TEMA 9. Radio GalaxiesAGN
Dr. Juan Pablo Torres-Papaqui
Departamento de AstronomıaUniversidad de Guanajuato
DA-UG (Mexico)
Division de Ciencias Naturales y Exactas,Campus Guanajuato, Sede Valenciana
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Radio Galaxies
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Radio Galaxies
A prototypical radio galaxy
Any size: from pc to Mpc
First order similar radio morphology (but differences depending on radio power, opticalluminosity & orientation)
Typical radio power 1023 to 1028 W /Hz
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Radio Galaxies
Why study radio-loud AGN?
Comparison of radio-loud AGN and optical AGN samples =>investigate origin of radio-loudness
Some radio and soft X-ray selected AGN show little or no line emission=> include AGN missed by emission-line selection in such surveys
Radio-loud activity provides an efficient means of feeding AGN energydirectly back into environment (cf. sound waves in Perseus cluster,from Fabian et al.) => role of AGN feedback
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Radio Galaxies
Why study radio-loud AGN?
Feedback of radio-loud AGNinto the surrounding IGM(seen through X-ray here).
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Radio Galaxies
Why study radio-loud AGN?
Radio galaxies & radio-loud quasars: the most powerful radiosources
(Usually) extended (or very extended!) radio emission with commoncharacteristics (core-jets-lobes)Typically hosted by an elliptical (early-type) galaxy
Amazing discovery when they were identified with extragalactic, i.e. faraway, objects
Unexpectedly high amount of energy involved!
Nevertheless, the radio contribute only to a minor fraction of the energyactually released by these AGNs. (ratio between radio and opticalluminosity ∼10−4)
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Radio Galaxies
Why study radio-loud AGN?They show most of the phenomena typical of AGNs (e.g. optical lines, X-ray emissionetc.) → very interesting objects in (almost) all wavebandsIn addition they have spectacular radio morphologiesBut they are quite rare!
Local Space Densities of Some ObjectsObject Gpc3
Spiral Galaxies MV < -20 5×106
MV < -22 3×105
MV < -23 3×103
Elliptical Galaxies MV < -20 1×106
(incl. S0) MV < -22 1×105
MV < -24 104
Rich Clusters of Galaxies 3×103
Radio Galaxies P1.4 Ghz > 1023.5 W Hz−1 3×103
P1.4 Ghz > 1025 W Hz−1 10Radio Quasars P1.4 Ghz > 1025 W Hz−1 3Radio Quiet Quasars MV < -23 100
MV < -25 1Sy 1 MV < -20 4×104
Sy 2 MV < -20 1×105
BL Lac P1.4 Ghz > 1023.5 W Hz−1 80Strong IRAS Galaxies LIR > 1012 L 300
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Radio Galaxies
How to find RGs?
Because of the variety of AGNs, there is also a variety of techniques tofind them (e.g. blue colours, strong emission lines etc.).
Here we focus on the way radio galaxies have been found: radio surveys
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Radio Galaxies
Some Radio surveys
Start: 3CR (Cambridge Telescope) → 328 sources with δ > - 5o fluxabove 9 Jy @ 178 MHz (1 Jy = 10−26 W m−2Hz−1)
4C 2 Jy 178 MHz Cambridge (+5,6,7C)
PKS ∼3 Jy 408 MHz Parkes Molonglo
B2 0.25 Jy 408 MHz Bologna (+B3)
NRAO 0.8 Jy 1.4-5 GHz NRAO
PKS 0.7 Jy 2.7 GHz Parkes
NVSS 2.5 mJy (45” res.) 1.4 GHz NRAO VLA Sky Survey
FIRST 1 mJy (∼5” res) 1.4 GHz Faint Images Radio Skyat Twenty centimeters
WENSS 300 MHz WSRT
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Radio Galaxies
Spectral Index/Power-lawEnergy Distribution
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Radio Galaxies
Deviations from a constant spectral index
1. Energy loss
2. Self-absorption in therelativistic electrons gas
3. Absorption from ionizedgas between us and thesource (free-freeabsorption) → torus!
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Radio Galaxies
Deviations from a constant spectral index
1. Energy loss
2. Self-absorption in therelativistic electrons gas
3. Absorption from ionizedgas between us and thesource (free-freeabsorption) → torus!
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Radio Galaxies
Energy loss
The relativistic electrons can loose energy because of a number of process(adiabatic expansion of the source, synchrotron emission, inverse-Comptonetc.).
→ the characteristics of the radio source and in particular the energydistribution N(E ) (and therefore the spectrum of the emitted radiation)tend to modify with time.
Adiabatic expansion: strong decrease in luminosity but the spectrum isunchanged
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Radio Galaxies
Energy loss
Energy loss through radiation: characteristic electron half-life time (timefor energy to half)
Eb =1.7× 108
B2tb
(Special case assuming p = 2)
After a time tb only the particle with E0 < E ∗ still survive while thosewith E0 > E ∗ have lost their energy.
For ν < νbreak the spectral index remains constant (α = α0)
For ν > νbreak → νbreak ∼ B−3 t−2yr → α = (α0 - 1/2)
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Radio Galaxies
Energy loss
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Radio Galaxies
Energy loss
These energy lost affectmainly the large scalestructures (e.g. lobes).
Typical spectral index ofthe lobes → α = 0.7
tb(Myr) = 1.6×103(B/µG )−3/2(νb/GHz)1/2
Typically 20 - 50 Myr for B =10µG , freq 8 - 1 GHz
Unless there is re-accelerationin some regions of the radiosource!
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Radio Galaxies
Self-absorption in the relativistic electron gas
Optically thick case: the internalabsorption from the electrons needs tobe considered → the brightnesstemperature of the source is close to thekinetics temperature of the electrons.
