1. Quasars and Other Active Galaxies

5. FIGURE 17-8 Seyfert Galaxy NGC 1566
This Sc galaxy is a Seyfert galaxy some 50 Mly (16 Mpc) from Earth in the
southern constellation Dorado (the Goldfish). The nucleus of
this galaxy is a strong source of radiation whose spectrum
shows emission lines of highly ionized atoms. (Anglo-
Australian Observatory)

6. Active galaxies are brighter, and emit more non-stellar radiation

7. In this chapter you will discover…
5-10% of galaxies unusually bright, called active galaxies
The most distant objects we can see: quasars
Supermassive black holes are central engines for radio galaxies, quasars, Seyfert galaxies, & BL Lac objects
Distribution of Quasars is a key to the evolution of the Universe, and to the Big Bang Theory of its formation.

8. Active Galaxy Types Seyfert Galaxies: Spirals with very bright centers
Radio Galaxies: Ellipticals with huge emissions of radio energy in “lobes”
Quasars: Very tiny, distant objects
All have very bright, active galactic nucleii

9. Active Galactic Nuclei
Seyfert Galaxies
spiral galaxies with incredibly bright, star-like center (nucleus)
they are very bright in the infrared
their spectra show strong emission lines
Circinus

10. Active Galactic Nuclei
Seyfert Galaxies
The luminosity can vary by as much as the entire brightness of the Milky Way Galaxy!!

11. Radio Galaxies

12. FIGURE 17-1 Cygnus A (3C 405)
Radio image produced from observations made at the
Very Large Array. Most of the radio emissions from Cygnus A come from the radio lobes
located on either side of the peculiar galaxy seen in the inset, a Hubble Space Telescope image.
Each of the two radio lobes extend about 160,000 light-years from the optical galaxy and contain
a brilliant, condensed region of radio emission. Inset: At the heart of this system of gas lies
a strange-looking galaxy that has a redshift that corresponds to a recessional speed of 5% of
the speed of light. According to the Hubble law, Cygnus A is therefore 635 million light-years
from Earth. Because Cygnus A is one of the brightest radio sources in the sky, this remote
galaxy’s energy output must be enormous. (R. A. Perley, J. W. Dreher, J. J. Cowan, NRAO; inset:
William C. Keel, Robert Fosbury)
Cygnus A Radio Image
635 Million light years away, and still one of the brightest radio sources in the entire sky!

13. Active Galactic Nuclei
Radio Galaxies |
galaxies – usually giant ellipticals - which emit large amounts of radio waves
the radio emission come from lobes on either side of the galaxy; not the galaxy itself

14. Radio Galaxy Lobes
These lobes are swept back because the galaxy is moving through an intergalactic medium.

15. FIGURE 17-10 Head-Tail Source NGC 1265
This active elliptical galaxy is moving at a high speed through the intergalactic
medium. Because of this motion, the two tail jets trail
the galaxy at its head, giving this radio source a distinctly
windswept appearance. (NRAO)

16. X-ray/Radio Image of Centaurus A
X-ray is blue; radio is red

17. FIGURE 17-9 Peculiar Galaxy NGC 5128
(Centaurus A) This extraordinary radio galaxy is
located in the constellation Centaurus, 11 million
light-years from Earth. At visible wavelengths a dust lane
crosses the face of the galaxy. Superimposed on this visible
image is a false-color radio image (green) showing that vast
quantities of radio radiation pour from matter ejected from the
galaxy perpendicular to the dust lane, along with radio
emission (rose-colored) along the dust lane, and X-ray
emission detected by NASA’s Chandra X-ray Observatory
(blue). The X rays may be from material ejected by the black
hole or from the collision of Centaurus A with a smaller
galaxy. Inset: This X-ray image from the Einstein Observatory
shows that NGC 5128 has a bright X-ray nucleus. An X-ray
jet protrudes from the nucleus along a direction perpendicular
to the galaxy’s dust lane. (X ray: NASA/CXC/M. Karovska et al.;
radio 21-cm: NRAO/VLA/J. Van Gorkom/Schminovich, et al.; radio
continuum: NRAO/VLA/J. Conden, et al.; optical: Digitized Sky
Survey U.K. Schmidt Image/STScI; inset: X ray—NASA/CXC/Bristol
U./M. Hardcastle; radio—NRAO/VLA/Bristol U./M. Hardcastle)

18. Superbright Elliptical Galaxy
BL-Lac Objects
Superbright Elliptical Galaxy
FIGURE BL Lacertae
This photograph shows fuzz around BL Lacertae (arrow). The redshift of this fuzz indicates
that BL Lacertae is about 900 Mly (280 Mpc) from Earth. BL
Lac objects appear to be giant elliptical galaxies with bright
quasar-like nuclei, much as Seyfert galaxies are spiral galaxies
with quasar-like nuclei. BL Lac objects contain much less gas
and dust than Seyfert galaxies. (T. D. Kinman, NOAO/AURA)

19. Quasars
FIGURE 17-2 Quasar 3C 48 For several years, astronomers
erroneously believed that this object is simply a peculiar,
nearby star that happens to emit radio waves. Actually, the
redshift of this star like object is so great that, according to the
Hubble law, it must be roughly 4 billion light-years away. (Alex
G. Smith, Rosemary Hill Observatory, University of Florida)
A peculiar Star-like object, emitting lots of radio waves? But not with a stellar spectral fingerprint!

