第八章活动星系

(1) High overall luminosities
Milky Way
Radio galaxies
Seyfert galaxies
Quasars (3C 273)
X-ray
luminosity
1
100-5,000
300-7×10 4
2.5×10 6
Optical
luminosity
1
2
2
250
Radio
luminosity
1
2,000-2×10 6
20-2×10 6
6×10 6

Bright extragalactic radio sources

Normal galaxies
Optical X-ray
Quasar PKS 1127-145

(2) Non-thermal radiation
Normal galaxies
Blackbody radiation with peak
energy at optical wavelength
Radiation mainly comes from
stars inside galaxies
Active galaxies
Non-stellar radiation
Thermal (infrared) radiation +
non-thermal radiation, with
peak energy at far-infrared
wavelength

(3) Unusual structure
Bright nucleus, jets and irregular appearance

(5) Strong emission lines and polarized
emission

Statistics of Active Galaxies
Only 2% of all galaxies are active galaxies,
bright active galaxies are more common at
greater distances.
Most active galaxies are ellipticals.
The high luminosities of active galaxies
implies a short lifetime, so active galaxies
must represent a passing stage in the
evolution of normal galaxies, rather than form
a separate class of galaxies.

2. Radio Galaxies
Active galaxies that emit most of their energy in the
radio portion of the electromagnetic spectrum.
Historical perspective
In 1960s, British radio astronomers finished
compiling the 3rd Cambridge (3C) catalog of radio
sources.
Many bright radio sources in the catalog were found
to have optical counterparts (galaxies).
They are both point-like and extended, and are
called radio galaxies.

Cygnus A with two-lobe structure

(1) Characteristics
Most active galaxies are radio galaxies.
Radio luminosities ( ~10 42 -10 45 ergs -1 ) much
higher than those of normal galaxies ( ~10 37 -
10 39 ergs -1 ).
Non-thermal synchrotron radiation.
Most of them are elliptical galaxies, usually
the brightest, largest ones in galaxy clusters.

Appearance
Core-halo type: the radio
images are comparable or
slightly smaller than their
optical images; most of the
radio emission comes form
the nucleus.
Double-lobe type: extended
structure (as long as 1 Mpc);
the radio energy is released
from the radio lobes.
M86
Centaurus A

M87
The giant elliptical galaxy in Virgo cluster.
The first galaxy found to have jets.
The jet is about 2 kpc long and is made up of a series
of distinct “blobs”.

NGC 1265: Head-Tail Radio galaxy

0313-192: The Wrong Spiral Galaxy

(2) Theoretical Model
Energy is fired out from
the nucleus in the form
of narrow, high-speed
jets that travel into the
intergalactic medium
and become extended
lobes.
Radio emission is
produced by highspeed
electrons in
magnetic fields through
synchrotron radiation.

Movement of radio galaxies causes different
appearance.

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.

Comparison: Seyfert Galaxies and Star Burst
Galaxies
Seyfert galaxy
NGC 7742
Starburst Galaxy M94

In type II, all allowed and forbidden lines are similar
and narrower (≤10 3 kms -1 ) .
Seyfert I
Seyfert II

The reason for this difference is thought to be that the
allowed lines are formed in denser gas near the
nucleus (broad line region), and the forbidden lines in
more diffuse gas further out (narrow line region).
In type II Seyfert galaxies the denser gas is missing
or obscured.

The activity of some Seyfert galaxies may result from
galaxy interactions.
Visible-light (left) and radio (right) image of galaxy pair:
Radio image shows gas streaming between galaxies.

Non-thermal continuum with very weak or no
emission lines.

Most energy emitted in γ-ray, with strong radio
and infrared radiation.
Usually elliptical galaxies.
3C 279

Rapid and violent variability within days to
months.

5. Quasars, QSOs
Optically appear almost
as point sources.
Quasars – quasi-stellar
radio sources
QSOs – quasi-stellar
objects.

The first quasar 3C273 with very strong,
wide, unknown emission lines
In 1963 Maarten Schmidt interpreted
the optical emission lines of 3C273 as
hydrogen Balmer lines redshifted by
16%.
Maarten Schmidt
HST+VLA

High-z Quasars Discovered in Sloan Digital Sky Survey

The Most Distant X-Ray Jet from Quasar
GB1508+5714

Quasar Absorption Lines
In addition to their own strongly redshifted spectra, many
quasars also show additional “forest” of absorption lines
starting at the wavelength of the quasar’s own Lyman-alpha
emission line and extending to shorter wavelengths.
These lines are interpreted as Lyman-alpha absorption features
produced by gas clouds in foreground structures (galaxies,
clusters, and so on) along the line of sight.
Lyman-Alpha Forest in the spectrum of quasar QSO 1422 + 2309

Observational Features of Gravitational Lensing
The lensing tends to amplify the light of the quasar,
making it easier to observe.
There is a time delay, ranging from several days to
several years, between different images.
Microlensing — lensing by individual stars in the
foreground galaxy — can cause large fluctuations in
a quasar’s brightness.

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Time Delay in Lensing Images of Quasar RX
J0911.4+0511
Differences in the variation
in each separate image
can be used to tell
information about the
gravity and the expansion
of the universe.

Abell 1835 IR1916, The Farthest Known
Galaxy with a Redshift of 10

Dark Matter in Distant Galaxy Groups Mapped
for the First Time

Implications from X-ray binaries
Black hole accretion jets

RX J1242-11

Evidence of the Dust Torus
Hydroxyl (OH) line emissions trace the obscuring material within
the circumnuclear environment of the galaxy Markarian 231.

Super Massive Black Holes in Galactic Nuclei