The opacity is larger at lower frequency→ plasma opaque at low frequenciesand transparent at high
τ >> S(ν) ∝ ν−5/2B−1/2dΩ
Frequency corresponding to τ = 1
νmax ≈ f (p)B1/5Smθ−4/5(1 +z)1/5GHz
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Radio Galaxies
Self-absorption in the relativistic electron gas
Affects mainly the centralcompact region or very smallradio sources
Higher “turnover” frequency→ smaller size of the emittingregion.
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Radio Galaxies
Polarization
Characteristic of the synchrotron emission: the radiation is highly polarized.
For an uniform magnetic field, the polarization of an ensemble of electronsis linear, perpendicular to the magnetic field and the fractional polarizationis given by:
p =3p + 3
3p + 7percent→ 0.7− 0.8 for 2 < p < 4 never!
Typical polarization from few to ∼20 % → Tangled magnetic field
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Radio Galaxies
Polarization
Polarization between 10 and 20 % (some peaks at ∼40 % around the edgeof the lobes)
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Radio Galaxies
Polarization
Example of polarization in radio jets.
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Radio Galaxies
Faraday rotation
Travel through a plasma+magnetic field (that can be internal or externalto the source) changes the polarization angle
∆θ = 2.6× 10−17λ2
∫NeBdl
where Ne is the electron density of the plasma, dl the depth, B thecomponent of the magnetic field parallel to line of sight, and
∫NeBdl the
Rotation Measure (RM).
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Radio Galaxies
Faraday rotation
Thermal electrons with density ∼10−5 cm−3
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Radio Galaxies
Faraday rotation
RM can be derived via observations at different wavelengths
If the medium is in front of the radio source: no change in thefractional polarization
If the medium is mix in the radio source: depolarization dependenceon wavelength (if due to Faraday rotation)
Depolarization happens also if the magnetic field is tangled on the scale ofthe beam of the observations.
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Radio Galaxies
Jets
Not well understood
Emitted from axis of rotation
Clues from Polarized light
Acceleration of charged particlesfrom strong magnetic fields andradiation pressure
Synchrotron Radiation
Produces radiation atall wavelengthsespecially at Radiowavelengths
Possible source of Ultra highenergy cosmic rays and neutrinos
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Radio Galaxies
Jets: Focused Streams of Ionized Gas
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Radio Galaxies
Shock waves in jets
Lifetimes short compared to extent of jets => additional accelerationrequired. Most jet energy is ordered kinetic energy.
Gas flow in jet is supersonic; near hot spot gas decelerates suddenly =>shock wave forms. Energy now in relativistic e− and mag field.
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Radio Galaxies
Different types of radio galaxies
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
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Radio Galaxies
Different types of radio galaxies
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Radio Galaxies
Different types of radio galaxies
The morphology does notdepend on size!
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Radio Galaxies
Effects of the interaction with the environment
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Radio Galaxies
Effects of the interaction with the environment
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Radio Galaxies
Two main types of RGs
Fanaroff-Riley type I and II
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Radio Galaxies
Two main types of RGsFRI FRII
Jets Large opening angle Very collimatedlow Mach number high Mach number
Magnetic Field Perpendicular Parallel to the jetto the jet
Hot-spot – Yes
Lobes Plume-like Backflow
Spectral index Steeper away from Steeper toward thein the Lobes the nucleus nucleus (from hot-spots)
The reason(s) for these differences is not completely clear; likely related to the nuclearregions (BH?).
Differences are seen also in other wavebands.
Possibly also environment: lower-power radio galaxies tend to be in clusters
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Radio Galaxies
Two main types of RGs: Optical/Radio
Strong separation in MB -F1400 space.
FR-II are much brighter in theradio for a given opticalluminosity.
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Radio Galaxies
What makes the difference?
Well known dichotomy: low vshigh power radio galaxies
Differences not only in theradio
WHY?
Intrinsic differences in thenuclear regions?
Accretion occurring at low rateand/or radiative efficiency? Nothick tori?
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Radio Galaxies
Two main types of RGs: Jets
Two flavors also for the jets:
supersonic and highlycollimated
subsonic with entrainment
This can explain the presenceof hot-spots and thecollimation of the jets.
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Radio Galaxies
Jets Collimation
Going very close to the BH tosee how the collimation of thejet works.
Rapid broadening of the jetopening angle as the core isapproached on scale below 1mas (0.1 pc).
The jet does not seem to reach a complete collimation until a distance ofmany tens of Schwarzschild radii (escape velocity = c)
Jet emanating from the accretion disk, not yet collimated.
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Radio Galaxies
Jets
Often the radio emission ismore symmetric on the largescale and asymmetric on thesmall scale
The core is defined based onthe spectral index: flat (α ∼ 0)
[to find which component isthe radio core is not alwayseasy: free-free absorption cancomplicate the story!]
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Radio Galaxies
Superluminal motions
Discovered (around 1970-80) inpowerful radio galaxies and quasars:
apparent change (on the VLBIscale) in the structure of somesources during a period of fewmonths.
the velocities appear superluminal
the components of the velocitiesand direction remain constant
there are no observed“contractions”
a flux outburst seems to beassociated with the appearance ofnew components
Case of 3C273 (quasar) apparentvelocity ∼10c
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Radio Galaxies
Superluminal motions
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 (Doppler Boosting)
These are among the methods used to find out the orientation of a source
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Radio Galaxies
Sub-luminal motions
VLBI observations ofCentaurus A (between 1991and 1996)
Apparent motion sub-luminalspeed ∼0.1c
However this does not seem tobe characteristics common toall lower power (Fanaroff-RileyI) radio galaxies
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Radio Galaxies
3C120: FR-I
Apparent motion of the components between 4 and 6 c but very complex.
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