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Energy Transport in Radio-loud AGN
Daniel Evans (Harvard), Julia Lee (Harvard), Martin Hardcastle (U. Herts), Ralph Kraft (SAO), Jane Turner (UMBC/GSFC), Diana Worrall (U. Bristol), Mark Birkinshaw (U. Bristol), Judith Croston (U. Herts)
Energy Transport in Radio-loud AGN
OverviewOverview
• Introduction to AGN and their importance• Concentrate on “radio-loud AGN”: those with jets
• The Central Engine• Black-hole accretion• Jets• The unified model
• Jets in AGN• Simulations• Emission mechanisms
• Hotspots: the points of jet termination• Environments of radio-loud AGN
• Cosmological importance of outbursts• Case study: 3C 321
Energy Transport in Radio-loud AGN
Energy Transport in Radio-loud AGN
What are Active Galactic Nuclei (AGN)?What are Active Galactic Nuclei (AGN)?
• AGN are the compact, luminous centers of certain galaxies (stellar appearance)
• Nonthermal emission, prominent from radio through -ray
• Luminosity exceeds sum of all starlight in galaxy (1010 Lsun)
• 10% of all galaxies host AGN
• 10% of AGN host powerful jets of particles
• Optical spectroscopy of quasars (a class of AGN) shows emission lines that are not close to the laboratory position of any element redshifted
• AGN have profound cosmological influence
Energy Transport in Radio-loud AGN
The AGN ‘Zoo’• AGN are principally divided into ‘radio-quiet’ (no jet) and radio-loud (with
jet) sources:
• Radio Quiet AGN: Seyfert Galaxies
Type 1 Type 2
Radio-quiet Quasars Radio Quiet
• Radio Loud AGN: Radio Galaxies
Narrow Line (Type 1) Broad Line (Type 2)
Radio-loud Quasars Blazars
• All likely to be powered by accretion onto a supermassive black hole• We will see how seemingly different classes can be unified into orientation-
dependent manifestations of the same phenomenon
Luminosity of unresolved ‘nucleus’ exceeds sum of
all stars
HST images of NGC 5548 (left) and NGC 3277 (right)
Energy Transport in Radio-loud AGN
Evidence for SMBHs and their ImportanceEvidence for SMBHs and their Importance
• Optical slit spectroscopy shows relatively high velocities (500 km/s) in unresolved (<10 pc) regions
• Motions of stars in Sag A*• Strong relationships between black-
hole mass and• Bulge mass • Stellar velocity dispersions
• Intimate connection between galaxy formation and black-hole growth, as yet poorly understood
Eck
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Ferrarese & Merritt (2000)
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Energy Transport in Radio-loud AGN
• Host galaxies are always spiral• Quasars are essentially high
luminosity versions• Type 1:
– Have both narrow (forbidden) lines and broad lines peaks in their optical spectra
– Broad lines imply fast moving (v>3,000 km/s) clouds
• Type 2:– These have only narrow (forbidden) emission
lines visible in their optical spectra
– Forbidden transitions implies low-density
Seyfert Galaxies and Radio-Quiet QuasarsSeyfert Galaxies and Radio-Quiet Quasars
Seyfert galaxy NGC 4258
Energy Transport in Radio-loud AGN
Radio-loud AGN (Radio galaxies and quasars)Radio-loud AGN (Radio galaxies and quasars)
• 10 % of AGN emit relativistic jets• Jets extend 100s kpc, up to Gpc• All host galaxies are elliptical – what
does this imply for the radio-loud/radio-quiet dichotomy?
