Presentation on theme: "Raffaella Morganti ASTRON Radio Galaxies Part 1. The plan 1.What are AGNs and radio galaxies - How to find them A prototype radio galaxy - Emission mechanisms."— Presentation transcript:
Raffaella Morganti ASTRON Radio Galaxies Part 1
The plan 1.What are AGNs and radio galaxies - How to find them A prototype radio galaxy - Emission mechanisms 2.Morphology of the radio emission: various morphologies, nuclear regions, highly collimated jets - hot-spots - radio lobes 4.A look at the host galaxy Ionized gas: from pc to tens of kpc 5. Origin and evolution of radio galaxies Unified Schemes for AGNs: trying to make sense of the different types 3.Gas in radio galaxies. Neutral hydrogen: looking at some data and what can we derive
What are the Active Galactic Nuclei Difficult to have an unique definition. huge amount of energy (up to 10 4 times more than a normal galaxy) emitted from a tiny region (<1 pc 3 ) An AGN can have a luminosity ranging from to erg/sec wide energy range covered wide range of different objects! The high energy released by an AGN is believed to originate from a supermassive black-hole (10 6 to 10 9 M sun in <<1pc) now even lower power AGN are found but the presence of a supermassive BH is not enough!
Some characteristics Comparison of the continuum emission from a Seyfert galaxy and a normal galaxy Optical emission lines for different AGNs radio optical X-ray UV Luminosity is not the only criteria: continuum emission (often quite blue) across ~13 order of magnitude in frequency emission lines radio emission (in about 10% of AGNs)
Very small angular size Wavelength/resolution dependent Radio VLBI, HST cores High luminosity Broad-band continuum Composite spectrum for AGNs Strong emission lines Variability Polarization Radio emission Powerful way to detect AGNs, BUT only a minority are strong radio sources and the radio accounts for at most 1% of the total energy output Observables to classify an object as AGN
There are many different types of AGNs, depending which characteristic is dominant radio loud vs radio quiet strong optical lines narrow vs broad core dominance weird & extremely variable objects
Optical Emission Line Properties Different types of AGNs: a summary Type2 narrow line Type 1 broad line Type 0 Radio quietSeyfert 2Seyfert 1 ?QuasarsBroad absorption line QSO Radio loud low powerBL Lac Narrow-line radio galaxies Blazar, OVV and many other weird objects high powerbroad-line RG lobe/core dominated QSR Decreasing angle to line of sight
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 > - 5 o flux above MHz 4C 2Jy178 MHzCambridge (+5,6,7C) PKS ~3Jy408 MHzParkes Molonglo B MHzBologna (+B3) NRAO 0.8Jy1.4-5GHzNRAO PKS 0.7Jy2.7 GHzParkes NVSS 2.5 mJy (45” res.)1.4 GHzNRAO VLA Sky Survey FIRST 1mJy (~5” res)1.4 GHz Faint Images Radio Sky at Twenty centimeters WENSS 300 MHz WSRT (1 Jy= W m -2 Hz -1 ) 85 mJy
Units that will be used for the radio data Radio flux in “Jansky” 1 Jy = W m -2 Hz -1 or 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 (10 7 to Hz)
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 power high 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 to W/Hz Lobes Core Jets Hot-spots
How a radio galaxy works Zoom-in of the central regions to hot-spots and/or lobes Supermassive Black 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 bowshock undisturbed intergalactic gas
Observable DiagnosticConstituentsDerived Properties Radio continuumRelativistic plasma Thermal plasma Energetic, Pressure, Jet propagation velocity, Internal magnetic field Ages, Faraday rotation, Magnetic fields Radio absorption Lines (21cm) Neutral gasColumn density, kinematics IR-mm continuumDustMass, Temperature IR-mm emission lines (CO)Molecular gasMass, 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 state kinematics Ly absorption Neutral gasColumn density Mass, covering factor X-ray emissionNon-thermal plasma Hot gas (10^7 K) Jet (and hot-spots) properties Cluster properties
Electron energy distribution is a power law: >>1 Relativistic electrons in a magnetic field The radio spectrum is therefore a power law: Typical ~0.8 p~2.6 For one electron, max frequency for slightly different covers the entire spectrum Assuming the emission from each can be added up (optically thin case)
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! 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) After a time t* only the particle with E 0 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!
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 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: for 2
Example of polarization Polarization between 10 and 20% (some peaks at ~40% around the edge of the lobes)
Example of polarization in radio jets.
Travel through a plasma+magnetic field (that can be internal or external to the source) changes the polarization angle If the medium is in front of the radio source: no change in the fractional polarization If the medium is mix in the radio source: depolarization dependence on wavelength (if due to Faraday rotation) Depolarization happens also if the magnetic field is tangled on the scale of the beam of the observations N e = electron density of the plasma dl = depth = component of the magnetic field parallel to line of sight Rotation measure (RM) RM can be derived via observations at different wavelengths Faraday rotation thermal electrons with density ~ cm -3
Energetics Magnetic field strength (B me ) and minimum energy density (u me ) 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/cm 3 Total energy (electrons and magnetic field) can be up to 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 in many different wavebands
Type 2 (narrow line) Type 1 (broad line) Type 0 (unusual) Radio quiet Seyfert 2Seyfert 1 ?QuasarsBAL QSO? Radio loud FRI BL LAC Narrow-line radio galaxies Blazar, OVV and many other weird objects FRIIBL radio galaxies Lobe dominated RQ Core dominated RQ Decreasing angle to line of sight Optical Emission Line Properties Different types of AGNs: a summary