Download 1. Seyfert Galaxies

Active Galactic Nucleii (AGN)
AGN, refers to the existence of energetic phenomena in the nuclei, or
central regions, of galaxies which cannot be attributed clearly and directly
to stars.
In the case of a typical Seyfert galaxy, the total energy emitted by the
nuclear source at visible wavelengths is comparable to the energy emitted
by all of the stars in the galaxy (i.e., ~ 1011 L ), but in a typical quasar
the nuclear source is brighter than the stars by a factor of 100 or more.
Historically, the appearance of quasars did not initially suggest
identification with galaxies, which is a consequence of the basic fact
that high-luminosity objects, like bright quasars, are rare. One is likely to
find rare objects only at great distances, which is of course what happens
with quasars.
At very large distances, only the star-like nuclear source is seen in a
quasar, and the light from the surrounding galaxy, because of its small
angular size and relative faintness, is lost in the glare of the nucleus.
Hence, the source looks ``quasi-stellar''.
1. Seyfert Galaxies
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First noted by Fath at Lick Observatory in 1908 who was taking
spectra of the nuclei of “spiral nebulae” and noted that NGC 1068
had strong emission lines
Slipher obtained a higher quality spectrum at Lowell in 1917, noted
the lines were similar to planetary nebulae
In 1926, Hubble noted 3 galaxies with strong emission lines: NGC
1068, NGC 4051, NGC 4151
In 1943, Carl Seyfert published a list of 12 otherwise normal spiral
galaxies which contain anomolous strong, broad high-ionization
emission lines and bright central nuclei. These galaxies are now
known as "Seyferts."
In some Seyferts, the central nucleus outshines the whole
surrounding galaxy.
The following figure shows successively deeper images of the
Seyfert galaxy NGC4151. In short exposures, only the bright
nucleus is apparent, but deeper images reveal the normal spiral
galaxy around it.
NGC 1068:
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The central nucleus light output varies on timescales of less than a
year
o So, the emitting region must be less than ~ a light year
across, as the source cannot vary coherently on timescales
shorter than this due to light travel time effects:
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Spectra of the nuclei in Seyferts are non-stellar. They
contain:
o
o
o
Non-thermal continuum emission
Narrow (=> low velocity), forbidden (=> low density
material) lines which do not vary detectably (=>
large emitting region)
Broad (=> high velocity broad, up to 8500 km s-1,
full width at zero intensity), permitted lines which
vary on fairly short timescales (=> small emitting
region)
o
o
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Also, strong emission in the radio, infrared,
ultraviolet, and X-ray parts of the spectrum.
Seyferts have only moderate radio emission (~10 40
erg/s) but strong x-ray emission (> 1042 erg/s)
Seyfert galaxies have been classified into two basic types:
o Seyfert 1 galaxies are bright in the optical, and have
both broad and narrow lines in their spectra.
o Seyfert 2 galaxies are fainter in the optical (but the
same as Seyfert 1s in the infrared), and they only have
narrow lines in their spectra.
o Intermediate cases with weak broad emission lines
also exist, and are classified as Seyfert 1.3, 1.6, etc.
The current paradigm for Active Galactic Nuclei explains the
variations between these different classes by the different
inclinations from which the galaxies are viewed:
o
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In the broad-line region (BLR)
 The Keplerian orbital speeds of the clouds
around the central massive body will be large =>
lines are Doppler broadened.
 Density is high => no forbidden lines are emitted
In the narrow-line region (NLR)
The Keplerian orbital speeds of the clouds will be
much smaller => lines are narrow
 Density is low => forbidden lines are emitted
o So, if the above Seyfert were viewed from above, you
would see:
 Broad permitted lines
 Narrow Forbidden Lines
 Bright continuum from the central engine
 i.e. a Seyfert 1
o If, on the other hand, it were viewed from the side,
you would see:
 No broad permitted lines (obscured by dust
torus)
 Narrow Forbidden Lines
 No bright continuum from the obscured central
engine
 except in the infrared and X-ray region,
which gets through the dust
 i.e. a Seyfert 2
Note we already have enough information to estimate the
mass of the central engine:
o H-beta emission lines (wavelength
= 486 nm) from
the BLR are typically broadened to a width of ~ 2 nm.
