energy transport in radio-loud agn daniel evans (harvard), julia lee (harvard), martin hardcastle...

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)


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


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


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)


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

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Ferrarese & Merritt (2000)

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• 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


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)


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


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


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]


III: Unification - The Circumnuclear TorusIII: Unification - The Circumnuclear Torus

Jet

NLRclouds

BLRclouds

1 pc

Accretion disk


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)


Accretion Processes In AGN

• Accretion flow surrounded by dusty torus• BB radiation from disk ‘big blue bump’



• Accretion flow surrounded by dusty torus• BB radiation from disk ‘big blue bump’• B-field loops optically thin corona



• 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


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)


• 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


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)


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


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


3. JETS IN AGN


Jets Are Outflows 5-GHz VLA observation of Cen A

1991


5-GHz VLA observation of Cen A

2002

Jets Are Outflows


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)


Hotspot

Core

Jet

Hotspot

Lobe/plume

High-power(FRII)

Low-power(FRI)

5-GHz VLA images: synchrotron emission


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)


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


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.


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


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


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)


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?


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)


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


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


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)


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


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


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


VLA+MERLIN

• Compact core• Knot of radio emission• Diffuse, flared jet


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



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




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)



• Extended UV emission• Approximately conical shape• Ground-based optical (Draper et al. 1993) [SII] and [NII] ratios indicate photoionization• Needs a powerful central engine


A Jet/Companion-Galaxy Interaction

Jet/companionInteraction point

Host AGNof 3C 321

Companion galaxy(also an AGN)

Disrupted jet

Evans et al. (2007c)


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

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rent

z


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

energy transport in radio-loud agn daniel evans (harvard), julia lee (harvard), martin hardcastle...

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