• Similar to Seyferts, some have broad optical lines in their nuclei (type 1, or BLRG); others have narrow lines (type 2, or NLRG); some have no lines (see afternoon talk)
Energy Transport in Radio-loud AGN
The Importance of Radio-Loud AGN• Radio-loud AGN (“radio galaxies”) are the 10% of AGN which emit twin jets of relativistic particles
• Jets transport energy from the Schwarzschild radius on which they are created, out to vast distances (up to Gpc)
• In order to understand the energetics of AGN, we need to know about
– Accretion
– Jet propagation and particle acceleration
– Jet/environment interactions and feedback mechanisms
Necessitates a multiwavelength approach:
– Radio: synchrotron emission from jet
– Optical: properties of host galaxy
– X-ray: Accretion process, particle acceleration to ultrarelativistic energies, hot-gas environments
Chandra and VLA observationof Cygnus A
Energy Transport in Radio-loud AGN
Energy Transport in Radio-loud AGN
I. The Accretion DiskI. The Accretion Disk
• Matter falls in with some angular momentum – it is orbiting the central black hole
• The orbiting material cannot fall all the way in, so a disk is formed
• Frictional forces internal to the disc heat it up, causing it to radiate
• They also transport angular momentum away from the center, so material can eventually fall in
Energy Transport in Radio-loud AGN
II. Broad-line and Narrow-line RegionsII. Broad-line and Narrow-line Regions
• Broad-line region clouds seen in Seyfert 1 galaxies are highly ionized, fast moving, and likely sit relatively close to the AGN power source
• Narrow-line region clouds are further out
• HST and ground-based optical observations resolves the NLR
• Takes the form of an ionization cone
Mrk 573 - HST [OIII]/VLA
IC 5063 - HST [OIII]
Mrk 78 - HST [OIII]
Energy Transport in Radio-loud AGN
III: Unification - The Circumnuclear TorusIII: Unification - The Circumnuclear Torus
Jet
NLRclouds
BLRclouds
1 pc
Accretion disk
Energy Transport in Radio-loud AGN
Observational Evidence for AGN Unification
1. Dusty disks:Commonly observedin all AGN (e.g. Jaffe et al. 1996)
2. Polarized light:Scattered light in Seyfert 2s shows broad emission lines
3. X-ray spectra:Type 1: UnabsorbedType 2: Heavily absorbed(NH>1023 cm-2)
Energy Transport in Radio-loud AGN
Accretion Processes In AGN
• Accretion flow surrounded by dusty torus• BB radiation from disk ‘big blue bump’
Energy Transport in Radio-loud AGN
Accretion Processes In AGN
• Accretion flow surrounded by dusty torus• BB radiation from disk ‘big blue bump’• B-field loops optically thin corona
Energy Transport in Radio-loud AGN
Accretion Processes In AGN
• Accretion flow surrounded by dusty torus• BB radiation from disk ‘big blue bump’• B-field loops optically thin corona• Isotropic X-rays from Comptonization of disk photons in hot corona• Power law X-ray spectrum
Energy Transport in Radio-loud AGN
Fe K Production• Fe K lines are the most commonly used accretion diagnostic• Width and centroid of Fe K line give location of fluorescing material w.r.t. black hole
Fe Kα
George & Fabian (1991)
Energy Transport in Radio-loud AGN
• The 6.4 keV Fe Kα line complex in general consists of a narrow line core, often accompanied by broadened emission
• If we can deconvolve the contributions from the two, we can probe AGN geometry
• Chandra High Energy Transmission Gratings Spectrometer best suited
• Narrow core always attributed to the circumnuclear torus
• What is the origin of the broad emission? (v≈0.3c)• Relativistically blurred diskline?• Unmodeled absorption?
