tema 9. radio galaxies - agn - departamento de …papaqui/fisica_agn/tema_1.09.pdf · tema 9. radio...

TEMA 9. Radio GalaxiesAGN

Dr. Juan Pablo Torres-Papaqui

Departamento de AstronomıaUniversidad de Guanajuato

DA-UG (Mexico)

[email protected]

Division de Ciencias Naturales y Exactas,Campus Guanajuato, Sede Valenciana

Radio Galaxies: AGN J.P. Torres-Papaqui Physics of AGN 1 / 43

Radio Galaxies


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


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


Radio Galaxies


Feedback of radio-loud AGNinto the surrounding IGM(seen through X-ray here).


Radio Galaxies


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)


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


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


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


Radio Galaxies

Spectral Index/Power-lawEnergy Distribution


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!


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


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)


Radio Galaxies

Energy loss


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!


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


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.


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


Radio Galaxies

Polarization

Polarization between 10 and 20 % (some peaks at ∼40 % around the edgeof the lobes)


Radio Galaxies

Polarization

Example of polarization in radio jets.


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).


Radio Galaxies

Faraday rotation

Thermal electrons with density ∼10−5 cm−3


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.


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


Radio Galaxies

Jets: Focused Streams of Ionized Gas


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.


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


Radio Galaxies



Radio Galaxies


The morphology does notdepend on size!


Radio Galaxies

Effects of the interaction with the environment


Radio Galaxies

Two main types of RGs

Fanaroff-Riley type I and II


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


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.


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?


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.


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.


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!]


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


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


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


Radio Galaxies

3C120: FR-I

Apparent motion of the components between 4 and 6 c but very complex.


tema 9. radio galaxies - agn - departamento de …papaqui/fisica_agn/tema_1.09.pdf · tema 9. radio...

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