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Radio Galaxies
Part 2
A prototypical radio galaxy
Lobes
Hot-spots
Core
Jets
 Any size: from pc to Mpc
 First order similar radio morphology
(but differences depending on radio power,
optical luminosity & orientation)
 Typical radio power 1023 to 1028 W/Hz
….but radio galaxies are not all the same!
The morphology of a radio galaxy may depend on
different parameters:
- radio power (related to the power of the AGN?)
- orientation of the radio emission
- intrinsic differences in the
(nuclear regions of) host galaxy
- environment
The morphology
does not depend
on size!
~200 kpc
~20 pc
Effects of the interaction
with the environment
Effects of age
Two main flavors
Edge-brightened
high radio power
Fanaroff and Riley type I and II
Edge-darkened
lower radio power
FRI
Jets
FRII
Large opening angle
low Mach number
(but not on small scales)
Perpendicular
to the jet
Very collimated
high Mach number
(relativistic on small scales)
Parallel to the jet
Hot-spot
--
Yes
Lobes
Plume-like
Spectral index in the
Lobes
Steeper away from the
nucleus
Magnetic Field
Backflow
Steeper toward the
nucleus (from hot-spots)
o The reason(s) for these differences is not
completely clear; likely related to the nuclear regions.
o Differences are seen also in other wavebands.
o Possibly also environment: lower-power radio galaxies tend to be in clusters
Radio jets
Two flavors also for the
jets:
 supersonic and highly
collimated
 subsonic with entrainment
This can explain the
presence of hot-spots
and the collimation of
the jets.
Simulations of an high Mach number radio jet
(velocity of the plasma >> sound speed)
Movie M = 10
Movie hot-spot
What makes the difference?
Well known dichotomy:
low vs high power radio galaxies
Differences not only in the radio
WHY?
Intrinsic differences in the
nuclear regions?
Accretion occurring at low
rate and/or radiative efficiency?
No thick tori?
Also important:
the resolution of the observations!
Importance of observations at different resolution
M87
radio emission at different
frequencies and resolutions
Often the radio emission is
more symmetric on the
large scale and asymmetric
on the small scale
The core is defined based on
the spectral index: flat ( ~ 0)
[to find which component is
the radio core is not always easy:
free-free absorption can
core
complicate the story!]
What are the characteristics of
the jets close to the AGN?
Superluminal motions
Discovered (around 1970-80) in
powerful radio galaxies and quasars:
 apparent change (on the VLBI scale) in the
structure of some sources during a period of
few months.
 the velocities appear superluminal
 the components of the velocities and
direction remain constant
 there are no observed “contractions”
 a flux outburst seems to be associated with
the appearance of new components
Case of 3C273 (quasar)
apparent velocity ~10c
v  v sin /(1   cos  )
These projection effects explain:
 the apparent superluminal motion
 the asymmetry between the two jets, also the flux of the approaching
and receding components are affected by projection
These are among the methods used to find out the orientation of a source
Not all the jets are
superluminal
VLBI observations of
Centaurus A
(between 1991 and 1996)
Apparent motion
subluminal speed ~ 0.1c
However this does not seem to
be characteristics common to all
lower power (Fanaroff-Riley I)
radio galaxies
3C120
Apparent motion
of the components
between 4 and 6 c
but very complex.
 The structure can be complicated, perhaps from the interaction
with the inter-stellar medium
 New components can be seen to appear
Going very close to the BH
to see how the collimation
of the jet works.
43 GHz
VLBI
rapid broadening of the jet
opening angle as the core is
approached on scale below
1 mas (0.1 pc).
~ 1 mas = 0.071 pc
M 87
The jet does not seem to reach a complete collimation until a distance
of many tens of Schwarzschild radii (escape velocity = c)
jet emanating from the accretion disk, not yet collimated
Optical Jets
 some radio jets have an
optical counter-part
Optical jet in M87
(NGC 4486 in Virgo)
 few more cases found by HST
(high resolution is needed)
 the more the jet is “beamed”
toward us the more is likely we
see the optical part
Origin of the optical emission:
likely an extension of the synchrotron spectrum.
Lifetime of the electrons very short
Acceleration mechanisms need to be present.
Jets in optical and X-ray
M87
Radio
Optical
X-ray
X-ray
optical
X-ray +
optical contours
 The new X-ray satellite Chandra has shown that many radio jet have also
an X-ray counterpart
Almost identical morphology from the radio to X-ray band:
the optical, UV and X-ray data (and spectral indices)
are consistent with synchrotron emission
The electrons must have high  (107) and very short life-time (<<yrs)
Other possibility : inverse Compton effect
Relativistic electrons in a radiation field. Because of the interaction with the photons,
the electrons loose energy while the outcome are photons with higher energy. This
interaction can take place between the relativistic electron producing the radio emission
and either the radio photons or the photons from the cosmic micro-wave background.
pink = radio
blue = X-ray
Radio and X-ray
of the jet in Centaurus A
Very detailed image, scale
of the arcsec (~18 pc).
Small offset between the radio and X-ray emission.
The X-ray trace the regions where the electrons with high  are
accelerated NOW.
An other well studied
case.
3C273 (quasar)
HST, Chandra, Merlin
o Also radio hot-spots are now found to have optical and X-ray counter-parts
A look at the nuclear regions
Many indications that the view
of the central regions is
“orientation dependent:
 superluminal motion of the
jets
 broad optical lines
 free-free absorption
Evidence for a circum-nuclear disk/torus
Seyfert galaxy
NGC4258
(from water masers)
 Evidence of absorption
from the X-ray spectra
(column density above 1024 cm-2)
 Evidence of obscuring “torus”
from the optical broad line
observed in polarized light
in narrow-line radio galaxies
Presence of ionized gas
The inner edge of the “torus” (<0.3 pc)
is likely to be completely ionized
from the radiation from the AGN.
The presence of this ionized medium will cause
“free-free” absorption of the radio emission
from the nuclear radio components.
This effect is prominent at longer wavelengths.
Component that “appears” only at high frequency
(absorbed at lower frequencies) REAL NUCLEUS!?!
Neutral hydrogen in the nuclear regions
HST image
From the absorption, the column density of
the neutral hydrogen can be derived.
Typical numbers are 1020 to 1021 cm-2
VLBI data
Structure of the circumnuclear torus
 1946+708
Mrk 231
Nuclear dust/gas disks very common in radio galaxies
(as we will see tomorrow!)
These characteristics
(different morphologies, superluminal motions, presence of
nuclear tori etc.) are extremely important when we will
try to “unify” different type of AGN.
A key parameter is the orientation of the
jets with respect to the line-of-sight.