Download DTU 8e Chap 17 Quasars and Other Active Galaxies

In this chapter, you will discover…
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bright and unusual objects, called active galaxies
distant, luminous quasars
the unusual spectra and small volumes of quasars
the extremely powerful BL Lac objects
supermassive black holes that serve as central engines
for radio galaxies, quasars, Seyfert galaxies, and BL Lac
objects
Cygnus A (3C 405)
(a) 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. (b) An X-ray
image of Cygnus A taken by the Chandra X-ray Observatory showing that some of the
jet particles from the central source have spread throughout the galaxy and, striking
other gas and dust, caused this debris to heat and emit X rays.
Active Galaxies and Quasars
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An active galaxy is an extremely luminous galaxy that has one
or more unusual features: an unusually bright, starlike nucleus;
strong emission lines in its spectrum; rapid variations in
luminosity; and jets or beams of radiation that emanate from
its core. Active galaxies include quasars, Seyfert galaxies,
radio galaxies, double-radio sources, and BL Lacertae objects.
A quasar, or quasi-stellar radio source, is an object that looks
like a star but has a huge redshift. This redshift corresponds to
a distance of billions of light-years from Earth, according to the
Hubble law.
To be seen from Earth, a quasar must be very luminous,
typically about 100 times brighter than an ordinary galaxy.
Relatively rapid fluctuations in the brightness levels of some
quasars indicate that they cannot be much larger than the
diameter of our solar system.
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 starlike object is so great that, according
to the Hubble law, it must be roughly 4 billion light-years away.
Quasar 3C 273
This combined X-ray and infrared view shows the starlike 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.
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.
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.
Brightness of 3C 279
This graph shows variations in the brightness of the quasar 3C 279.
Note the especially large outburst observed in 1937. These data
were obtained by carefully examining old photographic plates in the
files of the Harvard College Observatory.
Other Active Galaxies
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An active spiral galaxy with a bright, starlike nucleus and
strong emission lines in its spectrum is categorized as a
Seyfert galaxy.
An active elliptical galaxy is called radio galaxy. It has a
bright nucleus and a pair of radio-bright jets that stream
out in opposite directions.
BL Lacertae (BL Lac) objects (some of which are called
blazars) have bright nuclei whose cores show relatively
rapid variations in luminosity.
Double-radio sources contain active galactic nuclei located
between two characteristic radio lobes. A head-tail radio
source shows evidence of jets of high-speed particles that
emerge from an active galaxy.
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.
Peculiar Galaxy NGC 5128 (Centaurus A)
Radio galaxy NGC 5128 is 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 (blue)
detected by NASA’s Chandra X-Ray
Observatory. 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.
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.
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.
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 quasarlike
nuclei, much as Seyfert galaxies are spiral galaxies with quasarlike nuclei.
BL Lac objects contain much less gas and dust than do Seyfert galaxies.
Elliptical Galaxy M32
This small galaxy is a satellite of M31, a portion of which is seen at the left of
this wide-angle photograph. Both galaxies are roughly 2.5 million light-years
from Earth. (Inset) High-resolution image from the Hubble Space Telescope
that shows the center of M32. Note the concentration of stars at the nucleus
of the galaxy. The nucleus is only 175 light-years across.
Giant Elliptical Galaxy M87
M87 is located near the center of the
sprawling, rich Virgo cluster, which is
about 50 million ly from Earth.
Embedded in this radio image of gas
is the galaxy M87 from which the gas
has been ejected (bottom inset).
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.
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 in
the galaxy’s center created by stars and gas that orbit the black hole.
Supermassive Black Holes as Engines for Galactic Activity
(a) In the accretion disk around a
supermassive black hole, inswirling 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 doubleradio source located in the Virgo cluster,
about 100 million light-years from Earth.
A visible-light 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 HST image of the nucleus of
NGC 4261 shows a disk of gas and dust
about 800 light-years in diameter,
orbiting a supermassive black hole.
Focusing Jets by Pressure
(a) If a high-speed jet of gas or liquid encounters little pressure (from
the surrounding air, in this image), then it will spread out. (b) If the jet
encounters high pressure, such as occurs when it enters water, then it
will maintain longer its shape as a column.
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.
Supermassive Central Engines
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Many galaxies contain huge concentrations of matter at
their centers.
Some matter that spirals in toward a supermassive black
hole is squeezed into two oppositely directed beams that
carry particles and energy into intergalactic space.
The energy sources from quasars, Seyfert galaxies, BL
Lac objects, radio galaxies, and double-radio sources
are probably matter ejected from the accretion disks that
surround supermassive black holes at the centers of
galaxies.
Orientation of the Central Engine and Its Jets
BL Lacertae objects, quasars, and double-radio sources 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. It if is tilted farther, we see an active galaxy. If the jets are
nearly perpendicular to our line of sight, we see a double-radio source.
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 lightyears 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.
Summary of Key Ideas
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The development of radio astronomy in the late 1940s
led to the discovery of very powerful and extremely
distant energy sources.
Quasars and Other Active Galaxies
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
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An active galaxy is an extremely luminous galaxy that has one or
more unusual features: an unusually bright, starlike nucleus;
strong emission lines in its spectrum; rapid variations in
luminosity; and jets or beams of radiation that emanate from its
core. Active galaxies include quasars, Seyfert galaxies, radio
galaxies, double-radio sources, and BL Lacertae objects.
A quasar, or quasi-stellar radio source, is an object that looks like
a star but has a huge redshift. This redshift corresponds to a
distance of billions of light-years from Earth, according to the
Hubble law.
To be seen from Earth, a quasar must be very luminous, typically
about 100 times brighter than an ordinary galaxy. Relatively rapid
fluctuations in the brightness levels of some quasars indicate that
they cannot be much larger than the diameter of our solar
system.
Quasars and Other Active Galaxies




An active spiral galaxy with a bright, starlike nucleus and
strong emission lines in its spectrum is categorized as a
Seyfert galaxy.
An active elliptical galaxy is called radio galaxy. It has a bright
nucleus and a pair of radio-bright jets that stream out in
opposite directions.
BL Lacertae (BL Lac) objects (some of which are called
blazars) have bright nuclei whose cores show relatively rapid
variations in luminosity.
Double-radio sources contain active galactic nuclei located
between two characteristic radio lobes. A head-tail radio
source shows evidence of jets of high-speed particles that
emerge from an active galaxy.
Supermassive Central Engines

Many galaxies contain huge concentrations of matter at their
centers.

Some matter that spirals in toward a supermassive black hole
is squeezed into two oppositely directed beams that carry
particles and energy into intergalactic space.

The energy sources from quasars, Seyfert galaxies, BL Lac
objects, radio galaxies, and double-radio sources are
probably matter ejected from the accretion disks that surround
supermassive black holes at the centers of galaxies.
Key Terms
active galactic nuclei (AGN)
active galaxy
blazar
BL Lacertae (BL Lac) object
double-radio source
Einstein cross
Einstein ring
head-tail source
peculiar galaxy (pec)
quasar (quasi-stellar radio source)
quasi-stellar object (QSO)
radio galaxy
radio lobe
Seyfert galaxy