The gas must therefore have turbulent velocities of ~ v,
where v ~ 106 m s-1
o This emission varies on timescales of ~ a year, so the
emission must come from a region whose size r ~ 1
light year ~ 1016 m across.
o So if the motion of the clouds are bound orbits around
the central engine, we can estimate the mass from the
usual dynamical formula,
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i.e. M ~ 1038 kg ~ 108 M .
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All within a region ~1 light year across !!
The AGN Paradigm
The following two cartoons summarize the current paradigm by which the nature of
the various type of active galactic nucleus can be understood:
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Direct observational evidence in support of this model comes from an HST
image of the dust torus around the centre of the radio galaxy NGC4261:
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Further support comes from this HST image of the Seyfert galaxy NGC 5728:
Here, ionising ultraviolet radiation is emitted by the central source, but it
is absorbed by the dust torus, and can thus only escape from the central
region in a cone. This image is taken in the light of doubly ionized
oxygen, and so reveals the ionization due to the escaping ultraviolet
radiation.
Evolution / lifetimes of Seyferts. About 10% of Sa and Sb’s are
Seyferts – does a galaxy spend 10% of its life as a Seyfert or are
10% of spirals Seyferts? The nuclear emission must certainly last
more than 108 years, because Seyfert galaxies constitute about 1 in
100 spiral galaxies.
An extreme scenario is that galaxies which are Seyferts are always
Seyferts, in which case their lifetime is the age of the Universe
(1010 years). The opposite extreme is one where all spirals pass
through a Seyfert phase (or phases).
SED: big blue bump and infrared bump
Note on the optical spectra
The optical spectrum of the Seyfert 1 galaxy NGC 1275. The
prominent broad and narrow emission lines are labelled, as are strong
absorption features of the host galaxy spectrum. The vertical scale is
expanded in the lower panel to show the weaker features. The full width
at half maximum (FWHM) of the broad components is about 5900 km s-1,
and the width of the narrow components is ~ 400 km s-1.
The emission lines. The permitted lines - those that can be produced at
high densities by astronomical standards - show both broad and narrow
components. The strongest of these are the hydrogen recombination lines,
such as Lyman alpha at 1216 A, H-beta at 4861, and H-alpha at 6563,
plus the strong ultraviolet lines of C IV at 1549 and Mg II at 2800.
Other features produced only by very rarefied gas at densities of 1000
atoms per cubic centimeter or so - the forbidden lines, denoted by
brackets - arise in regions with less velocity structure and are narrower.
Some strong examples are [OIII] at 4959 and 5007 A, [O II] at 3727, [Ne
V] at 3426, and [S III] at 9060 and 9532.
NGC 4151 is a bit unusual in showing strong absorption in several lines,
especially Lyman alpha and C IV. The absorption is blueshifted with
respect to the line centers, so that it arises in some kind of wind or other
gaseous outflow.
The spectra of active galactic nuclei are noteworthy in showing species
with a large range in ionization at once, from neutral ions such as [O I]
and [N I] to highly ionized cases such as [Ne V] and [O VI].
Even hot stars cannot ionize gas as highly as these ions require, so that
both a strong source of hard radiation and a wide range in gas density
must be present to see such spectra.
Radio Galaxies
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New developments in radio astronomy in the 1950s:
A common pattern for radio sources away from the Milky Way (away
from the galactic plane):
A pair of radio sources, 100kpc – 10Mpc, straddling optical
galaxy.
Many associated with normal-looking elliptical galaxies, but a few
displayed odd features.
As techniques improved, not only could vast numbers of radio galaxies
be found, but their structures could be mapped in exquisite detail.
Interferometer arrays such as the Westerbork Synthesis Telescope
(WSRT) and Very Large Array (VLA) revealed that many displayed jets
of radio-emitting material tracing from the twin lobes of emission
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back to a tiny nuclear source. Whatever produced the radio
emission had to have a long memory, preserving its direction over
millions of years.
Emission-line spectra of radio galaxies similar to Seyfert galaxies
o Broad line radio galaxies (BLRG) ~ Seyfert 1
o Narrow line radio galaxies (NLRG) ~ Seyfert 2
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M87 showed a radio jet shooting thousands of light-years from its
core; had already been discovered as an optical jet as early as 1918.
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Centaurus A looked like en elliptical galaxy cut in two by a thick,
irregular absorption lane of dust and gas…here shown in radio, optical,
mid-infrared and X-ray:
 Cygnus A, at the then-remarkable redshift of z =
0.056, was a small fuzzy image with two main
lobes.Speculation: Colliding galaxy, splitting galaxy?
