Large-scale AGN Jetsgroup scales Molecular gas and dust –fuel? Absorption line/LINER spectra Broad...

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Large-scale AGN Jets Physical conditions and particle acceleration Robert Laing Oxford, July 3 2017

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  • Large-scale AGN JetsPhysical conditions and particle acceleration

    Robert LaingOxford, July 3 2017

  • Extragalactic jets


    n Jet power ~1041 – 1047 ergs-1

    n Relativistic bulk flow: pc-scale Lorentz factor Γ ~

  • Jets as particle accelerators


    Centaurus A radio + X-ray

    Mkn 501 average (Abdo et al. 2011)

    M87 TeV gamma rays

  • Radio Galaxy 3C31(RL et al. 2008)

    FRI jets: Low-luminosityDeceleration

  • FRII: Powerful, fast, well-collimated jets(Swain, Bridle & Baum)

    Why the difference ?

    I will talk mostly about FRI jets (and one transition case). Markos will discuss FRII’s.

  • Hosts: old, red and undeadMassive elliptical galaxiesSlow rotators; core profiles Form early; grow by dry mergers

    Very massive black holes

    Old stars (mostly)

    Hot plasma on galaxy andgroup scales

    Molecular gas and dust – fuel?

    Absorption line/LINER spectraBroad lines weak or absentNo evidence for obscuring torus

    Energy output dominated by jets


  • •Flux density in observer's frame at frequency ν•S(θ,ν) = D2+α∫ ε'(θ,ν) dV/d2

    •Doppler factor•Dj = [Γ(1- βcosθ)]-1 (approaching)•Dcj = [Γ(1+ βcosθ)]-1 (receding)

    •sin θ' = D sinθ

    •Approaching jet appears brighter and the two jets are observed at different angles to the line of sight in the flow rest frame θ'

    •Polarization depends on θ' – different in the two jets

    v ≈ c: Proper motions and aberration


    Meyer et al. (2015)βapp = 1.8 – 7.0

  • n What distributions of flow velocity, field geometry and rest-frame emissivity are consistent with observations?

    n Observe:n Deep, high-resolution radio images; IQU, corrected for Faraday

    rotationn Assume:

    n Symmetrical, axisymmetric, stationary, relativistic flown Power-law energy distribution, optically-thin synchrotron

    n Parametrised model of:n Geometryn Velocity field in 3Dn Emissivityn Magnetic-field component ratios

    n Calculate I, Q, U; optimise

    Jet Models

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    Ten FRI radio galaxies (Laing & Bridle 2014) + one transition case

  • Consistency check: Faraday rotation

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  • Consistency check: core flux

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    Core is the optically-thickbase of the jet

    Assume intrinsic ratio ofcore/extended emission isconstant

    Doppler beaming causesobserved ratio f to be anticorrelated with θ

    Fit is simple single-speedmodel

  • NGC6251: a FRI/II transition jet

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  • Looking deeper

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    Flying saucerNGC6252

  • I Model Fits (θ = 30o)

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    Sidedness ratio→ transverse velocity gradient

  • Polarization model fits

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    Colour: I

    Vector length p

    Vector angle χ0+90o


  • Inferred velocity field

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    Independent longitudinal velocity profiles for spine and layerNo transverse variation in either component

    Simplest model:

    Spine: constant β ≈ 0.89 (Γ ≈ 2.2)Layer: β ≈ 0.4 (Γ ≈ 1.1)

    .... but cannot exclude faster spine close to the AGN wherejets are poorly resolved in width

    β = v/c

    This is very different from jets in FRI sources ...

  • FRI jet velocities: deceleration

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  • FRI jet physics: velocity

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    Laing &Bridle (2014)

  • Deceleration and emissivity

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    Deceleration startsin the high-emissivityregion and stops where the jetsrecollimate.

  • In close up

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  • Particle acceleration

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    NGC315 spectral index Radio/X-ray (RL et al. 2006) (Worrall et al. 2007)

  • Distributed particle acceleration

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    Cen A X-rayemission

    (Goodger et al.2010)

  • M87 jet knot spectral energy distributions

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    Consistent with asingle energy distributionfor radiating electrons.

    Broken power law spectrumto a first approximation.

    Typical for FRI jets.

  • Spectrum and speed

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    Spectrum becomes flatter with increasing distance from AGNOpposite to effect of synchrotron lossesVelocity-dependent particle acceleration?

    Laing & Bridle (2013)

  • FRI jets: relativistic particles

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  • Field components for NGC6251

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    Longitudinal → Toroidal, just as in FRI sources ....

  • Field components in FRI jets

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    Longitudinal Toroidal

  • FRI jets: field structure

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  • FRI jet dynamics: conservation law analysis

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    Velocity fits from Laing & Bridle (2014)Conservation-lawanalysis followingLaing & Bridle (2002)



    B2 0326+39

    3C 296

  • Tails and lobes

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    Tails (Plumes) Lobes

    Speed at recollimation

    High Low

    ρc2/w at recollimation(energy density cold:relativistic)

    Low High

    Entrainment Jets keeprestarts on going

  • NGC6251 (if pressure confined)

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    Spine:- fast- (mildly) supersonic- very light- almost no entrainment- carries most of the energy

    Kinetic power 3 x 1037 W

    Layer:- mildy relativistic- transonic- carries more of the mass- but still very light

    Kinetic power 6 x 1036 WLayerSpine

  • n FRI jetsn Widen rapidly, decelerate from relativistic to sub-relativistic speeds

    and recollimaten Develop transverse velocity gradients as they decelerate, implying

    external entrainmentn Have energy fluxes up to ~1037 W (1044 erg s-1)

    n The jets in NGC6251 (a transition case)n Remain narrow and maintain roughly constant speedsn Show persistent transverse velocity structure, with a central

    supersonic spine (β ≈ 0.9) and a slower (β ≈ 0.4) outer layern Entrain very littlen Have energy fluxes of ~3 x 1037 W

    n I suspect that true FRII jets are faster ... but not fast enough to produce beamed iCCMB (see Markos’s talk)

    n Observations of FRII counter-jets are very hard, but we are trying ….


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  • • In FRI jet bases:– Low frequency spectral index is significantly steeper than 0.5 (energy index > 2)– Correlation with flow speed: faster flow → (slightly) steeper spectrum– Particle acceleration up to electron γ ≈ 107 for bulk Γ ≈ 2– Characteristic “broken power-law” spectrum– Acceleration not restricted to a few discrete sites (distributed/multiple small sites)

    • Shocks?– Complex, non-axisymmetric brightness structure at start of deceleration – No evidence for shocks where jets recollimate– Conservation-law analysis (no dynamically important field) → transonic flow– Weak relativistic shocks might explain velocity - spectral index relation (Summerlin

    & Baring)• Shear?

    – Evidence for velocity gradients– One example of association with spectral index

    Particle acceleration questions

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    Are any of the proposed mechanisms efficient enough?