20. Quasar Spectra Star-like objects spectra that look nothing like a star
Faint Hydrogen lines…
VERY red-shifted!

21. FIGURE 17-4 Spectra of 3C 273
The visible and infrared
spectra of 3C 273 are dominated by four bright emission lines
caused by hydrogen. This radiation is redshifted nearly 16%
from its rest wavelengths.

22. Quasar Observations emit light at all wavelengths
A hot dense source?
occasionally VERY strong radio sources
Associated with jets from galaxies in clusters

23. FIGURE 17-3 Quasar 3C 273
This combined X-ray and
infrared view shows the star like object associated with the
radio source 3C 273 and the luminous jet it has created. The
jet is also visible in the radio and visible parts of the spectrum.
By 1963, astronomers determined that the redshift of this
quasar is so great that, according to the Hubble law, it is
nearly 2 billion light-years from Earth. (S. Jester, D. E. Harris,
H. L. Marshall, K. Meisenheimer, H. J. Roser, and R. Perley)

24. Quasars Brightness varies in time!
FIGURE 17-7 Brightness of 3C 279
This graph shows variations
in the brightness of the quasar 3C 279. Note the
especially large outburst observed in These data
were obtained by carefully examining old photographic
plates in the files of the Harvard College Observatory.

25. Quasars Brightness varies in time!
FIGURE 17-7 Brightness of 3C 279
This graph shows variations
in the brightness of the quasar 3C 279. Note the
especially large outburst observed in These data
were obtained by carefully examining old photographic
plates in the files of the Harvard College Observatory.

26. Quasars Show enormous redshifts  VERY far away by Hubble’s Law
Show extreme variability  VERY small, in scales of a light-hours to light years ….and so….
Quasars must be some of the most powerful objects we know of in the universe!

27. Quasar Distribution
Seen at greatest distances (earliest history of the universe!)
Not seen nearby….
But…
Quasar behavior in some nearer clusters
FIGURE 17-5 Spectrum of a High-Redshift Quasar
The light from this quasar, known as PKS , is so highly redshifted
that spectral emission lines normally in the far-ultraviolet
(L and L) are seen at visible wavelengths. Note the many deep
absorption lines on the short-wavelength side of L. These lines,
collectively called Lyman-alpha forest, are believed to be created
by remote clouds of gas along our line of sight to the quasar.
Hydrogen in these clouds absorbs photons from the quasar at
wavelengths less redshifted than the quasar’s L emission line.

28. FIGURE 17-6 History of Quasar Formation
The greater the redshift of a quasar, the farther it is from Earth and the farther
back in time we are seeing it. By observing the number of
quasars found at different redshifts, astronomers can calculate
how the density of quasars has changed over time.

29. A theoretical model quasar
Must account for observations:
Small Size
Enormous energy output across spectrum
Source of Jets
Similar behavior in galaxies in clusters
Some radio synchrotron emission (indicating magnetic field)
Full spectrum emission

30. A quasar model… Supermassive Black Hole Engine
Formed as Galaxies are born…
Pulling in gas, dust, and stars into huge accretion disk

31. A quasar model…
Generating jets of X-ray radiation for millions of years
“Quiet down” as fuel diminishes
“Re-ignited” during collisions & mergers of galaxies in clusters

32. Quasar Energy Source?
The energy is generated from matter falling onto a supermassive black hole…
1.2 x 109 M for NGC 4261
3 x 109 M for M87
…which is at the center (nucleus) of the galaxy.

33. Quasar Energy Source?
Matter swirls through an accretion disk before crossing over the event horizon.
Gravitational energy lost like E = mc2
10 – 40% of this is radiated away
Process is very efficient

34. A quasar model… Works to explain quasars and…
Even Supermassive Black holes are TINY
Accretion Disk generates thermal spectrum, jets, magnetic fields
Highly variable as mass is pulled in
and…
Works for active galaxies, too!

