radio galaxies in the chandra era agn jet flows mark birkinshaw university of bristol

Radio galaxies in the Chandra era

AGN jet flows

Mark Birkinshaw

University of Bristol

08 July 2008 Mark Birkinshaw, U. Bristol 2


Outline

1. Critical questions

2. Low-power jets – two case studies and some thoughts

3. High-power jets – another case study with some thoughts

4. Key future observations with Chandra and other instruments



Santa Fe meeting, February 2004“Important questions in the field of dissipation of jets”

• Origin of magnetic field• Jet collimation mechanism• Jet composition• Particle acceleration processes• Understanding of location sites, beaming factors, radiation mechanisms• Dynamical/steady state structures in jets• Applicability of minimum energy, departure sites • Jet slowing and stopping mechanisms• Total energies associated with jets• Processes and timescales of energy transfer to ISM, IGM

D. Worrall



Sources of information1. Jet collimation mechanism – polarization, X-ray brightness of

atmosphere2. Jet composition – calorimetry, radio to IR to X-ray spectral

structure3. Particle acceleration processes – IR to X-ray spectral structure4. Radiation mechanisms – spectral structure5. Applicability of minimum energy – iC/synchrotron brightness

comparisons, pressure balance with ISM, spectral clues6. Total energies associated with jets – IR calorimetry of cores,

evidence for X-ray heating7. Processes and timescales of energy transfer to ISM, IGM –

measurement of X-ray heating, motions in gas and jets



Working model• Twin symmetrical jets• High-speed flows carrying energy and momentum in

particles and fields• Relativistic injection at core associated with a

supermassive black hole and accretion disk• Deceleration during propagation by entrainment/drag• Disruption through terminal shock or flow

interruption, or dispersion through mass loading• Visualization through synchrotron and inverse-

Compton radiation



Caveats

• We only see jets because they are lossy– Physics of loss processes is more complicated than physics

of conservative processes

– Scale of loss physics is that of particle/wave interactions, far below instrument resolutions

– Averaging over many interactions causes uncertainties in the conclusions we can draw

• We usually see only a single time-slice, and hence there can be ambiguities between flow steadiness and environment effects



Case study 1: Centaurus A

Low-power source

Radio: small-scale jet, knot motions

Infra-red: jet and dust

Optical: absorbed

X-ray: fine-scale structure, bright core

γ-ray: to come Combi & Romero (1997)




Radio contours, X-ray image of inner structure

Jet to NE

Shell to SW, looking remarkably like SNR shock front

X-ray: fine-scale structure in jet and in edges of bright radio emission.

Kraft et al. (2003)

Worrall et al. (2008)




Centaurus AIRAC

IR colour (white = star-like)

Jet and dust

Jocelyn Keene; Brookes et al. (2006)




Subluminal radio knot motions

X-ray knot motions not detected with Chandra (8 year baseline).

Different knot SEDs associate with different radio motions.

Jet intrusions vs moving knots?

Hardcastle et al. (2003)



Case study 1: Centaurus A • Knot motions at 0.5c suggest mildly relativistic flow in inner jet; knots

become less evident further out where jet should have slowed• Inner narrow radio jet before flare point – faster and disconnected from

external medium?• No obvious counter-jet, hence alignment from brightness ratio• X-ray/radio offsets implying particle acceleration sites with different

characteristics• Different knot properties, different motions – related to nature of particle

acceleration• Messy gas structure – expect ISM to be impacting jet, perhaps causing

bending• Observation of ISM heating in counter-jet region• IR spectrum of outer jet suggests not entirely synchrotron emission• All this structure would be unresolved at high z



Case study 2: M87

Low-power source

Radio: small-scale jet, knot motions

Infra-red: jet and dust

Optical: jet, motions

X-ray: fine-scale structure, variability

γ-ray: to come

Owen et al. (1999)



Case study 2: M87

Galaxy-subtracted Spitzer image, IRAC channel 4

Main jet shows bright synchrotron emission

Counter-jet not seen, but note patches of cool dust on counter-jet side: cooling plume?

Spitzer; 8 μm; galaxy subtracted



Case study 2: M87• Much small-scale structure

• High variability of HST-1

• Relativistic internal motions

• Polarization/intensity correlations implying a sheared flow

• Inner VLBI shows edge-brightening, helical?

• X-ray spectra steep: synchrotron radiation

• SED break frequencies drop along jet VLA, HST, Chandra, Chandra + smoothed HST;

Marshall et al. (2002)



Case study 2: M87 • Knot structures change at several c so relativistic internal

motions in knots • Variability (e.g., HST-1) consistent with synchrotron outburst in

moderate relativistic flow• No obvious counter-jet, hence alignment from brightness ratio or

possibly lack of symmetry? Counterjet HST-1 debeamed in core?

• VLBI shows inner structure has edge brightening (e.g., Ly et al. 2007) and collimates within about 100 RS of the black hole

• Radio and X-ray structure suggests convective plumes lifting core material (offset IR emission from dust on counterjet side)

• All this structure would be compact at high z



Many jets: e.g, 3C31 optical, IR

Residual R map, after subtracting E galaxy profile. 11 Jy feature to N is counterpart of the brighter radio jet. Core structure from AGN and disk.