Radio galaxies are typically found in elliptical
galaxies, with extended jets
Radio emission is non-thermal - synchrotron
radiation as electrons spiral through magnetic fields
at relativistic speeds (L>1042 erg/s)
There are 2 classes based on jet morphology:
 Fanaroff-Riley Type I (FRI) – edge darkened
radio jets, slower jet speeds, lower radio
power
 Fanaroff-Riley Type II (FRII) – edge
brightened radio jets, fast jet speeds ~0.1c,
higher radio power
Fanaroff-Riley (FR) type. Ordinary, symmetric double structure is seen
in Fornax A, 3C 219, and 3C 285. Hydra A (3C 218) exhibits an
interesting corkscrew form, sometimes seen as suggesting a long-term
precession of the jets feeding outwards from the nucleus, while 3C 449
shows a very long and extended set of helical twists more or less
symmetric about the core. The radio source of 3C 315 (the butterfly) is
tightly twisted.
The optical galaxy spans only a small part of the range of the radio
source. In Fornax A, it fills the gap between the two lobes, and in the
other cases the visible galaxies are much smaller compared to the radio
source extent. See, for example, this combined radio and optical
image of 3C219.
The radio image (NRAO Very Large Array) is coloured red and
yellow and is superposed on an optical image (blue) of the
parent galaxy. Thin jets of radio emission can be seen carrying
energy from the nucleus of the galaxy to giant radio lobes that
are much larger than the entire visible galaxy. the radio
emission is highly polarized, so we know that it must be
synchrotron emission.
A web resource containing images of radio galaxies and Quasars can be
found at: http://www.cv.nrao.edu/~abridle/images.htm
Summary: Fanaroff-Riley Type II radio galaxies
possess hot spots at their outer edges (edgebrightened). Type I do not possess distinct hot
spots or they are located within the lobe.
The FR Is are generally of lower luminosity and
the jet speed is lower.
Wide radio jets leading
to plumes are often visible in the FR Is.
In the FR IIs the jets are relatively weak.
One needs relativistic electrons, magnetic field
to produce the synchrotron radiation.
Hot spots: Assume bulk flow of energy within jets
powers the radio lobes! The energy is dissipated
where the bulk flow is disrupted. Usually
involves shock waves to halt the highly
supersonic flow.
L = 0.5 (dM/dt) vjet2
The mass outflow rate is given by 2L/vjet2
where L is the power.
So, given L = 1038 Watts and a jet speed of c/3 =
108 m/s, yields a mass outflow of
2 x 1022 kg/s.
1 solar mass per year is roughly 2 1030/ 3 107
kg/s
= 6.7 1022 kg/s.
Therefore, about 0.3 solar masses per year must
be ejected
Quasars
Some radio sources were not associated with galaxies, but with
stellar-like sources on optical images
In 1963, Maarten Schmidt made the breakthrough that the
spectrum of 3C273 had nebular emission lines at the then
unheard redshift of z=0.158
Soon afterward, 3C48 was found to be at a redshift of z=0.367
(thus began the race for the highest redshift quasar!)
Quasars are very luminous – L ~ 1013-1015L
These objects are AGN whose nuclear light dominates so that we
can’t distinguish the host galaxy
These objects are known as quasi-stellar radio sources (quasars);
there are also quasi-stellar objects (QSO’s) identified via the
spectra that are radio quiet
Radio quiet QSO’s outnumber quasars by 10-30x, the term
quasar is often used for both
Quasars are variable over small time scales – hours to days!
Current record for the highest redshift quasar is z=6.4 found in the
Sloan Digital Sky Survey. PKS 1117-248:
The entire spectrum of quasars:
Quasar host galaxies:
 Many are interacting – does
interactions trigger AGN activity?
Promote fueling gas into centres?
Note: not all galaxies with SMBH are
AGN (even the Milky Way)
Quasars were much (~1000x) more
numerous at z~3 than today
 Is an AGN a requirement for galaxy
formation? Does every galaxy have
a SMBH? (Remember -- lots have been
discovered in nearby ellipticals
and bulges!)
 More interactions in the past?