Fe KFe Kαα Lines and Reflection: Lines and Reflection: AGN GeometryAGN Geometry
MCG-6-30-15 (Lee et al. 2002)
1. Newtonian
2. SR beaming
3. GR redshift
4. Profile
Energy Transport in Radio-loud AGN
Fe KFe Kαα Lines and Reflection: Lines and Reflection: AGN GeometryAGN Geometry
Precision HETGS spectroscopy of the Fe K line can tell us:
1) The distance of the primary X-ray emission source from the black hole (e.g., Reyolds & Nowak 2003)
1) The distance of the primary X-ray emission source from the black hole (e.g., Reyolds & Nowak 2003)
2) The inclination of the accretion disk w.r.t. the observer(e.g., Reyolds & Nowak 2003)
2) The inclination of the accretion disk w.r.t. the observer(e.g., Reyolds & Nowak 2003)
3) The spin of the black hole (McClintock et al. 2007)
3) The spin of the black hole (McClintock et al. 2007)
Energy Transport in Radio-loud AGN
More on X-ray SpectroscopyMore on X-ray Spectroscopy
NGC 3783 - Several ionization stages of S and Si are all present (e.g., Kaspi et al. 2002; Kriss et al. 2003)
Netzer et al. (2003)
• Chandra HETGS spectroscopy has brought about a revolution in X-ray spectroscopy
• Detailed modeling of AGN spectra shows the presence of several layers of ionized gas in addition to neutral absorption
• Sometimes ionized gas is in the form of an outflow
• Evidence for more complex absorption than originally thought from simple ‘torus’ model
Energy Transport in Radio-loud AGN
Summary of Section 2: The Central EngineSummary of Section 2: The Central Engine
• All AGN are variants of the same theme, and are powered by accretion onto a supermassive black hole
• Ingredients• Accretion disk of hot gas• Jets (sometimes)• High velocity clouds (broad-line region)• Low-velocity clouds (narrow-line region)• Circumnuclear torus
• X-ray gratings spectroscopy tells us about the physical state of the accretion flow and torus, and can give information about the black hole itself
Energy Transport in Radio-loud AGN
3. JETS IN AGN
Energy Transport in Radio-loud AGN
Jets Are Outflows 5-GHz VLA observation of Cen A
1991
Energy Transport in Radio-loud AGN
5-GHz VLA observation of Cen A
2002
Jets Are Outflows
Energy Transport in Radio-loud AGN
Jets: The Basics
• Jets are collimated energy-carrying channels• Likely electron-positron or electron-proton in nature• Emit radio synchrotron emission• The one-sidedness in some jets is attributed to beaming:
• When an emitting body is moving relativistically the radiation received by an observer is a very strong function of the angle between the line of sight and the direction of motion
• Jets often have a series of knots in them that may be related to shock acceleration
• Jets are efficient accelerators of particles to ultrarelativistic energies (X-ray and -ray emission)
Energy Transport in Radio-loud AGN
Hotspot
Core
Jet
Hotspot
Lobe/plume
High-power(FRII)
Low-power(FRI)
5-GHz VLA images: synchrotron emission
Energy Transport in Radio-loud AGN
The Fanaroff-Riley Dichotomy
Is the dichotomy• Environmental?
• Interaction of the jet with ambient medium either causes the jet to decelerate (FRI) or propagate supersonically to large distances (FRII)
• Intrinsic?• Properties of the central engine govern
large-scale morphology (FRI/FRII)
Energy Transport in Radio-loud AGN
How do Jets Accelerate Particles?Clues from X-ray Observations
Chandra commonly resolves kpc-scale X-ray jet emission in nearby RL AGN:
• FRIs kpc X-ray emission synchrotron in nature (e.g., Worrall et al. 2001)
• Shock acceleration of electrons in magnetic fields to ultrarelativistic energies
• Energy-loss timescale for X-ray synchrotron electron ≈ 10 years
• FRIIs X-ray emission tends to be inverse-Compton (e.g., Sambruna et al. 2004)
• CMB photons upscattered to X-ray energies by radio synchrotron-emitting electrons, dependent on bulk speed of jet outflow
Energy Transport in Radio-loud AGN
More on Particle Acceleration
Arrows to jet indicate compact X-ray features (blue) with radio counterparts (red)
3,000 light years
The Centaurus A Million Second Exposure (Kraft et al. 2007)
• Modeling of radioX-ray spectra as synchrotron emission indicates:
1) X-ray emitting electrons are energetic (E > 10 TeV, γ = 107 – 108).
2) Loss timescales are short (10s of years)
• High energies => particle acceleration must be efficient• Short lifetimes => particle acceleration must be local• How much energy (radiative + kinetic) do jets output? See later.
