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Radiogalaxias Basado en una presentación de Raffaella Morganti ASTRON
Temas • ¿Qué son los AGNs y las radiogalaxias? - ¿Cómo encontrarlos? • Una radiogalaxia prototipo - Mecanismos de emisión • 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 Optical emission lines for different AGNs • 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) UV optical X-ray Comparison of the continuum emission from a Seyfert galaxy and a normal galaxy radio
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 • Amazing discovery when they were identified with extragalactic, i.e. far away, objects Unexpectedly high amount of energy involved! 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)
Why are interesting? 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!
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
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 Radio surveys (some of them….) 3CR (Cambridge Telescope) 328 sources with > - 5o flux above 9 Jy @ 178 MHz (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)
Resolution important for the identification • (radio surveys find not only radio galaxies!) resolution /D 21 cm, D = 64 m 11 arcmin 21 cm, D= 3km 14 arcsec 21 cm, D= 3000 km 1 mas • Difference in power limit for the different surveys 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
Confusion ‘Confusion’ can be resolved by imaging at higher spatial resolution with large interferometers (WSRT, VLA or ATCA) ATCA image, July 2001 HIPASS beam NGC 6580 (S0) IC 4933 (Sbc)
Optical identifications radio much larger than optical NVSS resolution ~45 arcsec ~ 45 kpc (1 arcsec ~ 1 kpc at z = 0.04)
Going deeper and deeper 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 power high radio resolution is required to find a very compact core (to distinguish non-thermal emission from thermal emission)
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 detections at full sensitivity with an r.m.s. noise of 9 microJy/beam. VLBI nondetection 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 Hot-spots Core Jets Lobes • 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
to hot-spots and/or lobes How a radio galaxy works torus (supposed to hide – for some orientation – the very central regions) Zoom-in of the central regions Supermassive Black Hole accretion disk (UV, Xray)
A prototypical radio galaxy backflow undisturbed intergalactic gas bowshock “cocoon” shocked jet gas splash-point
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 IR Continuum Stars Scattered AGN light Mass, Age, Star-formation rate Polarization properties Optical emission lines: Ly , H ,[OIII] Ionized gas (10^4 K) Mass, temperature, Ionized statekinematics Ly absorption Neutral gas Column density Mass, covering factor X-ray emission Non-thermal plasma Hot gas (10^7 K) Jet (and hot-spots) properties Cluster properties
Relativistic electrons in a magnetic field >>1 • For one electron, max frequency for slightly different covers the entire spectrum • Electron energy distribution is a power law: • Assuming the emission from eachcan be added up (optically thin case) The radio spectrum is therefore a power law: Typical ~0.8 p~2.6
Deviations from a constant spectral index 1. Energy loss 2. Self-absorption in the relativistic electrons gas 3. Absorption from ionized gas between us and the source (free-free absorption) torus! Reality Theory
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) After a time t* only the particle with E0<E* still survive while those with E0>E* have lost their energy. (Special case assuming p=2) For the spectral index remains constant For Single burst 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!
Self-absorption in the relativistic electron gas 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 Frequency corresponding to =1
Affects mainly the central compact 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: 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)
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 Angular size in arcsec, flux in Jy and frequency in GHz l = path length Magnetic field in Gauss and minimum energy in erg/cm3 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 spectrumcan 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 in many different wavebands