Croston et al. (2003)

More convincing in Spitzer 8 m data

Bliss et al.



Many jets; e.g., NGC 6251 IR

IRAC ch2

IRAC images at 3.6, 4.5, 5.8, 8.0 m all show jet extending to edge of frame.



3C 66B

Radio, IR, optical, X-ray jets similar

10 kpc

Hardcastle et al. (2001)



3C 66BOptical polarization in jet (to about 30%) – synchrotron emission with significant magnetic order.

No second spectral component.

Field vectors tend to follow jet edges, so field tied to flow dynamics. Stokes I, % polarization, outer/inner apparent B

vectors; Perlman et al. 2006



3C 66B

Spitzer 4.5 m image, galaxy subtracted.

Bliss et al.



Jet spectra: high degree of uniformityMany jet spectra are similar.

Break frequencies in IR or optical. Using equipartition fields, break energies in the 300 GeV - 1 TeV range

Spectral break by > 0.5, indicative of acceleration physics.

Break energy, break amount, similar in many jets.



Electron energies and spectra• Beq 15 nT.• Electrons at spectral breaks have E 300 GeV.• Lifetimes of electrons emitting synchrotron X-rays 30 years,

so spectra are from locally-accelerated particles• Break energies consistent with the cyclotron instability (which

should give electron and positrons to E 1 TeV) and B = Beq (e.g., Hoshino et al. 1992; Amato & Arons 2006).

• Unique spectral feature in continuum flows that may be useful check on relativistic flows

• Requires presence of heavy particles in flow• Consistent with properties of hot spots (Kataoka et al. 2007)



Jet spectra• Synchrotron spectra of jets are very similar between different

sources, with breaks > 0.5, not synchrotron ageing but change of dominant acceleration mechanism at about 300 GeV

• X-ray spectra steeper than radio spectra – not inverse-Compton radiation.

• Synchrotron jets, close to equipartition, with cyclotron instability causing electron acceleration to Lorentz factors ~ 103

• Nature of acceleration to higher energies? Shocks? Reconnection? Must be active between knots as well as within knots.

• We have the telescopes now to map better the changes in spectra between knots, and in diffuse regions, as a function of position in jets.



Core spectra: Spitzer

Sequence of spectral behaviours is same as sequence of radio powers, and FR types.

Relates to energy input into hot dust: AGN IR power defines source structure.

First 10 of set of 35 spectra.

Birkinshaw et al.



Core spectra: Spitzer

Timescale of dust cooling is much greater than the timescale of radio jet propagation: correlated IR and radio properties imply AGN do not undergo large luminosity fluctuations.

Birkinshaw et al.



Case study 3: PKS 0637-752High-power, one-sided jet in quasar at z =

0.651

• X-ray/radio ratio fairly constant (?)

• Only subset of radio knots are X-ray and optically bright. Bend effect?

• iC/CMB best explanation: jet highly relativistic to 50+ kpc

• Issues to do with ageing of electrons: since iC from low energies

Chandra, HST, ATCA; Lovell et al. 2003



Jet spectra• Some indication that break frequency from synchrotron (radio-

optical) part of spectrum is at higher frequencies as move further out in jets – suggests magnetic field increasing along jet, if the break is identified with acceleration by the cyclotron instability

• Break frequency is lower in quasars than in lower-power jets, suggesting that the magnetic field is lower – possible because of beaming of these jets suggested by iC emission.

• If X-rays are not iC (second synchrotron component), then why does this arise only in some parts of the flow?

• Can bending and adiabatic expansion solve the X-ray/radio ratio problem in quasar jets – iC X-rays should be evident where there are no high-frequency knots



Jet spectra• Emission not single-component synchrotron, since spectra too

complicated• Emission not SSC if system near equipartition (Chartas et al.

2000)• iC emission/CMB is possible, but issues

– X-ray decreasing/radio increasing down jet (OK if decelerations; Georganopoulos & Kazanas)

– Knot/inter-knot contrast is higher than expected in X-ray (should be less than in radio; combination of expansion and ageing?)

– Sources have huge sizes if beamed– Why no entrainment and slowing changing the properties?– Why no big infra-red bump from iC of cold electrons?

• Polarization measurements of high-energy component (optical in some cases) would resolve issue



Summary• Better radio – X-ray spectra important to test

acceleration models (GLAST will help test hadronic models)

• Should use existing high angular resolution to get better position-resolved radio – X-ray spectra

• Optical and higher-energy polarization data would check interpretation of hard component in spectra and map field configurations

• Thermal dust lifted by jets may be an indicator• Plus Herschel, ALMA, Chandra projects …



Herschel

• Herschel: launch in 2009• Higher sensitivity in mid and far IR: complete program of

calorimetry of cores



ALMA

• Sub-mm band for dust and synchrotron emission

• Spectroscopy will separate emission mechanisms

• Sub-arcsec structures of cores and bright inner jets



Chandra• Defines high-energy spectra and

investigates particle acceleration sites and physics (needs higher-quality optical data to match)

• Critical for investigating environments – confinement and heating from jets (needs higher sensitivity observations than most done to date)

• High resolution for studying jet spectral/spatial relationships – offsets acceleration processes

• Will be crucial when GLAST sees AGN outbursts

radio galaxies in the chandra era agn jet flows mark birkinshaw university of bristol

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