Post on 16-Dec-2015
Basado en una presentación de Raffaella MorgantiASTRON
Radiogalaxias
Temas
1. ¿Qué son los AGNs y las radiogalaxias? - ¿Cómo encontrarlos? Una radiogalaxia prototipo - Mecanismos de emisión
2. Morfología de la emisión de radio: distintas morfologías, regiones nucleares, chorros altamente colimados – regiones calientes - lóbulos
¿Qué son los Active Galactic Nuclei?
Es difícil dar una definición única.
grandes cantidades de energía (hasta 104 veces más que en una galaxia normal) emitida desde una pequeña región (<1 pc3)
Un AGN puede tener una luminosidad que va de 1042 to 1044 erg/sec
Se cree que la gran energía liberada por los AGN se origina en un hoyo negro supermasivo (106 a 109 Msun en <<1pc)
Se han encontrado AGNs de menor luminosidad
¡Pero la presencia del hoyo negro supermasivo no es suficiente!
Algunas características
Comparison of the continuum emission from a Seyfert galaxy and a normal galaxy
Optical emission linesfor different AGNs
radio
optical X-ray UV
La luminosidad no es el único criterio:
emisión de continuo (comunmente azul) a lo largo de ~13 órdenes de magnitud en frecuencia
líneas de emisión
emisión (en alrededor de 10% de los AGNs)
Radio galaxies
Radio galaxies & radio-loud quasars: the most powerful radio sources
(Usually) extended (or very extended!) radio emission with common characteristics (core-jets-lobes)
Typically hosted by an elliptical (early-type) galaxy
Nevertheless, the radio contribute only to a minor fraction of the energy actually released by these AGNs.(ratio between radio and optical luminosity ~10-4)
Amazing discovery when they were identified with extragalactic, i.e. far away, objects
Unexpectedly high amount of energy involved!
They show most of the phenomena typical of AGNs(e.g. optical lines, X-ray emission etc.) very interesting objects in (almost) all wavebands
in addition they have spectacular radio morphologies
But they are quite rare!
Why are interesting?
How to find them?
Because of the variety of AGNs, there is also a variety of techniques to find them
(e.g. blue colours, strong emission lines etc.).
Here we focus on the way radio galaxies have been found: radio surveys
Radio surveys (some of them….)
3CR (Cambridge Telescope) 328 sources with > - 5o
flux above 9 Jy @ 178 MHz
4C 2Jy 178 MHz Cambridge (+5,6,7C)
PKS ~3Jy 408 MHz Parkes
Molonglo
B2 0.25 408 MHz Bologna (+B3)
NRAO 0.8Jy 1.4-5GHz NRAO
PKS 0.7Jy 2.7 GHz Parkes
NVSS 2.5 mJy (45” res.) 1.4 GHz NRAO VLA Sky Survey
FIRST 1mJy (~5” res) 1.4 GHz Faint Images Radio Sky at Twenty centimeters
WENSS 300 MHz WSRT
(1 Jy= 10-26 W m-2 Hz-1)
85 mJy
Units that will be used for the radio data
Radio flux in “Jansky” 1 Jy = 10-26 W m-2 Hz-1
or 10-23 erg cm-2 sec -1 Hz-1
Radio power (usually estimated at a certain frequency e.g 1.4 or 5 GHz)
or integrated over a typical (radio) range of frequencies (107 to 1011 Hz)
)(4 12 HzWFDP
Radio power: source of 2 Jy flux (@ 1.4 GHz), z = 0.2 log P = 26.5 W/Hz source of 0.2 Jy flux, z = 0.2 log P = 25.5 W/Hz source of 10 Jy flux, z = 0.2 log P = 21.2 W/Hz
resolution /D 21 cm, D = 64 m 11 arcmin 21 cm, D= 3km 14 arcsec 21 cm, D= 3000 km 1 mas
Resolution important for the identification (radio surveys find not only radio galaxies!)
Difference in power limit for the different surveys
HIPASS beam
ATCA image, July 2001
NGC 6580 (S0)
IC 4933 (Sbc)
‘Confusion’ can be resolved by imaging at higher spatial resolution with large
interferometers (WSRT, VLA or ATCA)
Confusion
Optical identifications
NVSS
radio much larger than optical
resolution ~45 arcsec ~ 45 kpc(1 arcsec ~ 1 kpc at z = 0.04)
Radio galaxies are only found among the most powerful radio sources (together with radio-loud quasars).
radio emission from non-thermal synchrotron process
but (radio) AGNs can also be found at low radio powerhigh radio resolution is required to find a very compact core
(to distinguish non-thermal emission from thermal emission)
Going deeper and deeper
Green: WSRT finding chart at 1.4 GHz with an r.m.s. noise of 13 microJy/beam. Grey: NOAO optical R to a limiting depth of 26 magnitude.
VLBI nondetection at full sensitivity with an r.m.s. noise of 9 microJy/beam.
VLBI detections at full sensitivity with an r.m.s. noise of 9 microJy/beam.