Energy Transport in Radio-loud AGN
Summary of Section 3: Jets in AGN
1) AGN with radio jets (i.e., “radio-loud AGN”, “radio galaxies”) are rare
2) Fanaroff-Riley Dichotomy influenced by jet power and environment
3) Prominent radio emission (core, jets, lobes, hotspots) is synchrotron in nature
4) Relativistic beaming important5) X-ray synchrotron observations probe ongoing particle
acceleration to ultrarelativistic energies
Energy Transport in Radio-loud AGN
Energy Transport in Radio-loud AGN
What are hotspots?What are hotspots?
• Manifestation of a strong shock at the beamhead of a highly supersonic FRII jet
• Jet fluid passes through shock to inflate a cocoon (lobe) of plasma
• Bow shock driven into ambient medium
• Particle acceleration determines energy distribution of large-scale lobes that inject energy into ambient medium
5-GHz VLA image of 3C98
200 kpc≈ 600,000 ly
Energy Transport in Radio-loud AGN
Physics of Jet Termination and Particle AccelerationPhysics of Jet Termination and Particle Acceleration
• Hotspots emit X-ray synchrotron (in situ) and synchrotron self-Compton radiation
• Detection of multiple hotspots challenges conventional paradigm of single-point termination
• High-resolution radio, optical, X-ray spectroscopy starting to argue in favor of complex jet termination
“Shock-web” structure(Tregillis, Jones, & Ryu 2001)
Energy Transport in Radio-loud AGN
Energy Transport in Radio-loud AGN
The Cooling Flow The Cooling Flow ProblemProblem
• Cooling time of gas tcool T1/2/n
• If tcool < Hubble time, a cooling flow will be set up
• In clusters, 100s of solar masses of gas should be radiatively cooling massive star formation
• Happens first in the center where density is highest, then external gas flows in to maintain hydrostatic equilibrium
XMM and Chandra observations of cluster centers (McNamara et al.; David et al.; Allen et al.; Blanton et al.; Buote et al, Wise et al., etc.)
Where is the cold (T=107 K, 1-2 keV) gas?
Energy Transport in Radio-loud AGN
Environmental Heating by Environmental Heating by Radio Outbursts?Radio Outbursts?
• High-resolution Chandra images of the X-ray-emitting ICM (e.g., Fabian et al. 2002, Croston et al. 2004)
• Radio sources and the X-ray emitting ICM have a profound effect on each other
• We will see that radio sources blow bubbles in the ICM
• In turn, the ICM confines and distorts the radio lobes
Croston et al. (2004)
Energy Transport in Radio-loud AGN
X-ray (Chandra) and radio (VLA) observations of the Perseus cluster (Fabian et al. 2005)
Jets Blowing Bubbles: A Potential Solution
• Total radio outburst energy (pdV) may be a significant fraction of ICM binding energy (see afternoon talk)
• Gentle reheating of ICM offsets rapid cooling of gas
• Need to convert kinetic and particle energy into heat
• Via Turbulent Mixing with ICM
• Via Advection and Mixing of ICM
• Via Shocks in ICM
Energy Transport in Radio-loud AGN
Calculating the Energy
McNamara et al. (2005), Nature
100 arcsec
10-12
10-11
Pres
sure
(N m
-2)
2
9
16
Tem
p (1
07 K)
360 kpc
Radio outburst transfers 1/3 keV of energy per particle
Energy Transport in Radio-loud AGN
Ghost Cavities: Evidence for Ghost Cavities: Evidence for Duty CyclesDuty Cycles
• Chandra observations sometimes show ghost cavities
• New low-frequency radio observations often show larger scale (older) outbursts
• Can begin to establish duty cycle of radio galaxies (synchrotron spectral ageing, buoyant rise times, etc.)