35. Theory
FIGURE Supermassive Black Holes as Engines for Galactic Activity
(a) In the accretion disk around a supermassive black
hole, in-swirling gas heats and expands. Pulled inward, compressed,
and heated further, some of it is eventually expelled
perpendicular to the disk in two jets. (b) The giant elliptical
galaxy NGC 4261 is a double-radio source located in the
Virgo cluster, about 100 million light-years from Earth. An
optical photograph of the galaxy (white) is combined with a
radio image (orange and yellow) to show both the visible
galaxy, which does not emit much radio energy, and its jets,
which do. Inset: This Hubble Space Telescope image of the
nucleus of NGC 4261 shows a disk of gas and dust about
800 ly (250 pc) in diameter, orbiting a supermassive black
hole. (b: NASA; inset: ESA)
Observation

36. FIGURE 17-11 Binary Head-Tail Source
This combined radio and X-ray image of 3C 75 shows the head-tail sources
emanating from supermassive black holes in a pair of galaxies
that are in the process of merging. The black holes are separated
by 25,000 light-years and are 300 million light-years away
from Earth. (X-ray: NASA/CXC/D. Hudson, T. Reiprich, et al. [AIFA])

37. FIGURE 17-14 Giant Elliptical Galaxy M87
M87 is located near the center of the sprawling,
rich Virgo cluster, which is about 50 million light-years
from Earth. Embedded in this radio image of gas is the
galaxy M87 from which it has been ejected. Images at different
radio and visible wavelengths reveal a variety of details
about the structure of the jets of gas. M87’s extraordinarily
bright nucleus and the gas jets result from a 3 billion-solar-mass
black hole, whose gravity causes huge amounts of gas and an
enormous number of stars to crowd around it. (NASA and the
Hubble Heritage Team [STScI/AURA]; Frazer Owen [NRAO], John
Biretta [STScI] and colleagues)

38. FIGURE 17-18 Focusing Jets by Magnetic Fields
The hot, ionized accretion disk (red-yellow) around
the black hole rotates and creates a magnetic field
that is twisted into spring-shaped spirals above and below the
disk. Some of the accretion disk’s gas falling toward the black
hole is overheated and squirted at high speeds into the two
tubes created by the magnetic fields. The fields keep the gas
traveling directly outward from above and below the disk, thus
creating the two jets.

39. Quasar Jet Formation Magnetic fields twisted
Pull charged particles out of disk & accelerate like slingshot
Particles bound to magnetic field; focused in a beam

40. FIGURE 17-19 Orientation of the Central Engine and Its Jets
BL Lacertae objects, quasars,
double-radio sources, and active galaxies
appear to be the same type of object viewed from different
directions. If one of the jets is aimed almost directly at Earth,
we see a BL Lac object. If the jet is somewhat tilted to our line
of sight, we see a quasar. Tilted farther and we see an active
galaxy. If the jets are nearly perpendicular to our line of
sight, we see a double-radio source. The central region of the
system is shown in Figures 17-16a and

41. Model Quasar Accounts for Other Observations, too
Orientation determines what we see:
if beams points at us, see a quasar
if not, molecular clouds/dust of galaxy block view of nucleus
we see a radio galaxy or Seyfert
lobes are where jets impact intergalactic medium

42. Hubble space telescope shows us that quasars do live in galaxies…they are Active Galactic Nuclei!

43. If the theory is right --- all galaxies start with Black Holes!
FIGURE Sombrero Galaxy (M104)
This spiral galaxy in Virgo is nearly edge-on to our Earth-based view.
Spectroscopic observations indicate that a billion-solar-mass
black hole is located at the galaxy’s center. You can see the
bright region created by stars and gas that orbit the black
hole. (NASA and the Hubble Heritage Team [STScI/AURA])

44. Evidence Quasars are distant?
Hubble’s Law
Association with Galaxies in clusters
Gravitational Lensing
FIGURE Gravitational Lensing of Quasars
Image from the Hubble Space Telescope that shows
the gravitational lensing of a quasar in the constellation
of Pegasus. The quasar, about 8 billion light-years from
Earth, is seen as four separate images that surround a galaxy
that is only 400 million light-years away. This pattern is called
an Einstein cross. The diffuse image at the center of the Einstein
cross is the core of the intervening galaxy. The physical effect
that creates these multiple images is the same as that seen for
galaxies, as depicted in Figure (NASA/ESO)

45. One Quasar, Five Images
Gravitational lensing by galaxies
in the intervening cluster brings
five images of a distant quasar in
our direction. (The fifth image is
behind the large, orange galaxy.)
The cluster of galaxies is 7 billion
light-years away, whereas the
quasar is 10 billion light-years
distant. Three images of the
same galaxy are also visible.
(ESA, NASA, K. Sharon [Tel Aviv
University] and E. Ofek [Caltech])

46. Essay Questions to Know!
What are active galaxies? How do active galaxies produce their energy? How do we know?
What are quasars, and where are they found? What do they tell us about the universe?