(Morganti & Garrett, 2002, ASTRON Newsletter No. 17; Jannuzi & Dey, 1999, ASP Conference Series, 191, 111)
Deep Wide-Field VLBI Surveys
A prototypical radio galaxy
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
Lobes
Core
Jets
Hot-spots
How a radio galaxy works
Zoom-in ofthe central regions
to hot-spots and/or lobes
SupermassiveBlack Hole
accretion disk (UV, Xray)
torus (supposed to hide – for some orientation – the very central regions)
A prototypical radio galaxy
“cocoon” shocked jet gas
backflow
splash-point
bowshockundisturbed intergalactic gas
Observable Diagnostic Constituents Derived Properties
Radio continuum Relativistic plasma Thermal plasma
Energetic, Pressure, Jet propagation velocity, Internal magnetic field Ages, Faraday rotation, Magnetic fields
Radio absorption Lines (21cm)
Neutral gas Column density,kinematics
IR-mm continuum Dust Mass, Temperature
IR-mm emission lines (CO) Molecular gas Mass, density Temperature
UV/Optical/near IRContinuum
Stars Scattered AGN light
Mass, Age,Star-formation ratePolarization properties
Optical emission lines: Ly , H ,[OIII]
Ionized gas (10^4 K) Mass, temperature,Ionized state kinematics
Ly absorption Neutral gas Column density Mass, covering factor
X-ray emission Non-thermal plasmaHot gas (10^7 K)
Jet (and hot-spots) propertiesCluster properties
2
2
2
1
1
cv
cmE e
Electron energy distribution is a power law:
>>1
Relativistic electrons in a magnetic field
dEkEdEEN p)(
2
1
/
/
2/)1(2/)1( ........),(
ppBP
The radio spectrum is therefore a power law:
S 2/)1( p
Typical ~0.8 p~2.6
For one electron, max frequency
2 for slightly different covers the entire spectrum
Assuming the emission from each can be added up (optically thin case)
1. Energy loss2. Self-absorption in the relativistic electrons gas3. Absorption from ionized gas between us and the source (free-free absorption) torus!
Deviations from a constant spectral index
Theory Reality
Energy loss
The relativistic electrons can loose energy because of a number of process (adiabatic expansion of the source, synchrotron emission, invers-Compton etc.). the characteristics of the radio source and in particular the energy distribution N(E) (and therefore the spectrum of the emitted radiation) tend to modify with time.
Adiabatic expansion: strong decrease in luminosity but the spectrum is unchanged
Energy loss through radiation: characteristic electron half-life time (time for energy to half)
*2
8* 1064.1
tBE
After a time t* only the particle with E0<E* still survive while those with E0>E* have losttheir energy.
(Special case assuming
p=2) For the spectral index remains constant break
0
For break Single burstGHz~ 23 yrbreak tB
)5.0( 0 Continuous injection
These energy lost affect mainly the large scale structures (e.g. lobes).
Typical spectral index of the lobes = 0.7
Unless there is re-acceleration in some regions of the radio source!
)()(106.1)( 2/12/33* GHzGBMyrt break
Myrt
GHz
Myrt
GBGHz
break
break
50
1
18
108
*
*
Optically thick case: the internal absorption from the electrons needs to be considered the brightness temperature of the source is close to the kinetics temperature of the electrons.
The opacity is larger at lower frequency -> plasma opaque at low frequencies and transparent at high
Self-absorption in the relativistic electron gas
dBvS 2/12/5)(1
GHz)1()(~ 5/15/45/1 zSBpf mm Frequency corresponding to =1
Affects mainly the centralcompact region or very small radio sources
Higher “turnover” frequency smaller size of the emitting region.
Polarization
Characteristic of the synchrotron emission: the radiation is highly polarized.
For an uniform magnetic field, the polarization of an ensemble of electrons is linear, perpendicular to the magnetic field and the
fractional polarization is given by:
7333
(%)
pp
P 0.7- 0.8 for 2<p<4 never!
Typical polarization from few to ~20% Tangled magnetic field
Example of polarization
Polarization between 10 and 20%(some peaks at ~40% around the edge of the lobes)
Example of polarization in radio jets.
Energetics
Magnetic field strength (Bme) and minimum energy density (ume) Corresponding to equipartition of energy between the magnetic field And the relativistic particles in a synchrotron radio source
7/2
222.01.14 )1(1051.1
lS
zBme
Angular size in arcsec, flux in Jy and frequency in GHz l = path lengthMagnetic field in Gauss and minimum energy in erg/cm3
837 2
meme
Bu
Total energy (electrons and magnetic field) can be up to 1060 erg
Radio, optical, UV, X-ray ……
What is produced apart from the collimated radio jets:
UV radiation (likely coming from the accretion disk) that ionizes the gas optical emission lines
X-ray emission (also from the accretion disk)
The synchrotron spectrum can extend to the optical and X-ray wavelength. Life time of the electrons very short, needs re-acceleration
Gas around the AGN: HI, CO, etc. etc.
Centaurus A: example of emission inmany different wavebands