Quasars were also much more luminous in
the past
There is probably some combination of
luminosity and density evolution! Here
are the Luminosity Functions for
quasars at different redshifts
(actually starts to drop off above
z=2). This is the co-moving space
density!!! Co-moving volume = 1/(1+z)3
2df quasar luminosity function:
Blazars: BL Lacs
Catalogued as variable stars (e.g., BL
Lacertae)
Highly variable, highly polarized
Variability occurs on the time scale of
days
Virtually featureless continuous
spectra
Also optically violent variables (OVV)
quasars
Collectively OVV’s and BL Lac’s are
known as blazers
Interpretation
Variability on short time scales
implies a small area:
 R=ct ~ 7-10 AU !! (for timescale
variability on the order of hours)
Highly luminous!
Probably powered by a black hole!!
Schwarzschild radius is the radius
where the “escape velocity” equals the
speed of light around a black hole of
mass M:
 RSch=2GM/c2
Continuum emission powered by gas
falling onto central black hole, losing
potential energy and radiating
Since gas has angular momentum it forms
an accretion disk
Consists of an accretion disk around a
supermassive black hole
Gas clouds in surrounding galaxy are
ionized by this central engine
Clouds closest to the central engine
are dense and have high velocities
(otherwise they would fall in!). So
emission lines from this region are
broad (~2000 km/s) – the broad line
region (BLR)
Clouds farther away from the central
engine have a lower density and lower
velocities. Line emission is narrow
(500-1000 km/s) – the narrow line
region (NLR)
Dust torus surrounds the central engine
and BLR in the same plane as the
accretion disk. The orientation of the
dust torus relative to the line of
sight affects the appearance of the
AGN.
Summary
Seyfert 1: Broad/narrow emission, weakr radio, strong X-ray,spirals,
variable
Seyfert 2: narrow emission, weak X-ray, spirals, not variable
Quasars QSR: broad/narrow, strong radio, FRII, polarisation,variable
Quasars QSO: broad/narrow lines, weak radio, weak polarisation,
variable
Blazars BL lacs: strong polarisation, absent/weak lines, strong radio,
highly variable, mainly ellipticals
OVV quasars: powerful BL Lacs but with emission lines
Radio Galaxies BLRG: Broad/narrow lines, strong radio emission, FR
II, weak polarisation, elliptical variable, variable
Radio Galaxies NLRG: narrow lines only, strong radio, FR I/II, no
polarisation, elliptical, not variable
in Seyfert 1s we are observing the active nucleus directly. In
Seyfert 2s we observe it through an obscuring structure which
prevents us getting a direct view of the optical continuum
At higher luminosities, quasars take the place of Seyfert 1s, but
the corresponding 'quasar 2s' are elusive at present. If they do not
have the scattering component of Seyfert 2s they would be hard to
detect except through their luminous narrow-line and hard X-ray
emission.
High-luminosity radio-loud quasars can be unified with narrow-line
radio galaxies in a manner directly analoguous to the Seyfert 1/2
unification. X-ray evidence, where available, supports the unified
picture: radio galaxies show evidence of obscuration from a torus,
while quasars do not
The population of radio galaxies is completely dominated by lowluminosity, low-excitation objects. there is no evidence for a torus
in these objects at all. Most likely, they form a separate class in
which only jet-related emission is important. At small angles to the
line of sight, they will appear as BL Lac objects
Superluminal Motion
The term ``superluminal motion'' describes proper motion of
source structure (traditionally mapped at radio wavelengths) that,
when converted to an apparent speed vobs, gives vobs > c.
This phenomenon occurs for emitting regions moving at very high
(but still subluminal) speeds at small angles to the line of sight.
Relativistically moving sources ``run after'' the photons they emit,
strongly reducing the time interval separating any two events in the
observer's frame and giving the impression of faster than light
motion.
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With the advent of very long baseline interferometry (VLBI),
we can obtain very high resolution radio images of jets.
These images show that jets form coherent structures over 5
orders of magnitude in scale (~1 pc -> ~ 100 kpc)
Somewhat disturbingly, knots seen in the jets observed by
VLBI are sometimes moving along the jets at superluminal
velocities (v > c)!
o This effect can be explained geometrically if the jet is
directed nearly along the line of sight:
o Taking observations in 2 epochs:
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o
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By the time that light leaves from
position (2), light emitted while
blob was at position (1) will have
travelled a distance AC
So, the difference in arrival time
for the observer is
So, the apparent velocity as seen
by the observer is
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For example, if phi = 11 degrees
and v = 0.999c, then v(OBS) =
10.37 c
It is therefore quite possible
for a relativistic (but subluminal)
jet
to
appear
superluminal.
Other exciting subjects:
Relatistic beaming and one-sided jets
Gravitational Lensing
Lyman-alpha forests