENT DISTANCE LOOK-BACK TIME (millions of years) (Mpc) (106 ly) 0.000 0.000 0 0 0 0.010 0.010 46 149 149 0.025 0.025 113 369 365 0.050 0.049 222 725 708 0.100 0.095 429 1401 1336 0.200 0.180 804 2622 2400 0.250 0.220 974 3177 2853 0.500 0.385 1693 5523 4570 0.750 0.508 2251 7346 5697 1.0 0.600 2702 8815 6483 1.5 0.724 3390 11062 7492 2.0 0.800 3899 12720 8099 3.0 0.882 4612 15048 8775 4.0 0.923 5099 16637 9132 5.0 0.946 5459 17809 9347 10.0 0.984 6443 21022 9754 50.0 0.999 7933 25882 10001 100.0 1.000 8307 27101 10019 1,000,000 1.000 9224 30096 10029		Warning! These are estimates of look-back times, which depend on the value of the Hubble constant and the nature of the universe. These estimates assume a very simple FLAT universe. Astronomers are still trying to understand the true nature of the universe. We will discuss this in the next few lectures.



Important properties of Quasars
1. Very large redshifts
   • Some larger than 4
2. Very short period brightness changes
   • can be just days

These properties imply
1. Very great distances
   • From Hubble's Law
2. Very small size
   • Like the size of the Solar System

3C 273
• 273rd object in the Third Cambridge Catalogue of radio sources
• Strong radio source identified with a faint starlike object in 1963
• emission lines puzzled astronomers (could not identify)
  • Optical spectrum of the distant quasar 3C 273
• Then it was realized this was a red shifted spectrum
• The redshift indicates the quasar's enormous distance
• The width of the three hydrogen spectral lines (H-beta , H-gamma, and H-delta) implies rapid internal motion within the quasar itself

3C 275
• Optical photograph of 3C 275 (one of the first quasars discovered) and a field of quasars
  • 3C 275's starlike appearance shows no obvious structure and gives little outward indication of this object's enormous luminosity
  • However, 3C 275 has a much larger redshift than any of the other stars or galaxies in this image; it is about 7 billion light years (2 billion pc) away
  • The optical image of the field of quasars includes QSO 1229+204, one of the most powerful quasars yet discovered, shown enlarged
  • The starlike appearance of QSO 1229+204 shows only a hint of structure and gives little outward indication of the object's enormous luminosity
    • Like 3C 275, QSO 1229+204's distance from Earth is about 7 billion light years (2000 Mpc)

Although quasars are the most luminous objects in the universe, they are often rather unimpressive in appearance
• Optical image of a distant quasar seen close (on the sky) to a nearby spiral galaxy
• The quasar's much greater distance makes it appear much fainter than the galaxy

The bright quasar 3C 273 displays a luminous jet of matter, but the main body of the quasar is starlike in appearance
• The jet extends for about 100,000 light years (30 kpc)

Radio image of the quasar 2300-189 showing radio jets feeding faint radio lobes
• This quasar is some 1.3 billion light years (400 Mpc) away

The big mystery of Quasars
• How can so much power (10,000 times that of a normal galaxy) be created in such a small space (light year or less)
  • The volume is less than a trillionth of the volume of the nuclear bulge of the Milky Way Galaxy

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This "twin" quasar (designated AC114 and located about 7 billion light years (2 billion pc) away) appears as two separate objects
• Instead, the two large blobs are images of the same object, created by a gravitational lens
• These two blobs of light have striking symmetry
• The lensing galaxy itself is probably not visible in this image
  • the two objects near the center of the frame are thought to be unrelated galaxies in a foreground cluster

Gravitational Lens
• When light from a distant object passes close to a galaxy or cluster of galaxies along the line of sight, the image of the background object (here, the quasar) can sometimes be split into two or more separate images

The "Einstein Cross," a multiple-imaged quasar
• Hubble view, spanning only a couple of arc seconds, showing four separate images of the same quasar produced by the galaxy at the center

Blue arcs are created by the intervening galaxy which deflects the light from background galaxies

Active Galaxy Evolution

A possible evolutionary sequence for galaxies
• Beginning with the highly luminous quasars
• Decreasing in violence through the radio and Seyfert galaxies
• and ending with normal spirals and ellipticals
  • The central black holes that powered the early activity are still there at later times; they simply run out of fuel as time goes on

Long-exposure Hubble image of a distant quasar
• Clearly shows the young host galaxy in which the quasar resides
  • lending strong support to the view that quasars represent an early, very luminous phase of galaxy evolution
• This quasar has the catalog name PG0052+251 and resides roughly 1.7 billion light years (500 Mpc) from Earth.