• Periodic radio outbursts may provide sufficient energies to balance radiative cooling
Clarke et al. (2005)
Energy Transport in Radio-loud AGN
Summary of Section 5: AGN Environments
1) Radio-loud AGN (especially FRI sources) are often found in cluster centers
2) Gentle reheating of ICM gas by AGN outbursts may provide a solution to the cooling-flow problem
3) Ghost cavities and new low-frequency radio observations show evidence for periodic outbursts, helping to establish a duty cycle for radio-loud AGN
Energy Transport in Radio-loud AGN
Energy Transport in Radio-loud AGN
3C 321: An Extraordinary FRII• z=0.096 (d=440 Mpc) FRII radio galaxy• Bright radio synchrotron core• Prominent compact radio hotspots• Flared, one sided (FRI-like) jet
Hotspot
Hotspot
Core
153 kpc = 500,000 ly5-GHz VLA A-config image
31 kpc = 100,000 ly
1.4
GH
z M
ER
LIN
+ V
LA
Energy Transport in Radio-loud AGN
3C 321: An Extraordinary FRII
• Prominent knot of radio emission
• Flared jet, seemingly bent
• What is happening?• Necessitates a
multiwavelength approach
31 kpc = 100,000 ly
1.4
GH
z M
ER
LIN
+ V
LA
“Knot”
Seemingly disrupted jet
Energy Transport in Radio-loud AGN
VLA+MERLIN
• Compact core• Knot of radio emission• Diffuse, flared jet
Energy Transport in Radio-loud AGN
HST F702WVLA+MERLIN
• Bright galaxy crossed by dust lane; center coincident with radio core• Companion galaxy at similar position to radio knot• Evidence the two are merging• Spectroscopy shows redshifts are identical
• Compact core• Knot of radio emission• Diffuse, flared jet
Energy Transport in Radio-loud AGN
HST F702WVLA+MERLIN
HST STIS UV
• Extended UV emission• Approximately conical shape• Ground-based optical (Draper et al. 1993) [SII] and [NII] ratios indicate photoionization• Needs a powerful central engine
• Compact core• Knot of radio emission• Diffuse, flared jet
• Bright galaxy crossed by dust lane; center coincident with radio core• Companion galaxy at similar position to radio knot• Evidence the two are merging• Spectroscopy shows redshifts are identical
Energy Transport in Radio-loud AGN
HST F702WVLA+MERLIN
HST UV Chandra X-ray
• X-ray emission: point-like and extended components• Spectroscopy of point-like features both heavily absorbed and accompanied by Fe K lines• Both the main galaxy and companion host powerful AGN (Lx>1044 ergs s-1)
• Compact core• Knot of radio emission• Diffuse, flared jet
• Bright galaxy crossed by dust lane; center coincident with radio core• Companion galaxy at similar position to radio knot• Evidence the two are merging• Spectroscopy shows redshifts are identical
• Extended UV emission• Approximately conical shape• Ground-based optical (Draper et al. 1993) [SII] and [NII] ratios indicate photoionization• Needs a powerful central engine
Energy Transport in Radio-loud AGN
A Jet/Companion-Galaxy Interaction
Jet/companionInteraction point
Host AGNof 3C 321
Companion galaxy(also an AGN)
Disrupted jet
Evans et al. (2007c)
Energy Transport in Radio-loud AGN
A Jet/Companion-Galaxy Interaction
• Two possible scenarios: an interaction with the ISM of the companion galaxy, or an interaction with a ‘cloud’ in the companion galaxy
• Even a 105 solar mass blob can deflect the jet, but the jet continues to propagate
• Compact hotspots, also emit X-ray synchrotron emission in situ particle acceleration
• Jet disruption is temporary:• Time≈light-travel time to hotspots• Will continue for only as long as
companion galaxy in jet’s pathHydrodynamical simulation of a Mach 4 relativistic jet interacting with a dense blob of gas(Choi, Wiita, & Ryu 2007)
Den
sity
Pre
ssur
eLo
rent
z
Energy Transport in Radio-loud AGN
SUMMARY
• The energy output of radio-loud AGN is cosmologically important
• Derive their energy through accretion• Evidence for AGN unification• Jets accelerate particles as they propagate• Jets influence, and are influenced by, their
environments• 3C 321: A remarkable radio galaxy